Zalt: Working on the Bus-Spy board

It has been a while since I wrote about my home brew Z80 computer project now called Zalt. It is not that I haven’t made any progress – although at times it has been slow- I just did not write it down. So here is what happened.

After designing the Decoder board PCB and having it manufactured I thought making the Bus-Spy board would be a good pass time project while I was waiting for the Decoder board to show up in my mailbox.

The Bus-Spy board I ended up making, houses the System Controller (the Cypress PSoC5), some logic to drive 8 TIL311’s my pen-friend Ron was kind enough to sent me -and a duplication of the bus connectors to make it easy to hook on extra logic or the logic analyzer for debugging.

I decided to make this board by hand and used 30AWG wirewrap wire to do the connections. It was a little more work than I thought, although I have made these type of boards before, so I did not get it finished before my Decoder board showed up. But I decided to focus on the Bus-Spy board because that would allow me to do away with all the bread boards and temporary wire-mess.

When it was time to test it, I discovered that the 100mA current a TIL311 may draw on average is a little much for 30AWG wire and I had a considerable voltage drop across my power lines from my poor-man’s bench supply: an old 300W PC supply. So much so that I only had 4.4V on some of the IC’s on the CPU board. I dug in the parts bin for a beefier wire, put on some banana plugs on one side and soldered a connector to the other. I then routed the power directly from that connection on the Bus-Spy board to the TIL311’s supply pins with some thicker wire. That fixed it.

There was one goof-up I only discovered when I was staring at the TIL311’s. I had reversed the order of the LSB/MSB on the displays. I can’t believe I didn’t notice that earlier! Doh!

So from left to right, the first digit is A0-3, the 2nd A4-7 etc. instead of the other way around. Ah, well. I printed out a small piece of paper indicating what each digit is. If/when I create a real PCB for the bus-spy, I will make sure this is fixed.



This photo was taken before I fixed the power.

I used the two decimal points on the TILL311’s to indicate the status of some of the control lines (as you can see on the legend I printed out).

The top (left) IC is a GAL16V8 that is programmed to deliver the Strobe signal for the TIL311’s and the Load signal for the two 74-573’s that latch the control signals. There is a row of jumpers (just above the displays on the left) that tell the GAL16V8 what events to capture. This allows you for instance to concentrate on just memory-writes or instruction cycles (M1) etc. Each jumper turns on a specific event type. Removing the jumper turns it off.

You’ll notice that the size of this board is a standard eurocard size (10cm x 15cm). That means that the other boards can go on top and you’ll still be able to see the display digits.


Here I am performing a basic DMA test where the System Controller on the Bus-Spy board talks to the Memory on the CPU board. I have removed all other ICs to keep it simple.

That is when I discovered that I had a problem. The values I wrote to the memory were not read back correctly. After some digging I found that I had not seated the IDC flat-cable connectors properly. The middle wires were making intermittent contact. So after pressing them down firmly in the vice, it all started working. You’ll be amazed at how much force you need to press down 40 pins into the wires of the flat-cable…

With this out of the way, I could focus on the Decoder board.

Published in: on June 25, 2016 at 6:13 am  Leave a Comment  

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