Hi folks, STC Executel fun continues and I'm at the point where I'm fairly sure code is running but it's stuck in a tight loop waiting for something to happen.
The 8085A reference tells me a non-memory I/O is signalled by IO/M going high while it puts the port number on the address bus (0xE3 to 0xE8 in this case), 8 bits on the data bus and sets S0/S1 to be WRITE. If that's the case then this machine in its current state doesn't do any non-memory I/O - IO/M is the only signal that stays low. It's not a failed CPU since I have 3 different ones and they all do this. I've pondered if one of the 3 chips the IO/M signal goes to is pulling it down; presumably I can test this by lifting the IO/M pin out of its socket then briefly touching a 1Kohm resistor connected to +5V to the 3 inputs and watch what the outputs do while the machine is running? With my little logic analyser on the address bus it's continually active so it's not a stuck bit either, at least not at the CPU. I can watch the repetitive patterns while the code runs through 3 delay loops then the patterns alter while it's off doing....something. All the 4116 RAM chips seem to be OK too. I'm also guessing things would be a lot easier if I had a memory map... Cheers! -- Adrian/Witchy Binary Dinosaurs creator/curator Www.binarydinosaurs.co.uk - the UK's biggest private home computer collection?
