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Thread: Osi c4p

  1. #11
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    By “caps lock” I assume you mean “shift lock”?

    I assume you have tried pressing the “break” key, followed by the “C” key and then the “RETURN” key?

    Have you found the schematics for this beast? The logic can’t be that complex for the keyboard interface!

    Dave

  2. #12
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    Yes shift lock, sorry.

    Basically I can press the Break key, and get the C/W/M prompt. What I've determined looking at the key matrix, (last page in this document) something is disabling rows 2 and 4. If I hit a 'wrong' key I can clearly see the data being sent to the computer, and the computer, recognizing that it's the wrong key, blinks back to the prompt. But hitting any key in the affected rows yields no response at all - you cannot even see anything changing on the keyboard - to -video card connector.

    I had another Rev B 542 keyboard and swapped it in just to check, but it's exactly the same. Since I know the keyboard was working normally prior to my removal of the 163s on the video board, and since the keyboard interfaces to that board, I feel pretty confident I've buggered something up. Or I have more than one bad 163 in the mix. What is hindering me is that this 540 video board was substantially modified by the previous owner. They installed an RF modulator and some switches to control various video settings, then removed them. There are wire patches underneath and some cut traces up top. I should have taken more closeup photos, I'm wondering if there was a patch somewhere I failed to replicate.

    Just now I powered up and had a blank screen again. I swapped two 163s and suddenly it was back.. so I do think we've still got some bad parts in the mix here. But looking at the 540 schematic leaves me scratching my head.. I really don't see anything that the 163s could be doing to disable the keyboard in this way. It's a mystery to me.

  3. #13
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    Thanks for the pointer to the schematic at http://www.osiweb.org/manuals/OSI-540B.pdf.

    The 163’s have nothing to do with the keyboard.

    If you look at schematic 4 of 8 you will see a couple of 8T26 buffers at U3H and U3I that buffer the keyboard data bits. Are these in sockets and (if they are) what happens if you swap them over?

    Dave

  4. #14
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    They are, and I've tried the swap back and forth with same result.

    I don't know if this helps, but I probed J2 on the keyboard itself. On power up, all the D lines but D0 are low (D0 is high). When you hit Break, you can see pulsing on D0-D1 and D3-D7 but D2 seems to be without any pulse - it just registers as Low. Not sure if that suggests anything, I'm assuming there should be activity on all of D0-D7.

  5. #15
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    Found some better schematics (including the keyboard) at http://www.osiweb.org/manuals/SAMS-C...ice-Manual.pdf.

    It is getting a little late in the UK - but I will have a look tomorrow. It certainly seems that D0..D7 are used, but there is more going on here than a simple 8-bit parallel input. It seems as though the data bus is bidirectional and that the Read/notWrite, A0, A1, /KBE and a few other signals are communicated to the keyboard logic.

    The row/column data for the 'C' key is R2/C6 and the 'M' key is R2/C2 - so I suspect you are correct in that row 2 is not working. That appears to come from /KD2 so I think your suspicions are correct in that something is wrong with data line 2 between the video card logic and keyboard.

    If you have swapped keyboards - and it is not working and you have swapped the buffers, then you must be looking for either a bad joint or a short circuit on the PCB tracking to start with I would have thought?

    Dave

  6. #16
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    Yes, I think that's most likely it. And I may have just found it. I flipped the video board over and noticed that one pin had only the tiniest amount of solder, on one side. Interestingly, it is Pin 3 of the keyboard connector, which corresponds to D2. Just soldering it now.. cross fingers.

  7. #17
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    Nope. Definitely that pin is soldered in well now but.. no dice.

  8. #18
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    Too easy my friend !

    I notice that KD2 also disappears off to U4G (74175) on schematic 7 of 8 of http://www.osiweb.org/manuals/OSI-540B.pdf. A faulty 74175 input I wonder?

    Dave

  9. #19
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    I am tracking that down right now. I actually left the keyboard disconnected except Break, hit Break, and observed that we have activity on all pins except pin 3 still. I followed KD2 to the 8t26 at u3h, andi see signal on all the relevant pins. I did notice that KD2 branching off somewhere so I'm thinking our culprit is probably down that way.

  10. #20
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    Okay I retested.. it gets a bit confusing when you've been looking at the things all day.

    So I see signal on pin 13 of U3H (8T26). However 12 and 14 are 'stuck' low. U4G pin 13 is also low. This is the only one of the KD lines where this is happening. KD0-KD1, and KD3-KD7 all have signal. So now I'm going to hunt around and see if I can figure this out... are you saying a bad input pin on the 74175 could drag everything low?

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