SKTF -- Fall 1993

The Smith-Kettlewell Technical File

A Quarterly Publication of
The Smith-Kettlewell Eye Research Institute’s
Rehabilitation Engineering Research Center

William Gerrey, Editor

Issue: SKTF -- Fall 1993

Original support provided by:
The Smith-Kettlewell Eye Research Institute
and the National Institute on Disability and Rehabilitation Research

Note: This archive is provided as a historical resource. Details regarding products, suppliers, and other contact information are original and may be outdated.

Questions about this archive can be sent to
sktf@ski.org

TABLE OF CONTENTS

DIGITAL MAGNITUDE

COMPARATORS

A REVIEW OF THE MICRONTA

22-182 DIGITAL MULTIMETER WITH COMPUTER
INTERFACE

FOLDING CANES AND

MODIFICATIONS THEREOF

DIGITAL MAGNITUDE

COMPARATORS

name="digital">

These chips allow two binary numbers to be

compared.
Outputs indicate whether one is less than, equal

to, or greater than the other. Singly, these chips compare
4-bit numbers;
however, they can be cascaded to accommodate

large numbers.
Applications would include comparing a counter

with the output of
an A-to-D converter. Suppose you run a stepper

motor from the
clock of the counter; you could get to stop, or

change direction,
as directed by the A-to-D converter's output.

The Motorola
MC14585BCP

Two 4-bit "words," "A" and "B," are fed to
four inputs. As they define them, these inputs are: A0, A1, A2, A3,
and B0, B1, B2 B3.
(The editor prefers to think of these inputs as
A1, A2, A4, A8,
etc., but the notation is arbitrary.)

There are three outputs: "A less than B," "A

equals B," and "A
greater than B." The appropriate output goes

high for its given
condition.

There are three inputs used for cascading:

"A less than B," "A
equals B," and "A greater than B." For some

obscure reason, on
the least-significant chip, these are to be tied

"low," "high,"
and "low," respectively. On the other hand,

"less than," "equal
to," and "greater than" inputs of each

higher-order chip go
directly to those corresponding outputs of its

less-significant
neighbor. (If only one chip is used, these

inputs are still to
be committed with "equals" being high and the

others low.)

As you might expect, the outputs of the

highest-order chip are
the ones you pay attention to for the ultimate

comparison.

[The truth table describing the operation of

those cascading
inputs suggest that the "greater than" one is a

dead pin. If you
think about it, the chip could learn anything it

needs to know by
responding to states of the first two. A

frequent entry in the
truth table for the "greater than" input is

"don't care."
Another source of literature, "The CMOS Pocket

Guide," says to
tie the "greater than" input high, as in low

high high. One of
us ought to leave it open some time and see what

smokes--or
doesn't smoke.]

Specifications for the

MC14585BCP

These are CMOS devices. Their supply voltage

can be between 3V
and 18V. In quiescence, and without a load, the

current drain is
minuscule--0.005uA typical, 5uA maximum. (As

devoted students of
the article "Inside Gates" will remember, SKTF,

Summer 1981,
current drain of CMOS logic goes way up in

operation, since
complementary FET's simultaneously conduct while

changing state.)

Their outputs can drive two low-power TTL

inputs, or one low-
power Schottky input. (This means sinking

1.6mA.)

As far as speed is concerned, they give a

figure called "turn-
on/turn-off delay time." This is the time

measured from when an
input line is half-way up and the time when the

output state is
half-way up. It varies considerably with power

supply
voltage--at 5V, the maximum time is 860

nanoseconds (ns). At
10V, this maximum is 180ns; at 15V it is 130ns.

They show
typical delays of half these values, but the

prudent engineer
would plan on a delay of a microsecond at 5

volts.

Pin Assignments for the Motorola

MC14585BCP (16-Pin Plastic Package)

  • Power Connections:
    • Pin 8--VSS (ground)
    • Pin 16--VDD
  • Four-bit Word Inputs (logic high

    defined as "1"):

    • Pin 10--Input A0
    • Pin 7--Input A1
    • Pin 2--Input A2
    • Pin 15--Input A3
    • Pin 11--Input B0
    • Pin 9--Input B1
    • Pin 1--Input B2
    • Pin 14--Input B3
  • Inputs for Cascading chips

    (least-significant chip has pin 6 tied
    high; pins 4 and 5 are grounded):

    • Pin 5--Input A less than B
    • Pin 6--Input A equals B
    • Pin 4--Input A greater

      than B

  • Comparison outputs (go high when

    condition is met):

    • Pin 12--Output A less

      than B

    • Pin 3--Output A equals B
    • Pin 13--Output A greater

      than B

    The CD4063

    Once an RCA number, then made by Harris (and

    who knows in six
    months), this is a very similar device to the

    above. They are
    not pin-for-pin compatible, however.

    As with the Motorola chip, the

    least-significant device should
    have its "cascading inputs" as follows: A less

    than B low, A
    equals B high, and A greater than B low. (This

    input arrangement
    holds true if a single chip is used.) In a

    cascade of chips,
    these inputs go to the corresponding outputs of

    the less-
    significant neighbor.

    The sense of the 4-bit inputs is positive

    logic--high equals 1.
    The outputs: A less than B, A equals B, and A

    greater than B, go
    high when their condition is met.

    Pin Assignments for the
    CD4063 (16 pins)

    • Power Connections:
      • Pin 8--Ground
      • Pin 16--VDD
    • Four-Word Inputs (high equals

      1):

      • Pin 10--Input A0
      • Pin 12--Input A1
      • Pin 13--Input A2
      • Pin 15--Input A3
      • Pin 9--Input B0
      • Pin 11--Input B1
      • Pin 14--Input B2
      • Pin 1--B3
    • Cascading inputs (on

      least-significant chip, pins 2 and 4 are
      grounded; pin 3 is tied high):

      • Pin 2--Input A less than B
      • Pin 3--Input A equals B
      • Pin 4--Input A greater

        than B

    • Comparison Outputs (go high

      when condition is met):

      • Pin 7--Output A less than

        B

      • Pin 6--Output A equals B
      • Pin 5--Output A greater

        than B

      A REVIEW OF THE MICRONTA 22-182

      DIGITAL MULTIMETER
      WITH COMPUTER INTERFACE

      id="review" name="review">

      by Jay Williams

      ABSTRACT--

      As you may know, Radio Shack no longer sells

      a
      multimeter equipped with internal digitized

      speech. It has been
      replaced by the Micronta 22-182, which instead

      provides an RS232C
      serial port and associated PC-compatible

      software. In addition
      to voltage, current and resistance, this meter

      can measure
      capacitance and frequency. It also includes a

      simple continuity
      tester, a logic level indicator and a transistor

      beta checker.

      Introduction

      At $130, this is a good buy. However, if you

      do a lot of work
      with electronics, this meter should simply be

      one more tool, not
      your "be-all and end-all." For the blind

      builder there is no
      substitute for the traditional analog meter with

      auditory output
      when measuring dynamic parameters.

      Readings can be displayed in several modes.

      First, all
      quantitative measurements can be displayed

      relative to a
      reference of your choice. Second, the meter can

      be set to
      register maximum and minimum readings, and

      third, it can be set
      to hold a reading indefinitely. Because you can

      interface the
      meter with a computer, you can save a sequence

      of readings to a
      file.

      In this article I will discuss these features

      in detail and how
      the meter can be accessed by your computer.

      And, because no such
      device is perfect, I will point out the few but

      relevant flaws
      you will encounter when using it.

      Thank you, Susan Fowle, for writing some very

      useful programs
      that render the meter more user-friendly. Tom

      Fowle also
      deserves gratitude for his advice to a neophyte

      writer on
      computers such as I.

      Location of Essentials--

      Since this meter will not function without

      its battery and fuse, it makes sense to

      ascertain their
      whereabouts. Orient the meter so that its

      control panel faces
      downward and the end containing the display

      points away from you.
      Notice that the bottom cover has two rubber

      "feet" that extend
      nearly the width of the cover. One foot is

      right at the bottom
      end and the other is about two-thirds of the

      length distant. In
      fact, it defines the beginning of the display

      section.

      The door for the battery compartment -- which

      accepts the 22-
      182's single 9-volt battery -- comprises the

      bottom end of the
      cover. The Phillips screw that secures the door

      is a finger's
      width up from the center of the rubber foot.

      In order to gain access to the fuse, remove

      the four Phillips
      screws that secure the meter's bottom cover.

      The battery door is
      actually part of this cover so you need not

      remove it separately
      for this operation. Two of the screws will be

      found in the
      extreme corners at the bottom end of the cover.

      The other two
      are placed at each end of the rubber foot that

      defines the
      beginning of the display section. When the

      cover is removed, you
      will find the fuse holder adjacent to the

      battery and just before
      the flexible protective cover for the circuitry.

      All of these screws have unthreaded portions

      in the middle of the
      shanks which prevent their falling out of the

      holes.

      When removing this cover, be careful not to

      jerk the wires of the

      small loudspeaker which is force-fitted into a

      recess in the
      cover. You can remove this speaker quite easily

      with your
      fingers. This loudspeaker emits the various

      "beeps" that
      indicate the completion and status of many

      functions.

      The bottom cover also contains a stand that

      can be extended by
      grasping it from within the rectangular recess

      just beyond the
      battery door.

      Control Panel Layout--

      Orient the meter so the display points away
      from you. Below the display and proceeding from

      left to right
      are three rectangular rubber buttons.

      The first button stands by itself and is the

      "power on/off"
      button. Toward the right side of the panel is a

      pair of similar
      buttons. The first one toggles through four

      states: normal
      reading, data-hold, maximum, and minimum. The

      button to its
      right toggles between "normal" reading and

      "relative" reading.
      These are discussed in detail later.

      Below these buttons is a large, 30-position

      selector control. At
      its top-left are two rectangular holes, one

      above the other.
      These are the holes into which the leads of a

      capacitor can be
      inserted for measurement. At the selector's

      lower-right is a
      circular object. This is a socket into which a

      transistor can be
      inserted for checking.

      Below the capacitor socket is another

      pushbutton labeled "comm."
      When toggled on, the meter sends data

      continuously through the
      RS232 port. When toggled off, further readings

      are issued only
      when you press a key on your computer's

      keyboard.

      At the bottom of the panel are four holes

      going from left to
      right. These holes accept the connectors for

      the test probes.
      The probes are conventional "pencil-like"

      extensions with pointed
      tips. Their connections to the meter are

      hollow cylindrical
      plugs. The holes in the meter that accept these

      plugs have a
      protrusion that must engage the corresponding

      center hole in the
      plug. While these connections greatly reduce

      the likelihood of
      contact with your skin, they require frequent

      attention since
      they do not make connection until they hit the

      bottom of the
      hole. This is the first thing to check if you

      get no response.

      You will use the two right-most holes for

      most measurements. The
      farthest hole to the right is the "hot"

      terminal, the one to its
      left is "common." To the left of "common" are

      two holes (BOTH
      POSITIVE INPUT TERMINALS) for measuring current.

      First comes one
      for measuring up to mA200. This terminal has a

      2-amp fuse. The
      left-most terminal is unfused and is for

      measuring up to 20
      amperes. The manufacturer recommends a maximum

      "on-time" for
      such measurements of 30 seconds followed by

      fifteen minutes rest.

      The connection for the RS232 port is on the

      right side of the
      meter. If you feel down the side, directly to

      the right of the
      "hot" terminal, there is what appears to be a

      lengthwise
      "scratch" in the plastic, just above the seam.

      This is actually
      a row of seven small holes. The 7-pin connector

      on the supplied
      cable plugs into this "socket," so that the

      cable extends toward
      the top of the meter. The plug can be inserted

      in the opposite
      direction, but this will not harm the meter

      since its innards are
      optically isolated from the computer's serial

      port. The reverse
      connection of this plug results in an "I/O

      error" message.

      A Few Flaws--

      Unfortunately, the selector knob has no

      "STOP" that
      would prevent continuous rotation. Although you

      can determine
      the setting of this control from the information

      on your computer

      screen, we recommend that you apply physical

      markings to indicate
      at least the highest ranges of voltage and

      current. You will
      probably want to mark other positions you use

      often.

      The sockets that are used for capacitors and

      transistors are
      poorly designed, so you will have to use some

      ingenuity in order
      to ensure reliable connections between their

      contacts and the
      leads of the component to be measured.

      Although we managed to obtain a reading for

      transistor beta, it
      took much "fiddling." We suggest that, since

      this meter gives
      you no more significant information than you can

      get by checking
      it with a continuity tester (see Continuity

      Tester Uses, SKTF,
      Fall, 1982), ignore this feature of the meter.

      Meter Functions and

      Specifications

      Turn the meter on by pressing the "power"

      button. The meter will
      emit a long and a short beep if it "powers up."

      Pressing the
      button again turns off the meter. It utters a

      very short beep
      when it "powers down." It turns off

      automatically if no readings
      are taken for about ten minutes. Be sure that

      the test probes
      are plugged into the two right-most sockets.

      Since there is no "end stop" on the function

      selector, the most
      logical point of orientation is the "continuity"

      function.
      Rotate the selector so that the slotted end of

      its pointer is at
      the "nine o'clock" position, between the two

      sockets for
      capacitor leads. The continuity position is

      four clicks
      clockwise. You can confirm that you have

      selected this function
      by touching the probes together and waiting for

      the steady tone.
      Depending on the mental gymnastics the meter's

      performing, the
      tone may not appear for a second or two. The

      tone will not
      appear if the resistance between the probes is

      30 ohms or more.

      Resistance--

      The next six notches going clockwise are the
      following resistance ranges: 200 ohms, 2

      kilohms, 20 kilohms,
      200 kilohms, 2 megohms, and 20 megohms. No

      audible indication of
      "overload" is presented in the resistance and

      "logic low"
      functions. In all other modes the meter emits a

      continuous tone
      when an overload condition is reached.

      The display and your computer will show a

      message such as, ". ol
      kohm." The "PERIOD" can precede the "ol"

      immediately, or
      separated from it by a space, or be placed

      between the two
      letters. The PERIOD indicates the degree of

      resolution to which
      the meter is set within the function being

      monitored.

      D.c. Voltage--

      Proceeding clockwise, d.c. voltage is next.

      Its
      six ranges follow in this order: 20mV, 200mV, 2

      volts, 20 volts,
      200 volts, and 1000 volts. Positive and

      negative readings can be
      taken.

      A.c. Voltage--

      The a.c. voltage ranges are next and proceed

      in
      reverse order: 750 volts, 200 volts, 20 volts, 2

      volts, 200mV,
      and 20mV. The manufacturer recommends that you

      not use this
      meter in circuits whose common is more than 500

      volts above
      "earth ground." They also pointedly discourage

      the metering of
      three-phase a.c. circuits with this meter. They

      further state
      that if, however, you insist on doing so, do

      your calculations
      very carefully.

      A.c. and D.c. Current--

      Continuing clockwise are the a.c. current,
      then d.c. current ranges as follows. a.c.:

      20mA, 200mA, and 2
      amps. D.c.: 2 amps, 200mA, and 20mA.

      For measurements up to 200mA, plug the hot

      test probe into the
      hole to the left of "common." For measurements

      between 200mA and
      twenty amperes, insert the hot test probe into

      the hole at the
      far left. As stated earlier -- but worth

      repeating -- this jack
      is not fused, and the manufacturer recommends a

      maximum "on" time
      of 30 seconds to be followed by 15 minutes of

      "off" time.

      The high-current jack is functional only when

      the selector is set
      to the highest current ranges. The jack for the

      "hot" probe that
      you use for all other measurements is rendered

      ineffective in the
      "current" modes.

      Transistor Beta--

      Then comes the position for checking

      transistor
      beta, designated as h, with the subscript FE.

      As mentioned
      previously, taking a measurement in this mode is

      really more
      trouble than it's worth. The socket is the

      culprit. The holes
      that accept the leads are too large, and there

      are a lot of
      them--eight, to be precise. There is a separate

      semicircle of
      four holes each for NPN and PNP transistors.

      The manual does not
      make it clear as to how to use these four holes.

      It further
      states that bipolar transistors, not of the

      power variety, are
      the only ones for which the meter is designed to

      check the gain.

      Logic Levels--

      The next position on the selector is for
      determining logic levels. The display reads

      "ready" when this
      mode is first selected. Connect the test probes

      to the minus and
      plus power supply voltages of the circuit

      containing the logic
      levels to be tested. Then, press the "rel"

      button and start your
      search with the hot probe. If the point in

      question is at 70% of
      the power supply's voltage or greater, the

      display reads "high"
      and a continuous tone is emitted. If the point

      is 30% of this
      voltage or less, the reading is "lo" and no

      audible indication is
      given. If the voltage is between these two

      references, the word
      "float" appears.

      Capacitance--

      Clockwise from the logic mode is a

      three-range
      capacitance section. The progression is 2000pF,

      2000nF, and
      20uF. An overflow condition emits a continuous

      tone.

      Frequency--

      Lastly, there is a two-range frequency meter.

      The
      first range extends to 20kHz, and the second, is

      200kHz. I have
      gotten a reliable reading with a signal as low

      as 20 millivolts
      at 1kHz. The manual advises that you not

      measure a frequency
      whose voltage exceeds 250 volts RMS. Here,

      also, a tone is
      emitted when the frequency range is exceeded.

      Data Presentation Modes--

      Once you have selected a function, you
      have some choices as to how data is presented.

      Fortunately, the
      meter defaults to the conventional mode when it

      "powers up." The
      two buttons below the right-hand end of the

      display accomplish
      most of these. The left-most button toggles

      through four modes:

      Pressing it once initiates "data-hold"; this

      causes the meter to
      retain the most recent reading. Pressing it a

      second time causes
      the meter to display the minimum reading

      observed. Pressing the
      button again displays the maximum reading. The

      meter updates both
      maximum and minimum readings when monitoring

      either parameter, so
      when you change from displaying the minimum to

      displaying
      maximum, the latter has already been held in

      memory. These data
      are erased with any further change of function

      or when the
      meter's turned off. Pressing it the fourth time

      makes it give
      current readings, which is the default.

      Selecting the "Relative" Function--

      The right-most button toggles
      between "normal" and "relative." This function

      is used when
      determining logic levels and in situations where

      you wish to set
      a numerical value as a "zero" reference, such as

      a particular
      voltage. For example, you may want to observe

      the fluctuations
      of your household a.c. voltage during peak

      periods. You can
      start monitoring it at 5:00 p.m., press the

      "rel" button which
      causes the meter to read that voltage as "0."

      From there on, the
      meter reads the "difference"; you can take

      readings and the
      display will indicate their fluctuations either

      side of zero.
      Pressing the button again returns the meter to

      "normal."

      The "Comm" Function--

      The rectangular button located between the
      capacitor sockets and the panel that contains

      the test probe
      sockets toggles this function on and off. When

      active, it sends
      data continuously through the meter's serial

      port. When
      inactive, you must update the reading on your

      computer with a
      command from the keyboard. This function is

      useful if you choose
      to read the meter with your telecommunications

      software. It
      should not be used with either the software that

      comes with the
      meter or the software we have developed because

      the accumulated
      readings will clog the buffers quickly. These

      programs can be
      set to perform the same function.

      Reading the Meter via your

      Computer

      In addition to the test probes and the cable

      that connects the
      meter to the serial port, the meter comes with a

      3.5 inch disk
      that contains three programs and a "readme.txt"

      file. The
      programs are: Dmm.exe, Metdemo.exe, and

      Metdemo.bas.

      The program called "dmm.exe" displays the

      readings from the meter
      using graphics and is not accessible. On a

      computer with a
      monochrome display, it merely prompts you to hit

      "enter" and then
      displays an "illegal function" message and

      prompts you to "press
      any key to return to the system."

      The programs called "metdemo.exe" and

      "metdemo.bas" are source
      code and compiled versions of the same program.

      They will send
      continuously updated readings to the screen.

      The ".bas" file is
      written in interpreted basic such as the common

      "gwbasic"
      which comes with many versions of "dos". To run

      such programs,
      make sure that a program such as "gwbasic.exe"

      is in your search
      path. Then type "gwbasic" followed by the name

      of the program
      as, in this instance, "metdemo.bas."

      In addition, Susan Fowle has created some

      programs in Basic that
      tailor the readings for access by speech and

      Braille. These
      programs are public domain, so you may share

      them, but they
      should not be sold. We have included them on

      the disk along with
      this article. A brief description of each

      program follows.

      In order to run programs with the .bas

      extension, type "gwbasic"
      followed by the name of the program and follow

      the prompts.
      Brtdmm.bas takes one reading with each

      keystroke. The cursor
      remains on the same line as the reading, making

      this easy to use
      with either a Braille display or a speech

      program. With speech,
      you will want to set a window for this line.

      Focdmm.bas was designed to run with the Covox

      "Speech Thing"
      synthesizer, but it can be easily modified to

      run with a speech
      synthesizer that is driven as a DOS device,

      (that is, a stand-
      alone unit hooked to a serial or parallel port).

      By "digital
      focus" we mean that the program can be set to

      present selected
      portions of the reading to such a speech

      synthesizer, even though
      the entire reading appears on the screen.

      Rptdmm.bas is also designed to work well with

      a Braille display.
      It continuously accesses the meter and is thus

      akin to reading
      the meter with the "comm" function. Pressing

      any key stops the
      program.

      Finally, Stdmm.bas is another program

      designed to work with the
      Speech Thing, but will work with other

      synthesizers. It has an
      option that allows each character to be spoken

      separately.

      Specifications for Writing your Own

      Program--

      The following
      communication parameters are required: 1200

      baud, 7 bit ASCII;
      no parity; 2 stop bits.

      Data Format--

      Each reading consists of fourteen bytes

      numbered in
      the hexidecimal standard 1 through 9, and A

      through E. In the
      following two examples, a byte that is occupied

      by a blank space
      will be shown as an equals sign (for sign).

      Note that byte E is
      always a carriage return (

      ).

      • Example 1: dc-1.9999=v

        .
      • Example 2:

        ===1.9999mohm

        .

      Command Structure--

      1. The meter must receive the D command to
        activate data transmission,
      2. the computer must give the meter
        the C command to clear its memory, and
      3. the computer must give
        the meter the M command (memory call) to

        transmit data from the
        meter's memory to the software).

      Cable Formats--

      Since the demons of disorder dictated the
      arrangement of pins in the DB9 plug so that they

      bear no
      discernible relationship to those in a DB25, we

      present them here
      in Table I. Table II shows the relationship of

      the 5-pin plug
      that enters the meter to the DB9 socket on the

      other end of the
      cable.

      Table I. DB9 to DB25 pins:
      • 1-8 (Receive-Line Signal

        Detector)

      • 2-3 (Received Data)
      • 3-2 (Transmitted Data)
      • 4-20 (Data Terminal

        Ready)

      • 5-7 (Signal Ground)
      • 6-6 (Data Set Ready)
      • 7-4 (Request to Send)
      • 8-5 (Clear to Send)
      • 9-22 (Ring Detector)
      Table II. 5-Pin Plug to DB9 Socket

      (with the socket facing away
      from you); pin 1 is in the top left corner. On

      the 5-pin plug, I
      will call pin 1 the pin nearest the cable.):

      • 1-2, 2-4, 3-7, 4-3, and 5-5.

      As the serial port on this meter is optically

      isolated from the
      rest of the meter's works, the computer must

      supply power on the
      two handshaking lines on pins 4 and 7 of the Db9

      connector. In
      the provided basic programs, note the lines

      which read:
      OPEN "COM1:1200,N,7,2,RS,CS,DS,CD"

      Those used to "dos mode" setups for serial

      ports will see the
      addition of settings beyond the 2 indicating 2

      stop bits. These
      set up the handshaking lines, 1 high and 1 low,

      to provide power
      for the meter's serial port. Since most

      communications programs
      do not allow for these settings, you may have

      trouble using the
      meter with such. If you need to power the meter

      from another
      source -- that is, you have a serial port which

      can't be set up
      to provide power with those handshaking lines --

      a 9-volt battery
      in series with a 470-ohm resistor connected with

      the plus on the
      meter's pin 4 and the negative on pin 7 might do

      the trick.
      Don't forget the resistor for current limiting

      to avoid popping
      the optical isolators in the meter. Failure to

      provide this
      voltage will result in no apparent damage, but

      won't allow the
      meter to see its port.

      FOLDING CANES AND MODIFICATIONS

      THEREOF

      by Bill Gerrey

      [At one time, I intended to call this paper

      "Modifying
      Commercially Available Folding Canes for Use by

      the Blind."
      However, they have steadily improved, thanks to

      our involvement
      in their design, so that title would be

      unnecessarily snide
      today.]

      Introduction

      You would never know it from conversations

      with me or by my
      works, but I'm an opinionated guy. If you can

      get me to talking
      on a subject--and one word might do it--I have

      set ideas as to
      how things should be. On the subject of

      available folding canes:

      I hate rigidly affixed (not rubber-mounted)

      tips that stick in
      cracks and give you seven extra navels before

      you get home. I
      hate plastic tips that don't provide a good

      sound source for
      echoes--not making good tapping sounds--so you

      needlessly walk
      into trees, and you miss doorways that you are

      looking for. And,
      oh, I suppose, a gooey rubber handle is nice

      when you're walking,
      but the fact that it increases the size of the

      folded instrument
      by 50% makes it bust your pockets; isn't the

      purpose of a folding
      cane to get out of your way?

      The result is, when people see my cane, they

      invariably ask,
      "What earthly kind is that?"

      Telescoping

      Canes

      There have been various brands of these over

      the years. One,
      made of light aluminum, was so flimsy that it

      would break under
      the force of a hard rain. Others used chucks

      for each section,
      making a cane that was durable, but whose

      collapsed length was no
      more convenient than the extended instrument.

      Finally, fiber-
      composition canes whose sections are held

      together by static
      friction have become popular. (I confess to

      having worked on
      such a one.)

      There is a friction-fit cane on the market

      which is reported to
      be moderately durable. Two other advantages are

      that it is very
      light-weight, and that it has a rubber-mounted

      metal tip that
      makes good sounds for echoes and which rarely

      sticks in cracks.

      From my experiments, I have concluded that

      all friction-fit
      joints have a mode of failure about which I am

      duty-bound to warn
      you:

      Between solids, "static friction"

      ("sticktion") is greater than
      "dynamic friction." Users of canes held

      extended by static
      friction are advised to twist the ends as the

      cane is pulled
      taut; dynamic friction will be in play until the

      operator stops
      twisting, and when motion stops, it will take

      more force to
      dislodge the sections than was applied in

      "setting" them.

      This sounds just dreamy until one experiences

      conditions where
      sticktion is "broken" and a joint shifts

      slightly. Once dynamic
      friction is the only force holding the joint

      together, it is
      likely to collapse.

      Where are those trouble spots that make your

      telescoping cane go
      flaccid on you? Places where vibration causes

      slight movement in
      a joint. What sort of terrain causes vibration?

      Particularly
      asphalt.

      Therefore, a friction-fit cane is most likely

      to collapse in the
      middle of a street--quod erat demonstrondum. As

      trucks bear down
      on you and you are trying to find the safety

      island, you must
      pull the cane taut and try again knowing that

      this risk will be
      no less probable a second time.

      As a second spare when you're traveling,

      however, you might
      consider adding the NFB carbon-fiber telescoping

      cane to your
      collection.

      Multisection Canes Held Rigid by

      Elastic

      Various companies, Mahler, Hicor, WCIB, and

      foreign-made models
      exist which have four or more sections that are

      pulled together
      by an elastic cord when the cane is allowed to

      fall from your
      hand. My favorite of these is the AFB

      "SuperFold." (You may
      have to order this quickly, since the Foundation

      itself has
      spread the rumor that they are going out of

      business in the area
      of "aids and appliances.")

      All such canes are fitted with my pet peeves:

      they have rigid
      tips (mostly nylon), and their bulbous handles

      make you look like
      you are carrying a collapsible weapon. [Bay

      Area police
      forcefully subdued an innocent blind bus

      passenger who extended
      his automatic weapon in a confident way; it sure

      hit the news out
      here.]

      For durability, most of these models are the

      same diameter over
      the entire length. In my opinion, durability

      requires this.
      However, the resultant instrument feels much heavier than tapered
      solid or telescoping canes; their center of

      gravity is lower.

      When I purchase folding canes (usually four

      at a time), I buy a
      few extra plastic tips. These can be turned on

      a lathe to create
      a 3/4-inch-long peg of 9/32-inch diameter. This

      peg accommodates
      a press-on rubber-mounted steel tip intended for

      canes of the
      Rain-Shine umbrella company (my favorite solid

      cane, by the way).

      In order to modify the handle, it is

      necessary to take the cane
      apart. Such canes are made in various ways, so

      specifics cannot
      be given here. Suffice it to say that in most

      cases, the elastic
      should be temporarily removed.

      Taking a knife to that rubber defense device

      on the top piece, I
      strip down to the bare aluminum (giving that

      delectable morsel to
      the neighbor's dog). My choice is to replace

      the rubber with
      heat-shrinkable tubing; you need some insulation

      there in cold
      weather.

      You have two choices of handle styles. One

      is to fit the top end
      with a "crutch tip" from the hardware store and

      provide a loop an
      inch or so from the top. The other is to allow

      a loop of the
      main cord to emerge through the top, as many

      already do. The
      latter arrangement is simpler, but it is not my

      favorite.

      [By the way, the purpose of such a loop is to

      hold the sections
      in a bundle when the cane is collapsed. Never!, ever!, think of
      this loop as a "wrist strap," as it is sometimes

      advertised. If
      your cane gets in a jam, such as getting trapped

      under a wheel of
      a vehicle backing up, you've got to be able to

      instantly
      disengage yourself from the cane and move to

      safety. Any
      suggestion that a cane has a wrist strap is

      irresponsible
      thinking.]

      The rubber handle may have been the mechanism

      for holding the top
      end of the elastic cord; this is true for the

      Superfold, which
      uses the small hole in the end of the rubber to

      trap a knot in
      the cord. Often, the top section is just a

      piece of tubing,
      perhaps flared at its lower end to mate with the

      next section.
      To trap the main elastic, I borrow a trick from

      the old AFB "Aluminoid Cane" as follows:

      Find, or make, a washer whose diameter is a

      close, but loose, fit
      in the tubing. The hole in the washer should be

      large enough to
      accommodate the cord. Perhaps halfway down the

      tube, make five
      or six deep dimples around it with a center

      punch. A knot in the
      cord will keep it from pulling through the

      washer; the dimples
      will trap the washer at that point in the tube.

      In order for the washer to seat properly on

      the dimples, they
      must be on the circumference of the precise

      cross section; you
      don't want the washer to cock at an angle such

      that it could flip
      on edge and pass through the restriction created

      by the dimples.
      For this reason, wrap tape around the tubing to

      serve as a guide
      for the center punch.

      Now you can chose your loop configuration.

      If the elastic has a
      loop which, before, emanated from the top, you

      can position the
      retaining knot such that the loop still does so.

      I prefer to use
      smaller elastic cord to make a loop 2 inches

      down from the top.

      To accomplish this, I drill the diameter of

      the tube at this 2-
      inch distance. Eventually, I poke the two ends

      of my small

      elastic into the tube from either side, fish the

      ends out the
      top, tie the ends together in an "over-hand

      knot," and pull the
      loop so that the knot disappears into the tube.

      (The main cord
      has to have been installed first, and the shrink

      tubing must also
      be applied first.)

      Once the top section has been dimpled and

      drilled, heat-
      shrinkable tubing can be installed. Most

      American canes have an
      outside diameter of one-half inch; the

      appropriate shrink-tubing
      size is 5/8- or 3/4-inch. (See "Using

      Heat-Shrinkable Tubing,"
      SKTF, Spring 1983.) This tubing can be gotten

      in various colors;
      it is my inclination to stay away from black or

      gun-metal blue,
      as I don't want a beating like that other blind

      guy got.

      The tubing should reach the bottom of this

      section; however, its
      position at the top depends on your style of

      loop. For example,
      for the arrangement where a separate loop is

      positioned two
      inches down, you will want bare aluminum for the

      top three-
      quarters of an inch so that a crutch tip can be

      fitted to the
      top. If your loop is intended to emerge from

      the top, I
      recommend the following procedure:

      Thread the washer onto the cord; then feed

      the cord through the
      section from the top end. Determine the

      position of your knot so
      that the washer traps the elastic with the right

      amount of loop
      emerging. Making sure that the washer stays

      somewhere in the
      tube, pull the cord out again until the knot is

      an inch or so
      outside.

      Cut the heat-shrinkable tubing one inch

      longer than the section.
      Match the bottom ends up, positioning the extra

      length at the
      top. Apply heat so as to shrink from the bottom

      up (turning the
      assembly so as to distribute the heat). When

      you get to the top
      with the heat source, shrink the extra length;

      it will shrink
      down to perhaps 5/16 inches. Then, while the

      shrink tubing is
      still warm and malleable, pull the cord so that

      the knot folds
      the shrink tubing inside the aluminum tubing.

      This will produce
      a finished end which won't hurt your palm when

      the cane
      encounters something.

      Some canes may not require all of these

      steps. For example, the
      Superfold cane has plastic fittings at the

      joints; the one in the
      top section is perfectly adequate for retaining

      a knot in the
      cord, and all that is necessary is to position

      the knot further
      down. Depending on how far the modified handle

      piece has moved
      the position where the elastic is retained, you

      may find it
      necessary to shorten the cord to get the tension

      back up where it
      belongs.

      If holes in the handle have been drilled (for

      installation of a
      loop, for example), a tapered reamer is the best

      tool for opening
      the shrink tubing, although careful work with a

      pointed knife
      would do as well.

      Earlier models of canes had few

      sections--four being typical.
      One of my tricks was to order five shorter

      canes, use one for
      parts, and make four long ones having five

      sections. You can
      shorten the bundled-up cane by two or three

      inches that way.

      Notes on the AFB

      Superfold

      When it works, the Superfold is my favorite

      folding cane (see the
      first paragraph of the previous section). It

      has Delrin fittings
      at each joint that carry a pair of rubber

      O-rings. When pressed
      together, the cane does not wobble at the

      joints, and it does not
      rattle when in contact with rough terrain.

      These O-rings make it possible to make a

      6-section cane that
      outperforms canes with metal-to-metal joints.

      Unfortunately,
      these joints are somewhat vulnerable to

      breakage. Also, fittings
      can come loose of the pieces they are supposed

      to stay with.

      Out of four Superfolds, I got three joints

      (15% of them) in which
      the plastic fittings had to be cemented into the

      aluminum with
      SuperGlue.

      Right out of the package, the Superfold is

      very hard to press
      together and disassemble. The O-rings squeak

      and refuse to seat
      properly, making a wobbly assembly and causing

      internal
      hemorrhages when pulling them apart. There is

      an easy solution
      for this, and I'm surprised it is not done at

      the factory.

      The same snugness of fit can be had if the

      O-rings are "glazed"
      with Vaseline. Generously smear each O-ring

      (two per fitting)
      with Vaseline; then, press the joint together

      and twist it back
      and forth a few times. Next, pull each joint

      apart and wipe it
      off with a paper towel.

      Both the O-rings and the Vaseline are

      petroleum products, and
      they "take to each other." The glaze will

      remain, even though
      the fitting will not be greasy enough to soil

      your pocket. (They
      could glaze the batches of O-rings before they

      put them on; they
      would go on easier and it would save us the

      trouble.)

      On the Appropriateness of

      Folding Canes

      If you are a long-cane traveler, your cane is

      most important for
      your safety. You are more valuable than an

      inanimate stick, so
      if a stick must be sacrificed now and then, so

      what. On the
      other hand, it must be there for you when the

      going gets rough.

      I know people who swear by the most dainty of

      canes; they say, "I
      only have to replace it every once in a while,"

      or, "You just
      have to take care of it." Balderdash!

      Codswallop!

      When I was a kid, there were product

      advertisements that said,
      "This makes an excellent 'theater cane.'"

      Pardon my opinion
      here, but a theater is full of oddly spaced

      tiers, seats that can
      trap your cane, people who, with a small

      misstep, can render you
      caneless--unforgivable.

      If you need a cane, you need a cane. Travel

      styles and
      situations differ, however. I travel in benign
      environments--smooth, wide sidewalks--most of

      the time, and I am
      fairly gentle with my canes; I usually break

      them when I am
      abrupt with them. Some of you live in towns

      with no
      sidewalks--perhaps with car after car parked in

      your way (the
      bumpers of which eat folding canes). Still

      others are heavy
      handed with canes; they don't solder for a

      living, so why
      shouldn't they be.

      When you test drive a cane, solid or hollow,

      folding or not, put
      thought in your confidence in it. If your cane

      is too delicate,
      you will travel as if you are protecting it.

      Protecting your cane is dangerous. In

      bailing it out of trouble,
      you can side-step and fall off an edge, you can

      unexpectedly back
      up and be hit by a quiet bicyclist, or worst of

      all, you might be
      avoiding putting it where it best

      belongs--taking the hits for
      you.

      I need subscribers. I cannot afford to lose

      you, so do me this
      favor. At the slightest inkling that you might

      be favoring your
      cane, cast it aside and choose a mightier staff.

      Products and

      Numbers

      The AFB Superfold is available from the

      American Foundation for
      the Blind Product Center as the C601xx (where

      "xx is replaced by
      the length; my choice being the C60156). Extra

      tips are No.
      C61100. AFB Product Center, PO Box 7034, Dover,

      DE 19903-7044;
      Phone: (800) 829-0500.

      The telescoping fiber cane is an NFB product.

      The length in
      inches follows the prefix ACG, with the suffix T

      meaning it
      telescopes. Thus, mine would be an ACG57T.

      Order it from the
      National Federation of the Blind Materials

      Center, 1800 Johnson
      St., Baltimore, MD 21230; Phone: (410)

      659-9317.

      My favorite tips (and canes, for that matter)

      come from the
      Rainshine Company, PO Box 5615, Madison, WI

      53705; Phone: (608)
      437-8018. Their canes are noncollapsible solid

      fiberglass, and I
      still have the one I got in high school. It has

      survived being
      tripped over and run over by trucks, as well as

      being bent in a
      20dg arc so as to stuff it into my gym locker

      hundreds of times.