Zon ZX-81 Programmable Sound Generator expansion for ZX81PLUS35 with working SD-Card

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Project ZON X-81 PSG for ZX81+35
Sound & edge connector expansion board example.png
Programmable sound generator for my ZX81 clone
Status In progress
Contact Mahjongg
Last Update 2016-06-26

The ZON X-81 is a sound generator based on the famous AY-3-8912 PSG.

This is a remake for my ZX-81+35 clone. It features stereo sound outputs, and also enables the GPIO's, to which an SD-card slot is connected, which hopefully will enable, with help of some software expansion in ROM, to read SD-cards. Adittionally the board serves (can serve) as an interface to a classical edge connector, or to another ZX81+35 expansion board (with pinheader connector).

I started by investigating the original ZON X-81, and soon found out there were two verslons, the original ZON X-81 and the later, "Spectrum compatible" ZON X. For neither a schematic diagram seemed to be available, and there seemed to be some confusion about the I/O ports used. In the end I decided the best course of action seemed to be to use the available information to draw up my own schematic, and ask for input from the community. So I eventually came up with this. File:ZONX sound expansion REV 2,0.pdf

Sound expansion (preliminary)-1.png

The ZON X (and presumably the ZON X-81 also) used just the minimum amount of logic to interface directly to the AY-3-8912 PSG, using just two 8-bit registers, the data register and the index register. The data register is used to write data to one of the 18 registers of the chip, the index register is used to choose which one of the 18 registers is used. Some registers control GPIO ports of the chip, depending on the version of the chip there could be zero, one or two of these, the 8912 used has one eight bit port. But on the ZON X you can only write to any of the IO ports,due to the simple decoder logic used.