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

  1. #1

    Default Imsai 8080 cannot examine to RAM adress...


    These days i´´ve got an Imsai 8080 in an optical great condition. It is one of the first versions with exp-22 motherboard but it had only 10 sockets. And it has the old Signal transformer 8063-A. I think, this is 10A version ?! I don´t know.
    Ao, i checked the voltages with oscilloscope, everything fine. I tested the small testprogramm.

    IN FF
    OUT FF
    JMP 00h

    I think, this is everybody´s first programm on imsai with toggle switches ?! It works.

    I am using 64kb of RAM. I mounted 2 32kb salota mixed memory cards to the boards. The boards are tested and i bought them from a friend here in germany. But i´ve got a problem. There are the 16 switches, when i switch 11110000 00000000 and press examine, the it should jump to memory adress F000 and the first four of the 16 LEDs should be on. This does not work !
    I removed the frontpaned and cleaned everything, i removed the cpu card and cleaned everything. Some of the logic chips in front panel i checked with my chiptester successfull.

    Do you have an idea ?



  2. #2


    When the processor is stopped, things are static. The processor is halted and the data on the address buss is static. This means you should be able to trace most problems with a meter. At worst a logic probe with a pulse detector.
    I don't know how much you know about the way the IMSAI and the ALTAIR front panels work. Both work basically the same. When you enter an address there are a number of open collector buffer ( 7406 as I recall ). These do 3 thing in sequence. First they jam a C3 onto the processors data bus. The processor sees this as a JUMP instruction. It then expects to see the address on the next 2 memory cycles. These are provided from two other sets of 8 open collector buffers, on the front panel, that go through the address switches on the front panel. Since you were able to enter a program, I'd suspect that this is mostly working. You should see any address you enter from the front panel on the processors address line as a static level, easily read from a meter.
    The next thing to do is to put all of the address line high and see which address lines if any are working. Also, try some other combinations. We need to see what works and what doesn't. The fact that you were able to enter an sequence of address from reset doesn't tell us anything about entering addresses. Any of the stepping functions just jam a NOP instruction onto the CPUs data buss and it just sequences the address. The front panel is not generating that address, the CPU is.
    You see, it isn't really the front panel doing all the work. The CPU is creating the address, on the bus, while the front panel only tells the CPU to go to that address and the CPU creates the address. We need to determine if the address isn't getting from the CPU to the address buss or the front panel's address isn't getting to the CPU.
    So please try some other addresses so we can see which bits are missing.
    Use care when handling things if you have the AC switch on the front panel. Put a couple layers of insulating tape over them to keep from making sparks or killing your self.

  3. #3


    There is an easier to read schematic at:


  4. #4


    Thank you ! I have tested logic chips in examine circuit. All 74123n are damaged. I cannot believe. I tested with expensive elnec beeprog2 programmer. I can test 74123 but there is not writtem 74123n. All habe them same error. What is the meaning of the n in 74123n. This means negative ?
    The 74107n is also damaged says my programmer. I think, i have to doublecheck with logic probe and oscilloscope.
    Btw, i tried different ram cards, always the same.
    So, you think, there could be also a problem with cpu card ?



  5. #5
    Join Date
    Dec 2010
    Seattle, WA


    The N suffix in the 74123N part number just means it is a PDIP package. A D suffix part number would be an SOIC package.

  6. #6


    It is possible that the 74123s are bad but on analog timing parts, I would not trust a static logic tester. As gslick says the N is a package type indicator is not a negative indicator. As a general rule, I don't trust logic testers as they do not actually test the part at real speeds. I prefer to diagnose the problem first. These testers just do a static test. If it says the 74107 is bad, that is more likely bad, unless the tester has a bad setup for that particular part.
    When we ask to run a test, we'd like to hear the results. Many of us are good troubleshooters and we can help a lot with finding problems. You are our eyes and ears. We like to hear I did "this" and the unit did "this".
    Still based on your results, I'd replace the 74107 first. 74123s are notorious for failing but I'd not replace them based on a simple logic tester.
    As far as, is it the CPU or the front panel, we do not have enough symptoms yet, that is why we ask for test.
    Last edited by Dwight Elvey; October 3rd, 2019 at 06:02 AM.

  7. #7


    You have said that you did the I/O test. That means the Deposit Next is likely working. That would indicate that the CPU was not the likely failure.

  8. #8


    So, i tested with logic probe and your great schematic.

    U23 74123:

    Voltage supply 5.22 V ok
    Pin 9 high while examine
    Pin 7 pullup 5V ok
    Pin 15 pullup 5V ok
    Pin 6 to pin 7 capacitator nearly 100nF ok
    Pin 12 Signal ok

    U13 7402

    Pin 11 signal from U23 ok
    Pin 1 low in direction U18
    I swapped this chip for trying, not works

    U18 74107

    Pin 12 signal from U13 ok
    Other measurements in examine circuit on this chip not working, so i have to swap.

    I have no 74107 chips in stock, so i decided me to order all of the logic chips in front panel for reserve. I think, a new 74107 will fix the problem ?! I let you know.

    Can you tell me something about the power supply with signal transformer ? My Imsai has serial number 1258. Is this 10 Ampere version ?



  9. #9


    What I asked for was do any of the address light go high when you try to examine them with the switches high. I'm trying to determine where to begin looking?
    I don't know from the serial number which supply you have. A picture of the capacitor area in front of the transformer will tell if you have the newer or older supply. The newer supply with the higher current has the capacitor's screw terminals directly attached to the power supply PCB. The older type of supply has the screw terminals facing up and wires connecting the capacitor's terminals.

  10. #10


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