Actually, I built exactly this many years ago (1990s) to operate a cash draw for dumb
terminals on Unix systems, used on counters as point of sale devices...
The existing solution used a processor, ram, rom, double sided board etc and was too
expensive, so I designed a replacement with a real UART and a finite state machine
consisting of a EPROM and 8 bit latch that simply monitored the RS232 data passively and
when the appropriate character sequence was matched, it triggered the solenoid to open the
It decoded a long 14 character code sequence easily and reliably and used 5 chips in total
on a smaller single sided board.
Nowadays, a small microcontroller is the obvious way to go for cost and ease of
An easy way to implements is to use a small box to contain the two DB25 connectors and
simply tap into the receive data line and run a short cable to the monitor circuit,
probably built in and powered off what you want to operate...
From: W2HX via cctalk <cctalk(a)classiccmp.org>
Sent: 11 November 2022 21:16
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Cc: W2HX <w2hx(a)w2hx.com>
Subject: [cctalk] Inline Serial Device?
I am looking for a device that sits transparently in an RS-232 serial line and upon seeing
a particular code go over the serial line ((or sequence of codes) will actual a relay (or
a transistor). Something with two DB25s or DE9s and is configurable to what code will
trigger the output? Some kind of box?
Does anyone know of such a thing? I guess it could be cobbled up with a microcontroller,
but hoping to just get something "off the shelf."
73 Eugene W2HX
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