Lo-tech Tandy Compatible Sound Adapter

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Lo-tech Tandy Compatible Sound Adapter PCB

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

Part Description Part No. Qty Mouser
C1-C4, C7,C8 0.1uF Ceramic K104K15X7RF53L2 6 594-K104K15X7RF53L2
C5,C6 47uF Electrolytic UVR1C470MDD 2 647-UVR1C470MDD
C9,C10 470nF Ceramic SR205E474MAR 2 581-SR205E474MAR
C11 47nF Ceramic K473K15X7RF53L2 1 594-K473K15X7RF53L2
C12,C13 330uF Electrolytic ECA-1CM331 2 667-ECA-1CM331
IC1 74LS688N CD74HCT688E 1 595-CD74HCT688E
IC2 74LS74N SN74LS74AN 1 595-SN74LS74AN
IC3 SN76489AN SN76489AN 1
IC4 LM386N-3 LM386N-3/NOPB 1 926-LM386N-3/NOPB
JP1 1x05 Pinhead 61300511121 1 710-61300511121
JP2,JP3 1x04 Pinhead 61300411121 2 710-61300411121
R1 10R 1/4W Resistor CFR-25JR-52-10R 1 603-CFR-25JR-5210R
R2,R3 10K 1/8W Resistor 270-10K-RC 2 270-10K-RC
VR1,VR2 10K Preset Potentiometer 36WR10KLF 2 858-36WR10KLF
X1 3.5mm Jack STX-3100-5N 1 806-STX-3100-5N
2.54mm Jumper 2 806-SX1100-A
ISA Slot Bracket Keystone 9202 1 534-9202
Screws Keystone 9900 2 534-9900


Address Decoder:
  • Jumper wire required between pins 7 and 14 of IC1 (layout isolation error)
  • IC1 pin 2 should be belt out (so it doesn't make contact with the socket) and jumpered to IC3 pin 5 (/IOW signal). This modification masks the Chip Select signal generated by the '688 with the ISA IOW signal because the SN76489 de-asserts READY immediately on response to Chip Select, which differs from the ISA use of this signal in response to the command (IOR, IOW, MEMR or MEMW).
Amplifier Section:
  • 10k pull-up resistor is required on PC speaker input
  • 2k2 resistors are required in parallel to the preset potentiometers to approximate audio taper pots (otherwise level adjustment is very limited)

See Also