Whilst working on the ZAViouR v1.02, I discovered a kit for a Z80 based micro-computer designed by Spencer Owen. Spencer called it the RC2014 since it was for the Retro Challenge in 2014. It’s a nice, simple modular design which can be built on stripboard but Spencer sells a kit of parts (and all the different modules separately) on Tindie.
I ordered the RC2014 (full monty) kit along with the additional Digital I/O board. All of which arrived in a couple of days in a well packed padded envelope. There are no instructions included, however, the boards are pretty much self explanatory and there are details on the website.
Construction went well and I configured the SIL (Single In Line) connectors in the same way that is illustrated in the Tindie images. The only real problem I had was with the tact switches on the Back Plane and Clock boards. The through holes didn’t quite align with the switch leads and required a little ‘persuasion’. Spencer assures me that he uses a better footprint on his later designs.
After I completed construction, I checked my soldering and that I’d got all the chips orientated correctly and also double-checked I’d plugged the boards in the correct way and with all the ‘PIN 1’ contacts lining up.
I had an FTDI module sourced from eBay which I connected up to the FTDI connector on the Serial I/O board. This produced a sporadic start-up. One in every six or seven starts would result in the machine firing up. I re-checked everything but i could find nothing amiss. It took me a while to find the ‘fault’.
Very kindly, Spencer included a few extra boards for me and I’d used one of his newer designs, not yet in general circulation. It is a Z80 board with jumpers for the Z80 control bus that are not included on the Back Plane. Although I’d soldered in the headers, I’d forgotten to add the jumpers meaning that several lines on the control bus were floating. After a standard ‘Doh!’ slap on the forehead and adding the jumpers, the RC2014 started up reliably every time!
Now the machine was up and running, I built up the Digital I/O board. I was very careful to seat the tact switches properly! Once built, I realised there was a small layout problem with the FTDI module I’d used. The solution was to make a right-angle adaptor with a 6-way header plug and it’s associated socket.
This allows the Digital I/O board to be plugged in whilst still using the FTDI module. Sadly, it does partially obscure the LEDs on the Digital I/O board but is good enough for testing. I quickly wrote a BASIC program to output 0 to 255 on the LEDs and came across a strange problem. When the value exceeded 70 or so, the RC2014 would crash. I had included the 7805 option on the Back Plane so I reasoned that the LEDs were pulling too much current and hence, creating a partial shutdown of the 7805. To test this, I bridged over the connections where the optional header pins were to bypass the 7805 and tried again. I might add, I was powering the board from a variable PSU.
From there, I discovered a strange thing. The board works perfectly, LEDs displaying 0-255 with ease but only if I keep the supply voltage under 5.0 volts. If supplied with even a small amount over 5 volts, the RC2014 crashes. I have spoken to Spencer about the issue and he can’t replicate the problem. However, at 4.8 volts supply, I have a fully working RC2014.
I have now ordered the Pi Zero Terminal board and the Universal Micro Keyboard to add to the project. These will allow use of the RC2014 without the FTDI module.
I really enjoyed building this little machine, despite the small issues and look forward to experimenting with it in the future.