The Center for Really Neat Research
( CRNR)
Totally Neat Gadgets
TNG-3 is
the latest in a succession of serial interface boxes that work with PCs.
The earlier versions
are TNG-1 and TNG-2 (pron. "Thing 1" and "Thing 2",
as in "The Cat in the Hat" by Dr. Seuss), which are
respectively a) an
interface device specialized for myoelectric (or electromyographic or
EMG) signals and
b) a general purpose interface device with four analog
input channels. TNG-3 has 8 analog and (optionally)
8 digital inputs.
All three TNGs include a DB9 serial port connector for use on a PC.
Together with
appropriate sensors, transducers, and switches, they
provide a flexible system for human-computer interface
applications,
notably for persons with severe disabilities.
The standard TNG-3
includes eight 1/8" stereo jacks (four on each side) with the following
signal and power
connections: sleeve (shell)=ground; ring = 5V; and tip =
signal (0-5V range). The 5V power is derived from
some of the serial
port handshaking lines using a method analogous to that used to power a
serial mouse.
The internal power budget requires that each input device
should draw, on average, no more than about 1 mA.
The 5V can be used to
power passive (usually resistive) devices, subject to this limitation.
Alternatively,
one can power sensors separately (e.g. Hall Effect
magnetic sensors which require extra circuitry and need
more power than
TNG-3 can offer). In this case, one should ensure that the analog signal
to TNG-3 is
confined to the 0-5V range. When resistive devices are used
(potentiometers, CdS photocells, conductive-
plastic pressure sensors,
etc.), they are normally configured as a voltage divider with a 5k pullup
resistor
between the stereo plug signal line (tip) and Vcc (ring). A
typical choice for a potentiometer (connected
in a 2-wire arrangement as
a variable resistor) is then 5k.
The heart of TNG-3 is a
40-pin Microchip PIC16C74A micro-controller integrated circuit, which
sells for about
$10 in single quantity (PIC stands for peripheral
interface controller). Such micro-controllers (computers-
on-a-chip) are
widely used in consumer electronics: in stereo components and in their
infrared remote controls;
home appliances; digital thermostats;
automotive electronics (fuel injection, electronic ignition), etc.
An option for TNG-3 is an 8-channel digital input port (wired to
pins 33-40 or "Port B" of the PIC chip).
This can be provided as a
9-wire ribbon cable (the 9th line is for the ground) or as a bank of
eight 2-wire
phone jacks (3/32") on the rear.
The
communication between TNG-3 and the serial port of a PC is at 19.2kbps.
The data stream includes eight
bytes for the ananlog channels, one
additional byte for the 8 digital inputs, and either of two separator
(punctuation) bytes of 055h and 0AAh (85 and 170 decimal), which
alternate on successive cycles. See
properties of TNG-3 module in
NeatTools software.
