Unsafe PSG on MSX Engine

By Shinobi

Expert (125)

Shinobi's picture

16-09-2020, 19:52

Hi

Any danger consequences of using unsafe psg ROMs on MSX AX 200 with s3257 Yamaha MSX Engine???

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By _ThEcRoW

Expert (82)

_ThEcRoW's picture

16-09-2020, 21:10

Be careful or you could be frying something inside. I recall that psg has also connection with joystick port, so it could be doing a real mess on the machine. I wouldn't do it. If anyone has any more info is appreciated.
Those roms could be dangerous to a one chip msx machine or similar clone? Iask to those who maybe know.

By pgimeno

Master (230)

pgimeno's picture

16-09-2020, 21:57

Shinobi wrote:

Hi

Any danger consequences of using unsafe psg ROMs on MSX AX 200 with s3257 Yamaha 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.

By gdx

Prophet (3684)

gdx's picture

17-09-2020, 16:14

I believe that there is danger for the hardware only on certain MSXs and with certain devices in the joystick port. So the risk of damage is low. It can also cause wrong joystick detection in some cases. I noticed this on turbo R.

By Shinobi

Expert (125)

Shinobi's picture

17-09-2020, 20:36

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..

By Shinobi

Expert (125)

Shinobi's picture

17-09-2020, 21:34

Links of two images from Datasheet here:

https://ibb.co/k55QG67
https://ibb.co/wzmJPX1

By pgimeno

Master (230)

pgimeno's picture

17-09-2020, 22:10

Shinobi wrote:

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.

By Shinobi

Expert (125)

Shinobi's picture

18-09-2020, 00:53

Thanks pgimeno, you seem very sure about this. Did you try such ROMs on a s3527 machine???

By pgimeno

Master (230)

pgimeno's picture

18-09-2020, 13:06

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.

By Shinobi

Expert (125)

Shinobi's picture

20-09-2020, 02:11

Ok pgimeno, thanks for your help. My AX-200 is safe now.

Thanks.