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Thread: Kaypro keyboard fault

  1. #1

    Default Kaypro keyboard fault

    One thing about vintage computers...you are never short of a project!

    So here I was feeling very pleased with myself. I'd successfully replaced the caps in my Kaypro PSU, and I'd just run the unit through a disk drive and memory diagnostic test recently grabbed from the walnut creek archive. The Kaypro passed with flying colours on both tests. The memory test was done after the disk one. It took a long while and the machine was on for about an hour or so just ticking away.

    After I finished I rebooted and...eh? No keyboard input!




    Here are the symptoms...
    • The machine boots to the A> prompt as normal
    • There is no response to any of the keys at all
    • The keyboard is getting power. The caps light comes on.
    • The keyboard beeps faintly as you press each key (I think it did this when it was working). If you hold a key down it beeps continuosly as if some find of buffer is full?
    • A check on the cord shows there is continuity of all lines.
    I have the technical manual and haven't yet had a good close look at this problem but I thought I'd post, in case anyone has come across this before. Any ideas?

    Could it be the hex inverter IC (I've reseated it) between the keyboard and the Z80 SIO? What signal should you see on the serial data in /data out lines when a key is pressed? I'm ignorant as to how these types of keyboards work?

    ..and how do you get into the keyboard? I've undone all the screws but the thing seems clamped together?

    As there is no response at all, I suspect some fault in the 74LS04N Hex converter IC, a possible dry solder joint either in the keyboard or at the connector on the motherboard (wriggling the cord doesn't make a difference though) or a fault with whatever IC's lie in the keyboard itself (if there are any).

    Do these sound reasonable? Do the symptoms suggest anything different? Any hints on what I should look for?

    Tez
    Last edited by tezza; January 6th, 2010 at 02:25 AM. Reason: Z80 is the SIO not the CPU
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
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  2. #2

    Default

    Just some follow up info before I go to bed. Measured on the connector to the motherboard, the serial data in line (i.e. from keyboard) shows a slight drop in voltage when a key is pressed. Voltage is about halved when a key is held down. However, serial data out line (to keyboard) shows a steady 4.9v regardless of keypress.

    Does this suggest anything?

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  3. #3
    Join Date
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    Default

    Kinda sounds like the problem isn't in the keyboard at all, but the computer not accepting input. Maybe Andrew will read this; he has experience with Kaypro keyboards, having converted one to parallel output.

  4. #4

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    Quote Originally Posted by MikeS View Post
    Kinda sounds like the problem isn't in the keyboard at all, but the computer not accepting input.
    This is my hunch too. However, take a look at the attached schematic which shows the keyboard I/O on the Z80 SIO. Note that the output line (J3-4) says "beep". I take this to mean the beep in the keyboard only occurs when the computer acknowledges keyboard input. Well, the keyboard certainly beeps. I'm puzzelled that there is no voltage fluctuation that I can see but maybe I need to attach the scope to it.

    kaypro-keyboard-i-o.jpg
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  5. #5

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    Hi! The KayPro keyboard is about as simple as can be. What you are describing sounds like the keyboard is working as it is sending characters to the KayPro main unit. I recommend tracing the circuit out with your logic probe. I'd also try swapping out the U71 74S04. Any 74x04 style device should work at least temporarily. Try using the KayPro monitor and see if you can access the KayPro SIO chip registers. If you can access them at all odds are it is working. I'd also considering writing a simple Z80 program to program the SIO to send out characters as fast as possible and see if it is causing the SIO and keyboard connectors output signals to wiggle.

    Thanks and good luck!

    Andrew Lynch

  6. #6
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
    Posts
    7,137

    Default

    Quote Originally Posted by tezza View Post
    This is my hunch too. However, take a look at the attached schematic which shows the keyboard I/O on the Z80 SIO. Note that the output line (J3-4) says "beep". I take this to mean the beep in the keyboard only occurs when the computer acknowledges keyboard input. Well, the keyboard certainly beeps. I'm puzzelled that there is no voltage fluctuation that I can see but maybe I need to attach the scope to it.
    I'm not intimately familiar with the Kaypro, but is it possible that the keyboard can generate its own keyclicks as well as receiving beep commands from the main unit? Alternately it's possible that the main unit is not processing or displaying input but is still smart enough to beep.

    If you've still got Philip's scope, have a look at the data and beep lines; it sounds like the keyboard's sending something all right, but perhaps not in the correct format.

    I'm not sure how you'd implement Andrew's suggestion to use the monitor or write a simple program without a keyboard though...

  7. #7

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    Quote Originally Posted by tezza View Post
    I'm puzzelled that there is no voltage fluctuation that I can see but maybe I need to attach the scope to it.
    Yeah, even a relatively slow serial signal is going to be too fast for a VOM to pick up. You're talking about rapid fluctuations which average out to almost nothing over time.

    Maybe take a look at the other side of the PIO while you're there. If the "beep" line is indeed responsible for the key clicks, could it be that the keystrokes are getting far enough to trigger a click, but not through to the CPU?

  8. #8

    Default

    Hi Mike! What I would try is swapping the keyboard with a similar unit. I have three KayPro 10's in the basement and a little "swaptronics" will help fault isolate in a hurry. Maybe something similar would be of help in this situation.

    If that is not an option, try burning a replacement EPROM with a "scream" type program. The program initializes the SIO and sends characters out serial port as fast as possible. It makes it easy to debug the serial port since you can obviously trace the signal with a logic probe. Since the KayPro uses a TTL serial line it is pretty simple to trace.

    AFAIK the KayPro has an audible "key click" so I recall it sends data back to the keyboard. You should be seeing a "blip" on the KayPro keyboard output with every key press. If not, the inverter or SIO is probably dead. I am betting on the 74S04 myself although I've seen SIO/DARTs keel over and die too.

    Thanks and have a nice day!

    Andrew Lynch

  9. #9

    Default

    Thanks for those comments guys,

    Andrew, yes a keyboard swap would be a quick way of isolating the fault. Unfortunately I only have one and don't know anyone here who has another. They are not that common in NZ. I don't have an EPROM burner either. Although I'm moved some way from total newbie to competent and fully equipped techie like yourself, Steve and Mike, I still have a long way to go (-:

    >AFAIK the KayPro has an audible "key click" so I recall it sends data back to the keyboard.
    >You should be seeing a "blip" on the KayPro keyboard output with every key press.
    >If not, the inverter or SIO is probably dead. I am betting on the 74S04 myself although
    >I've seen SIO/DARTs keel over and die too

    Yes, this is the puzzelling thing. The circuit diagram would suggest the click (beep) is an output to the keyboard in response to input. The keyboard certainly beeps with every keypress just like it did when it was working.

    I take your point about the voltage drops being hard to detect Steve. On the serial input line from the keyboard they are hard to detect with a DMM with single presses but if you hold a key down, there is a definite voltage drop. I would have expected the same on the output line (to the keyboard) when holding a key. Certainly the keyboard beeps furiously when you do this but there is no drop in voltage I can see. There is no "blip" I can see on the output line like I can on the input line. However, I'll check both lines with a scope.

    >I am betting on the 74S04 myself although I've seen SIO/DARTs keel over and die too.

    These are also my two suspects. Unfortunately I don't think I have a spare 74LS04..I might have to scour some of my parts machines and if I find one unsolder it. I think I have got a spare SIO Z80 somewhere. Luckily all the ICs in the Kaypro are in sockets.

    Andrew, I can't find a circuit diagram showing just what's inside the keyboard. What ICs are in there if any? Also, do you know how to open one up? I've undone all the screws. Does it just need brute strength to prise the top off?

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  10. #10

    Default

    Hi Tezza,
    As I recall, the KayPro keyboards are held together with screws and are pretty easy to take apart. Remove the screws and the top of the keyboard should pop right out. I would verify all the connections are OK in the keyboard and verify continuity to the connectors on the KayPro.

    Upon power on, are you seeing characters on the KayPro display or blank screen? Does the caps lock light work on the keyboard?

    The guts of the KayPro keyboard are really simple. I believe it was a very simple microcontroller (8048?) reading the 8 bit keyboard and sending the TTL 3 wire serial back to the KayPro. It is 300 bps, 8, n, 1 as I recall. I don't recall if there is a UART or not but I think it was a just single uC chip and maybe one or two inverters.

    To me it sounds like the devices on the KayPro motherboard have a problem. You could substitute temporarily almost any TTL inverter like a 74LS04 or 74LS14. Most any family should work at least for a while although it would make sense to me to replace the 74S04 with another 74S04. They are faster, have more drive ability, and use a lot more power than typical 74LSxx parts.

    Thanks and have a nice day!

    Andrew Lynch

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