Unsafe PSG on MSX Engine

There's no danger in that machine, or in any other machine that uses the S3527. This chip does not honour the port direction bit of port A in the PSG; it's set to always be an input regardless of what the user sets.

I saw the datasheet for S3527 MSX Engine, here are two sections talk about it:

and

They only say keep Bit 7 and 6 as 10, why?? what will happen?? they don't say..

In chips that have an S3527, nothing. But in machines with AY-3-8910, setting bit 6 to 1 transforms an input port into an output, and that may short circuit the pin and potentially burn that output or do more damage.

The S3527 is designed specifically for MSX. The makers of the S3527 would not say that you can set it to whatever you want, because that would mean they don't care for other MSX machines. They say that it must be set to "1 0" in order to work fine in any MSX, not just in the machines that use the S3527.

Yes. I've just tried Athletic Ball Ghost Flipper on a Philips NMS 8250; the joystick works without problems. If this issue happened, the joystick could not possibly work.

Before that, I verified in openMSX with the debugger, that when running that game, register 7 of the PSG has bit 6 set most of the time. You can do that by launching the debugger, connecting to openMSX, and selecting View > Add Debuggable Viewer > PSG Regs. The register at address 7 has FE most of the time, which means that the first two bits are 1 1. In particular, the second "1" means that the joystick port should be in output mode, meaning the joystick couldn't be read if the bit was honoured, because there'd be no access to the inputs.

Don't try this in e.g. a Philips VG 8020/00.

I would have tried in the AX-150, but the joystick port needs to be resoldered first.