Sardis Technologies ST-2900 Serial Port Expander (SPE-1)

The ST-2900 Serial Port Expander (SPE-1) allows a serial port on the ST-2900 CPU board to connect to three serial devices (although only one at a time). It also adapts the CPU board's 14-pin serial port header into a standard DB-25F serial connector, with flexible pinouts.

SPE-1 Adapter features:

Power (+5V, +12V, -12V) for the SPE-1 is supplied by the ST-2900 CPU board via the 14-pin serial port connector.

Each of the three DB-25F connectors has an associated 14-pin socket (J1, J2, J3) that can hold a custom personality module in the form of a wired 14-pin DIP header. It allows connecting the four signals from the CPU board's serial port (TxD, RxD, /RTS, /CTS) plus +12V and GND to any of these nine pins on the DB-25F connector: 1..8, 20. This allows changing the pinout, including for null modem connections, while still using unmodified straight-through serial cables.

Jumpers J4, J5, J6 are normally shorted. J5 and J6 allow loopback testing, with TxD connected to RxD, and /RTS connected to /CTS.

If I remember correctly, I paid someone to professionally create the PCB artwork, because I was considering putting the SPE-1 into production. But I only made one prototype PCB. It is double-sided, but the holes weren't plated through on this prototype, which made for tricky soldering. That's why you see many individual "naked" IC pin sockets instead of normal IC sockets, so I could more easily solder each pin on both sides of the board.

I used my SPE-1 so I could connect my ST-2900's Port B (P2) to my SwTPc computer (which had an EPROM programmer installed, and talked to my printer via its parallel port) and to a Novation CAT 300 bit/s acoustic-coupled modem, without having to swap serial cables. I connected a SPST toggle switch between each of the two control lines (A1, A0) and GND to select which port to make active. But those control lines could instead have been connected to spare output pins on the ST-2900 (using pins 1..4 on the 14-pin connector) to allow software control.

(Click on a picture for a larger version.)
Pictures of top and bottom of assembled SPE-1 board (414-667KB .jpg).

ST-2900 Serial Port Expander (SPE-1) schematics.
(273KB .png)

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Last revised 2020-Mar-26 21:50 PDT.
Copyright 2020 by David C. Wiens.

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