Denford Software & Machines

Hi Guys,

I have a problem with the display on my Orac. It looks like shift is pressed as it were. It's easier to describe in a photo:



I was wondering if anyone has any pointers as to where to start looking. I don't have the schematics for the video board or the processor board, which will make finding this fault a bit of a nightmare. Does anyone have these schematics at all?

Any help greatly appreciated. I tried programming one of the examples, but unfortunately I get an error reported on every page, even pages like mm/inch where I can't really go wrong, so I wonder if this problem is present on a memory bus, thus the input is also being corrrupted. I don't know how it is all hooked up as yet.

These diagrams may help...

viewtopic.php?f=15&t=400&start=0

Hi,

Thank you for the link. I have those diagrams, but it doesn't include schematics of the syntel board or video board. I shall start downloading component datasheets and get the logic analyser out.

Can't wait to get it working properly so that I can run a program on it!

Best Regards,

Brian.

Okay, I found the problem. One of the RAM chips on the video board has a bad data bit. As the memory is 4-bit on an 8-bit bus I swapped two of the RAM chips around to absolutely make sure that the RAM chip was duff. I confirmed via the ascii table that bit-5 was the original problem, and when I swapped both RAM chips around, sure enough the bit that is now duff is bit-1. The photo below shows this. Bizarrely I managed to find replacement MCM2114 RAM on eBay! So as soon as that arrives I should have a working display again.



I'm not sure how this will affect the programming problem though.

Okay, fixing the problem with the MCM2114 RAM on the video board fixed both the programming and video display issues. Brilliant! So I have now run my first program and everything worked fine! I'm really looking forward to designing some bits now that everything's fixed.

Interestingly, this must mean that the program pages must be stored in the video ram, so if anyone else stumbles across this thread with the same problem at least you'll know fixing one problem will fix both.

My first program ran fine

Good news! Thanks for the info