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                    - IR / RS-232 Transceiver -
                             revision 4


97/05/14 19:00


FILES:     ir_box4.txt      - this file
           ir_box4.ps       - complete plans


    Size of ir_box4.ps: 99234 bytes
UNIX sum of ir_box4.ps: 2850
UNIX sum -r ir_box4.ps: 03477


The IR / RS-232 Transceiver module will convert the signal from a
UART port into infrared (IR) format, and from IR back to UART
format.  It will handle baud rates from 300 to 38400 baud.  The
IR format supports half-duplex communications only.  The IR
signal is compatible with the HP48 series calculators.  Although
I have not tried, the IR signal closely resembles the IrDA
format, and I suspect this module should be able to talk to
laptop computers and other devices with an IrDA port.  When used
with a HP48, the module will communicate with any Kermit, XMODEM,
or YMODEM program.

The RS-232 port is a DTE port, which means it can be plugged directly
into a modem or serial printer.  For connection into a PC computer, a
null modem cable is required (the wiring diagram is included).  The IR
format dictates half-duplex communications.  The module will enforce
this by disabling the IR receiver while the IR transmitter is
transmitting data.

The module can use either a 6V battery or a 9V battery.  A 6V
battery (like 4-AA cells) will last longer, but the regulator is
bypassed.  I find a 9V battery is more portable.  The current
drain is 60mA idle, 70mA during file transfer.  Don't plug the
9V battery into the 6V connector.  That would be bad, very bad!

The basic design is based on the document at
"http://www.ee.ualberta.ca/~charro/cookbook/optic/HP48SXIR.asc"
by Tony Duell (ARD@PVA.BRIS.AC.UK).  The IR transmitter and IR
receiver runs on a 16x clock source for the desired baud rate.
U1A, U1B, U2A form the cristal oscillator.  C2 is critical.  The
2.4576MHz crystal do not like to oscillate at its fundamental
frequency (at least the ones I tried don't).  C2 will suppress
the overtones.  U3 and U6A form the 16X baud rate generator.  The
baud rate is selectable using S1.  The IR transmitter is quite
simple.  It will send a 1/2-bit wide pulse for every SPACE on the
RS-232 port.  The IR receiver uses U1, C4 to shape the IR pulse
going to U6B pin 4, and to create a reset pulse to synchronize
U5A with the UART on the IR side.  When the IR transmitter is
active (D1 is on), Q1 is disabled by Q2.  For a short duration
after the D1 switches off, Q1 will still be saturated and require
a recovery period.  This is provided by C3, R5.

Four LED's show the status of the RS-232 DTE interface.  The Tx,
Rx, CTS, and DSR lines are monitored.  DTR and RTS lines can be
pulled high, or left unpowered.  This is selectable using S1.  I
chose a DTE pin-out for the RS-232 port because this allows me to
plug the module directly into a modem or serial printer.  A null
modem cable is required to connect to a PC.  The wiring diagram
for this is included.

My next project will be a DAT cassette size serial/parallel
converter so I can use my EPSON printer from the HP48.

Enjoy,

Tom Wong (tomwong@bnr.ca / tomwong@nortel.ca)
- - - -----------------------------------------------------------------
<Standard disclaimers here; freeware; no warranties; not
responsible for anything; email your comments; etc. etc.>

Changes:
- - --------
ir_box4: fixed errors with Q2 and Q3 in schematic
ir_box3: added a missing wire to the schematic
ir_box2: added a warning about 6V/9V connector
ir_box:  original copy


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