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• Failure and Success

  Stephen Moody • 02/19/2024 at 17:53 • 0 comments

  It's been a year since the last update on this project. 

  The PCB has been built for quite a while now but no work had been done due to lack of time and I was missing a few ICs on the decode logic.

  I have finally started looking at this again and attempting to get the board up and running. I had performed some basic testing when I built the PCB, checking for obvious shorts and making sure that the quadrature clock was working correct. Until I populated the decode logic ICs I couldn't run any code.

  Several months later I have everything I need and some spare time so thought it was time to get the board up and running. Initial attempt was to try and write some simple code to write a message out of the SC26C92 UART serial port.

  I was not expecting everything to work on first attempt and was not disappointed, nothing happened. Time to fire up the oscilloscope and try and work out what's happening. 
  

  Looking at the decoding logic, a lot of signals seemed to be active even when they should not be used. Addresses bus also was showing a lot of activity on address lines that should not be used on a simple looping code. It looked like the CPU was just free running. Checking the data lines showed that some of them were changing between 5V and 2.5V so looks like a short along them somewhere. Checking for shorts and removing all devices connected to the databus except for the ROM and DUART didn't fix the issue. 

  Removing the DUART and putting a very simple loop ROM in and there seems to be a bit of life in the computer. Logic analyser seemed to confirm that it's running code at least.

  Plugging the UART back in caused issues so found the obvious culprit. Probing the signals with the scope wasn't showing what I expected, no clock signal from the oscillator and every other signal looked strange. Checking with the datasheet I discovered the problem fairly quickly. This IC has 2 different pinouts for 44 pin parts, the PLCC version that I'm using and the QFP which I used the pinout for in the schematic and has a totally different pinout!

  I was going to go the dead bug route of swapping out the UART and soldering a lot of wired but a friend suggested using the expansion port to add it. Still a lot of soldering wires but a much neater solution in general. The first addon board for the computer before it's even fully running.

  It's currently connected to a TTL serial to USB adaptor but will add some connections back to the MAX208 on the board.

  
  I required a few changes to my serial port code but it now seems to be working great.

  A stupid mistake caused a lot of head scratching but at least managed to find a solution so I'll call that a win. Next steps are to test the RAM and then to get a simple monitor up and running to allow me to test the rest of the computer.

• PCB has arrived

  Stephen Moody • 02/26/2023 at 13:40 • 0 comments

  A long soldering job ahead I think.

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