[GreenKeys] Parallel to Current Loop Conversion

[Jones, Douglas W]

On Dec 4, 2014, at 2:15 PM, Cory Heisterkamp wrote:

> Before I go and reinvent the wheel, I was wondering if anyone had a proven old school circuit (no micros) for converting parallel 8-bit data to ASR-33/35 friendly 110 baud equivalent.

The first time I did this was in 1975 or so.  We had a parallel
port and we needed to push serial data.  The solution was a
shift register that was always shifting (day in and day out),
with stop bits shifting in the far end, so the output of the
shift register was an idle condition.

The clock came from a Schmidt trigger, trimmer resistor,
capacitor oscillator.  Tune it for the baud rate with a
screwdriver.

The parallel port set the data into the parallel load port
on the shift register, including the data bits and the start
bit.  The first clock edge after the load operation cleared
the parallel load bit, and the ready bit (enabling the next
parallel load) came when the entire register was back into
the idle condition.  It took about 4 TTL ICs (oscillator,
8-bit parallel load shift register, 8-input nand gate to
generate the busy/ready condition, and edge triggered D
flipflop to handle the load condition.

We used an op-amp to do TTL to RS232 conversion, but you could
just as well use a fast-acting relay to do TTL to current loop.

		Doug Jones
		jones at cs.uiowa.edu

PS:  Why?  Well, we had PLATO IV terminals and we needed
to drive a GE Terminet 120 printer as a peripheral.  The original
design didn't need full support for handshaking (acknowledge
data, ready) because we could only send 60 bytes per second
to the PLATO terminal and we were running the RS-232 line
at 1200 baud (120 bytes per second).  Those were the days
of relatively easy interfacing and delightful kluges.