Retro&Reverse

 


This is a blog where I’m documenting my assorted electronics projects and their backgrounds. As the name implies, most of my projects are somehow dealing with retro electronics and reverse engineering things. I’m trying not to get limited to any specific area, but often I find myself tinkering with yet another project related to music, games or robotics.

My main motivation in all projects is that the result should be as useless as possible. Taking non-usefulness as the main objective opens up a whole new world in regard to project management: you don’t have schedules or deadlines, because nobody is expecting the result to become ready. You don’t have requirements, because nobody is wanting the result to function in any specific way. You don’t have a budget, because nobody is willing to pay anything for the result anyway. You have complete freedom to do whatever you like, and not to do things you don’t like. Sounds ideal project to me.

If an electronic device would have even a faintest possibility to be useful, meaning somebody might be willing to pay something for it, Chinese factories are already mass-producing it and selling it at a price lower than its component cost. Considering this fact for a while, the conclusion cannot be anything else: the only electronic devices worth building yourself are the totally useless ones, because they are not readily available.

This is my “declaration of non-usefulness” and I’m trying to keep this as a guiding star in my projects. Projects which all have the same project plan: it is ready when it is ready and it does what it does. 

 

Projects currently in the blog

Analog drums - a fully analog drum machine in the spirit of 70's

Yamaha PSR-70 keyboard reverse engineering - for documenting the sound chips YM3806 and YM2154

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Secrets of the YM2154

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  After a break, I returned to the PSR-70 reverse engineering project: t he YM2154, a.k.a. RYP4 is still an unknown territory. The time has come to reveal its secrets. R YP4 is used a bit more widely than OPQ. Besides PSR-60/70 it is used at least in PSR-80 and RX- 11/15/21 drum machines. This time I did remember to check for service manuals, and found one for RX-11: https://elektrotanya.com/yamaha_rx11_sm.pdf/download.html . This contains a full schematic and other useful information.   T racing register writes Because I had good results with OPQ by tracing what the PSR-70 firmware is writing to the chip registers, this would be a good starting point also for RYP4. The chip has 128 registers which reside in Z80 I/O address space at addresses 80H...FFH. Looking at the disassembly listing reveals quickly that there is no single function which would take care of all the register writes, like OPQ had. There are many IN and OUT instructions accessing the RYP4 register
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Reversing the PSR-70 hardware

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  First step in the actual reverse engineering is to start studying the PSR-70 hardware. This is quite service-friendly (meaning hacker-friendly) device: all circuit boards are single-sided and copper side silkscreen has IC numbers and jumper wire positions marked. There are also many useful looking signal names marked, like “D7”, “A12”, “IOWR” etc. It would be possible to draw the whole schematic from one high-quality photo of the circuit board. My intention is not to draw the whole schematic, but at least the address maps should be figured out. It will make the interpretation of disassembly much easier. All address decoding is done with 7400-series discrete logic, no PAL-circuits or other nuisances used, this is very good.   Block diagram Most of the electronics is on the large main board . The other cards are: Power + audio board contains the audio amplifiers, some kind of analog chorus-effect based on a BBD-circuit, power amplifiers feeding the speakers, and voltage
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Digging into YM3806

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In the previous post I got the PSR-70 original firmware modified so that it dumps to serial port all register writes done to OPQ chip (YM3806). Now I’m getting the data I wanted and plenty of it! A simple keyboard key-down or key-up produce only under 20 register writes but changing sound from the front panel results in about 300 writes. Why 300, there are only 256 registers? Yes, but there are many registers which are written several times. T he raw dumps from the modified firmware are quite hard to read, they just list the register address and written value in hex in the order they happened: 1F=20 5E=1F 56=1F 4E=20 46=20 5E=81 56=81 4E=81 46=81 9E=1F … To make some sense out of this I wrote a small python script to analyze the dump. It lists all registers in numerical order and all writes done to that register. In the script output the first column is OPQ register number, second column is the number of writes to that register and then a list of the written values.
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