Sega Saturn modchip service?

Well, I've talked to the guy, but he prefers to remain anonymous - he did make a few interesting comments, though.

The first one was about timing - the initial design used two PALs, one to deserialize the incoming command stream and the other to serialize the faked response stream - they had their clock pins tied to opposite phases of the on-board xtal oscillator. The reason for this was that the generator and receiver had timing constraints that worked in opposite directions - on the receiver, you want a late clock so that the signal has as much time as possible to stabilize, and on the transmitter you want the clock as early as possible for the same reason. Since a PAL only has a single clock domain, if you try to put them in the same device you end up having to trade off timing margin between the TX and RX side. Using the split phase clock with a 8.4MHz xtal provided about 60ns of timing margin that made the whole thing basically a non-issue.

But they never actually manufactured that version - when they started making it, they merged the logic in the two PALs into one to save costs, and hence ran both sides from the same clock signal, eliminating the designed-in timing margin. This is the reason they are so sensitive to clock phase. So basically, you can blame the timing sensitivity of Saturn modchips on the decision of someone at Mako Toys to save a couple of bucks on each board - they also removed most of the decoupling caps on the basis that it seemed to work without them (insert facepalm here).

They were pretty quickly cloned (this is HK, after all) - and after a while you could buy both the programming files and the (regenerated) source code - which is why they got to the point where everyone was making them. He wasn't especially worried, since he did the design as a contract job and had already been paid.

 

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He's pretty sure he hasn't got it any more, but has promised to have a look through his old backups just in case. This also got me thinking about it, and CPLDs have much better clock routing than PALs, so if you used one of them it should be pretty easy to generate the necessary early and late clock signals.