General Stuff(tm)

[stephen_usher]

I think the board works, but has sockets so that I can swap in broken bits to test with my diag ROM.

That's the plan anyway.

[rubber_jonnie]

I think the board works, but has sockets so that I can swap in broken bits to test with my diag ROM.

That's the plan anyway.

Cool :)

[stephen_usher]

And... some PCBs have arrived...

IMG_4488.jpg

On the left are the recreated William Stuart Systems sound card for the ZX81, reverse engineered from the wrecked one I got with the ZX81 bundle from my old school friend a couple of months ago.

On the right is a replacement keypad PCB for the Hornby Zero 1, using surface mount switches rather than the carbon covered rubber mat and contacts.

[stephen_usher]

OK, so lesson learnt... On a ZX81 expansion always put the components facing AWAY from the machine!

IMG_4489.jpg

[stephen_usher]

Well, after removing the sockets and soldering in the chips (resoldering in 'cos the first time I soldered them in upside down!!!) it fits and doesn't cause the ZX81 any problems.

IMG_4490.jpg

However.... and it's a big HOWEVER....

I didn't realise that ZX81 BASIC doesn't have IN and OUT and I don't know Z80 assembler to write a program to test the card!

I'm a bit dead in the water.

[stephen_usher]

I've been tinkering with the z88dk C cross compiler for Z80 machines so as to see if I'll be able to write a test program for the ZX81 sound card.

Looks pretty easy to use actually and this evening even managed to load a program (which doesn't do much) onto the ZX81 directly from the MacBook Pro.

IMG_4491.jpg

[stephen_usher]

More "fun" with the board.

Firstly KiCAD's symbol for a double pole double throw switch is nuts! You would think that both switch sides would look the same as each other with pins the same side of the switch being the same sense... Erm, no. On the symbol the pins are switched so that opposite sides of the switch will be on/off if you use the diagram rather than carefully checking the pin numbers and how they relate to the footprint. How to fail big time... (Could have been worse, it could have swapped pins between switches.)

Secondly, I couldn't understand why the inputs to a 74LS27 were floating high-ish. They weren't connected to anything other than a couple of jumper pegs. Even putting a 10K pull down only brought them down a few tens of millivolts. Eventually I got them down low enough to produce an output... Which was only 1.5V. So I added a 10K pull-up... 1.6V. I think that I have a duff 74LS27. If it had been in a socket I could have easily swapped it for my other 74LS27, which is also supposed to be new.

Anyway, I've run out of time now so I can't desolder it until at least Sunday now.

[stephen_usher]

Well, I'm not getting very far with the sound card, well, other than verifying that the I/O port addresses I worked out were correct, but the wrong way around for the register and data ports. I've done this using the oscilloscope.

The problems I have now are:

• I don't know if the Ali Express AY-3-8910 chips are good or are even AY-3-8910s. I have no way of testing them in a known working system.
• I don't know if the 74LS27s are actually LS chips. Given that they have the same date codes and other text but one is marked with laser etching and the other is printed suggests that these are also AliExpress specials. I think I got them in with a hoard of other chips so I don't know their provenance. I've new ones from Farnell on order. 74LS27s shouldn't be strongly pulling up their inputs to 1.5V for a start. The output also shouldn't be 1.5V either.
• I don't know if my test program is programming the AY chip correctly. If anyone who's an AY-3-8910 expert can look over the attached code I'd appreciate it. (z88dk C compiler.)

So, I have too many variables to actually diagnose anything. Even once I've replace the 74LS27 variable on Tuesday there's no way of knowing if I'm just chasing my own tail.

I s'pose I could use a logic analyser but the only 16 channel one I have generated spurious events so I can't trust it. I'll need a 16 channel one as I need to monitor A8, /A9, BC1, BC2, R/W and the 8 bit data bus to see if the data is getting to the AY chip correctly.

[rubber_jonnie]

Well, I'm not getting very far with the sound card, well, other than verifying that the I/O port addresses I worked out were correct, but the wrong way around for the register and data ports. I've done this using the oscilloscope.

The problems I have now are:

• I don't know if the Ali Express AY-3-8910 chips are good or are even AY-3-8910s. I have no way of testing them in a known working system.
• I don't know if the 74LS27s are actually LS chips. Given that they have the same date codes and other text but one is marked with laser etching and the other is printed suggests that these are also AliExpress specials. I think I got them in with a hoard of other chips so I don't know their provenance. I've new ones from Farnell on order. 74LS27s shouldn't be strongly pulling up their inputs to 1.5V for a start. The output also shouldn't be 1.5V either.
• I don't know if my test program is programming the AY chip correctly. If anyone who's an AY-3-8910 expert can look over the attached code I'd appreciate it. (z88dk C compiler.)

So, I have too many variables to actually diagnose anything. Even once I've replace the 74LS27 variable on Tuesday there's no way of knowing if I'm just chasing my own tail.

I s'pose I could use a logic analyser but the only 16 channel one I have generated spurious events so I can't trust it. I'll need a 16 channel one as I need to monitor A8, /A9, BC1, BC2, R/W and the 8 bit data bus to see if the data is getting to the AY chip correctly.

Is it not possible to breadboard the AY-3-8910 and force it into producing some sound output? IIRC many moons ago I used a latch card with my ZX81 to force a breadboarded AY-3-8910 into producing sound. It was VERY rudimentary but I'm sure you should be able to test it that way.

Always difficult getting stuff from Ali Express, not always certain you're going to get what you expect, very annoying.

[stephen_usher]

Is it not possible to breadboard the AY-3-8910 and force it into producing some sound output? IIRC many moons ago I used a latch card with my ZX81 to force a breadboarded AY-3-8910 into producing sound. It was VERY rudimentary but I'm sure you should be able to test it that way.

Always difficult getting stuff from Ali Express, not always certain you're going to get what you expect, very annoying.

Potentially I could try to breakboard it, but I'd need to generate a clock.

I think that I've found at least part of the problem. The "fake" 74LS27 wasn't outputting a high enough voltage for the AY to notice the A8 and BC2 (connected together) going high (needs at least 2.2V).

It was suggested that I test the "fake" chip in an TL866 (or similar) but set the voltage to 3.3V instead of 5V. If it worked at that voltage then it definitely wasn't a TTL chip... Well, it worked at 3.3V... and almost worked at 1.8V! Definitely not LS series logic then. I've got new stock coming from Farnell, hopefully Tuesday.

In the meantime, as I only need an inverted /IOREQ line to drive A8 & BC2 I hacked a 74LS04 onto the board. That's got me further in that I'm getting SOMETHING out of the AY, but not the right things yet. I'm wondering if I've somehow got the data bus back to front somehow.

I've run out of time now. I can't do anything until at least Tuesday as I'm busy tomorrow.