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Thread: Imsai 8080 cannot examine to RAM adress...

  1. #81


    I am NOT reading random data. I only see FFs on the data lines at the CPU, and the 8212 and at both 8216's
    Addressing does work, when using examine next or deposit next, addresses increment correctly.
    I just tried depositing using C0 but still no change. Both buffers still show FFs.
    I looked at DIEN (not) on both 8216's (B8,B9) both are low, and DBIN on the CPU is high.
    Just checked out A2 and A3 using probe, good MPU and Bad MPU have same signals on all pins.

  2. #82


    Earlier, you said you were reading random values. I guess it was not what I meant. FF is not random. When you do a write the data has to go onto the cpu side of the buffers.
    I'll try to describe the actions.
    First, when you load enter an address, the front panel board writes a JUMP instruction to the CPU over the cable ( not through the bus ). It then sequences the two address bytes of the cable. All of the other operations, the CPU is doing a NOP instruction. Any data transferred to the data bus has to go through that same cable, either, writing or reading. The CPU increment the address for each operation because that is all it knows to do.
    The front panel sees the address that the CPU wrote to the address lines on the S100 bus but anything to do with address is controlled by the CPU, not the front panel. When there is any increment, it is just the CPU doing a NOP instruction.
    To write or read data, the CPU is held waiting for an instruction until the write or read completes. What we don't know is, that was the data not written to the s100 buss or was the data not being read from the data bus. We can't tell from the front panel which was which because we don't directly see the data bus. We only see what is on the CPU's data lines. When nothing is happening the lines go high from the pullup resistors and the bus drivers are suppose to be in the input mode to the CPU to overdrive the pullup resistors if the data bus has a 0. When an operation, like incrementing the address, the signals from the front panel have to momentarily isolate the S100 data bus and force the data signal on the cable to zero for the CPU to read as an instruction. When done, it is suppose to go back to having the bus buffer go back to reading the S100 data bus. This action of sending the NOP happens really fast. You will not see the actions of doing the bus transfers, on a meter, because they will be really quick. So, the actual write of the S100 bus does not involve the CPU, other than the static address driven to the S100 bus. The display that you see as a static signal should be the value on the S100 bus, driven through the buffers. The buffers need to be turned around to do the write. This happens really fast so the static levels of the DBIN and DIEN are not part of the write. The DBIN has to change momentarily to transfer the switch data to the bus.
    It is possible that we are not seeing the data S100 data bus but just seeing the pullup resistors. At this point we need to do an experiment to see what is happening with the bus data. With the memory board removed, we should be able to drive the data in and see the result when we look at the data lights at any memory location. You can drive, low, any of the DI pins of the S100 bus and see that value immediately on the data LEDs. You can do this with a wire to ground a DI pin. This will tell us if the bus buffer is working. Since the DIEN and the DBIN are the same as the working CPU we should see it work.
    You didn't mention the CS pin of the bus buffers. Please check it? It should be low when you are doing nothing as we should be seeing the DI signals to the front panel data LEDs. It should only go high when we are sending a NOP or a JUMP+Address for the CPU to execute.
    Anyway run that experiment first and then we can go from there.


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