WonderTANG on the Philips VG8235

[herraa1]

When trying to use the WonderTANG on a Philips VG8235 two problems have been observed:

• problem 1 is that on every power on (not reboot) the caps led turns on briefly and the cassette relay clicks briefly too
• problem 2 is that the Philips VG8235 freezes on boot after loading Nextor (or MSXDOS) and shows an A:> prompt even if there is an autoexec.bat present on boot media. It freezes too if the boot media does not contain Nextor nor MSXDOS and the system boots to Basic and shows a "Ok" prompt even if there is an autoexec.bas present on inserted media.

Problem 1 is still been investigated and it is thought to be related to the initial electrical state of the tn20k signals just immediately after power on.

Problem 2 has been traced down to a problem in the current board design which incorrectly handles nINT (edge cartridge connector pin 8) as a push-pull output when it should be an open collector output. Although this board design mistake does not prevent many MSX machines from working correctly with the WonderTANG as is, the Philips VG8235 can't keep the nINT signal HIGH if a WonderTANG is inserted and thus nINT is asserted continuously and the machine freezes.

[herraa1]

See screenshots of problem 2 on comment from user Papipapito in a different issue

[herraa1]

Finally, after proper testing, the root cause for problem 1 has been identified.

It is not related to the electrical state of the tn20k but it is caused by the WonderTANG holding the nWAIT signal during boot until it is ready for service, which prevents the Z80 from executing the BIOS code for a second or so, and in particular the code that initializes the S3527 msx-system/msx-engine (which, as seen in the Philips VG-8235/00 service manual, is in charge of the "CAPS LED DRIVER" which manages the CAPS led, and the "CASSETTE REMOTE CONTR" which manages the cassette relay). After power up and without the msx-engine initialized, the CAPS led seems to be lit by default and the cassette relay energized.

[neurohide]

I confirm with the last changes on the schematics and on the firmware that the keyboard does not freeze on the 8245 and nextor boot without any issue. Thanks.