Accessing the BUSRQ and BUSACK Pins on CPU 1.0 Modules

If you have a v1.0 CPU Module and wish to use the SD Memory Dump Module, you will need access to the BUSRQ and BUSACK pins.

BUSRQ is pin 25 on the Z80

BUSACK is pin 23 on the Z80

The good news is that the BUSACK pin is not connected to anything else and is fairly easy to solder a connector to on the PCB.

BUSRQ, sadly, is not so easy.  The 5v supply from the backplane goes via pin 25 and off to 3 other pins.  You have got a couple of options from here; Modify the PCB or solder to the CPU directly.

To modify the PCB you will need to cut 3 tracks and solder 3 wires before you can connect to pin 25.  See images below;



Luckily, the tracks shown in green are on the back of the PCB, so aren’t too hard to get to with a sharp knife.  You’ll need to cut each of the connections around pin 25 and then solder links to rejoin pin 11, 17 and 24 to 5v


Good luck!

Retro Challenge 10/2016 – Still a work in progress

So, who remembers my entry for Retro Challenge in January?  It was quite devoid of effort and results I think you’ll agree.

Well, this time around, I’ve taken that theme and pushed it even further!  Even less effort and much less result.

Since January, I have had a few opportunities to carry on with the ZX81 Module.  Because I wanted this to be a Retro Challenge project, I deliberately avoided doing any work on it, so I could save it all for October.  Well, October started and I was overwhelmed with non-Retro Challenge stuff.  But after a couple of weeks, I had a spare afternoon, and decided to dedicate this to the ZX81 Module!

So, I dug out the PCBs I had manufactured in January and fired up the soldering iron;


Time to wrack my brains and work out what my plans had been 9 months earlier.  I had failed to put any component values on the PCB, or even my Kicad schematics, so had to rifle through original ZX81 schematics to work out what these should be.

Now I knew what components I needed, it quickly dawned on me that I didn’t have very many of these at all.  This didn’t deter me though.  I fitted everything I had; IC socket, pin headers, some resistors and some transistors that may or may not be compatible;


And that’s about it.  I have ordered the rest of the parts now, but not had an opportunity to fit them.  yet.

One thing I have learned from this experience though, is that if I get any opportunity or urge to work on this, then I will seize it.  Not wait until the next Retro Challenge.  If I’d done work on this months ago when I had the chance, then this would more likely be a trouble shooting / tweaking / developing challenge.

One other good thing that has come out of this though, is that when I started this in January, I had a few design changes or additions required for the RC2014 for it to work.  The Backplane 8 has the ability to add resistors between the CPU and the RAM.  The Universal Micro Keyboard has the right connections to work with a real ZX81 or this ZX81 Module (including diodes).  The CPU Module also now has BUSRQ, WAIT, NMI and WAIT pins broken out which is required for this.  These changes should all make things much easier.

Despite my poor efforts this time around, I have thoroughly enjoyed seeing what everybody else has achieved with their Retro Challenge.  Good work everybody else!