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...and what it's showing is a constant /SRQ assertion, or Service Request. Unfortunately the OS9 code can't deal with this and just loops. Does anyone know anything about seemingly the only Positron floppy controller to exist in the world? 😆 /n

Here it is: binarydinosaurs.co.uk/Museum/Posit... Main ICs, top to bottom: TMS9914 GPIB controller HD68B44 DMA controller 6809 CPU 2K RAM EPROM TMS9909 floppy controller. The ROM is good and I can test the CPU and RAM in other machines. DMA stuff though, 🤷🏻‍♂️ Time to move some analyser clips 😁

(It DID work with the Positron 900 'datapulse exchequer', and actually did a seek with the 9000 controlling it 2 weeks ago. Nothing since)

There’s only 1 data bus on this machine but it’s buffered in 5 different places through 74LS244 and 74LS245 drivers, and these drivers have a habit of expiring.

The iron rule of big logic analysers is that, invariably, just one more pod is definitely going to cut it.

What are those IC clamps called ? I need those as well 😀

They're just called 'IC Test Clips' and are available in all popular sizes. Beware though that there are 2 main connector types - standard component which you slide your test leads onto, and 'nail heads' which you can't attach test leads to without using grabbers.

Once I had to do a full bus state analysis of a dual CPU (Z80/8088) system. Data getting lost between the two. I think I got away with only 5 pods, though! That being said my 16555D stack goes to 12 pods, just in case.

Haha MOAR PODS 😄 I have a similar issue with the Posi 900 because it's a twin 6809E machine and it's also losing data between the CPUs. I discovered earlier in the year that these analysers can debug 2x 6809 machines fully with ICE pods but I've not found any yet.

In my case it was a carbon composite resistor, in the 8088 clock source duty-cycle circuits. A few nanosec too long, and it would occasionally eat CAS cycles on memory access right after refresh. But only if the card was colder than 22 degrees C!

Have you tried poking around with an engineer's finger? That was my go-to piece of test equipment back in the '90s 😊