The Lo-tech Tandy Compatible Sound Adapter is an 8-bit ISA audio board for 8-bit IBM compatible PCs providing an SN76489AN Programmable Sound Generator (PSG). The board is equipped with a simple mixer-amplifier, enabling the PSG output to be mixed with the system-board generated PC speaker sound, the combined output fed back to the internal PC Speaker or output via a 3.5mm socket to external speakers.

Configuration & Resources

The board is configured with JP1. Two jumpers are required.

• For 0C0h (original Tandy port mapping), short pins 1-2 and 4-5
• For 1E0h (later Tandy port mapping), short 1-2 and 3-4
• For 2C0h (see note), short 2-3 and 4-5

In the IBM PC 5150, ports <01Fh are reserved by IBM. It is not possible to read values from these ranges from adapters installed in the system. The Lo-tech Tandy Compatible Sound Adapter requires only write access to the SN76489AN and therefore should work, however the implementation of these ranges on clones may vary. Therefore, a ‘safe’ option 2C0h is also provided.

Note that it is not possible to programatically detect the presence of the SN76489AN, except perhaps by utilising the execution delay introduced by writing to the SN76489AN due to it’s call on the ISA IORDY line. Software will likely need modification to utilise the SN76489AN as a ‘Tandy’ sound chip.

PC Speaker Mixer

The IBM PC System Speaker output can be connected to the board at JP2. The header connector has the same pinout as the header on the system board, i.e. 4-pin with key. Any audio output to the PC speaker will then be mixed with output from the SN76489AN, the relative levels of the two signals controlled with VR1 (PC Speaker) and VR2 (SN76489AN). The mixed output is provided at JP3, for connection back to the system speaker, and on the 3.5mm jack for connection of external speakers (the internal speaker is silenced when external speakers are connected).

Bill of Materials

Errata