I think I ned help to sort out this IVC video card issue I have. I have attached a circuit diagram below.
When I first got the machine earlier this year everything worked fine. However, after a week or so I was left with a blank screen with the occasional faint line type interference. As far as I can tell the IVC connected keyboard doesn't work either but this could be a red herring.
The basics have been done e.g.
- All socketed chips have been inspected, cleaned and reseated.
- The CPU has been changed as the original had failed.
- The CRTC chip was tested in a BBC Micro and was fine.
- The three RAM chips were swapped as I happened to have three brand new identical devices.
- The ROM contents have been compared to ROM images and both are OK.
So out came the logic probe and scope. I have removed the rack's right hand endplate and placed the IVC in the right hand most socket so I can probe around reasonably easily.
- The 4 Mhz clock signal from the backplane is good and appears where it should do on the various components.
- There is activity on the data and address bus when checked on the CPU pins.
Things that seem odd to me are...
- There is no activity on the CPU output IORQ which I know is used in conjunction with the clock to strobe data into or out of the CRTC.
- Pulling the CRTC and the Z80 allowed me to isolate and test IC31, this is a D-Type flip flop used to strobe the data as mentioned above. This did not appear to work. It is now socketed and has been swapped.
- There are vsync pulses output from the CRTC but no hsync signals.
Removing IC31 from its new socket allows me to monitor the IORQ CPU output and there is nothing to see. This seems to me to be the thing to investigate. The trouble is I am not sure how.
Julie, on this forum suggested that I program up a custom ROM that is just an infinite loop of IN instructions, however, rather than introduce another variable, I am wondering if I am missing something, perhaps there is something else I can probe or try out.