For a while, I’ve been working on an update to a popular diagnostic board for the Sinclair Spectrum range of computers. The original design was by Dylan Smith and has been through several periods of development in the past. I have had one of the older versions for a few years and it is really useful, especially when loaded with suitable diagnostic software such as Brendan Alford’s.

I asked lots of the more knowledgeable Spectrum users in the Facebook groups what features would be useful in an updated board and the most popular was indicators for the voltage lines. The original board, pictured in yellow above, has 8 LEDs to indicate if a Bit in lower RAM is faulty, if any, even if there is no display available. They also display interrupt addresses. The LEDs are driven by software. The other update requested was +2a/2b and +3 compatibility. The older board only works on 16/48K Rubber Key and Plus, 128K Plus “Toast Rack” and +2 grey machines.

For the update, I thought it would be useful to monitor ongoing activity on the data bus. While I was at it, I reasoned, surely it would be useful to see activity on some of the Z80 control lines too? I asked around again and compiled a list of the 8 most requested. They ended up being: M1, NMI, ROMCS, IORQ, MREQ, RD, WR and RESET.

To test the operation of the data bus and control line monitors, I used a simple 555 board (published on www.projectvir.com) to pulse the CLK lines appropriately. It worked very well and I eventually incorporated an SMD 555 circuit directly on the board. The switch in the top left of the board selects either Software control or the 555 oscillator as a clock for the data line indicators. The control line indicators are driven from the 555 oscillator all the time.

I had already included the switch to change the clock lines for the data bus LEDs over to the Spectrum’s CLK line but this pulsed way to fast to see any changes in the LEDs, so the 555 fixed that problem. Although the 555 Pulse circuit doesn’t allow real-time monitoring, it does show activity on each of the lines adequately for most diagnosis needs. It is also customisable for duty cycle at build time.

Running Brendan’s software, there is a small issue when connected to the +2a/2b and +3 machines. The software runs correctly until it gets to paging back the Spectrum’s usual ROM and dropping in to the BASIC Menu. Instead it crashes the machine, showing the typical screen indicating no ROM is available.

Development will continue in order to track down the fault and once fixed and working, the design files will be available on the projects page.