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Thread: Heathkit H8 not sending anything out via serial

  1. #1
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    Default Heathkit H8 not sending anything out via serial

    I've got an H8 with the H8-5 serial/cassette board. The cassette seems to work fine, the H8 loads programs. However the serial interface doesn't seem to do anything at all. I've determined that it is wired correctly, and that it is set for 9600 baud. Heahtkit has a little test program that outputs As to whatever is connected to the serial port, but alas, nothing gets to the terminal.

    The board has two Intel 8251s.. one handles the tape I/o, the other handles serial. I've switched them back and forth just to see if there's any difference -- but tape keeps working no matter which 8251 is installed while serial refuses to do anything.

    What I have on hand to diagnose this is a logic probe. Could I use that somehow to probe the outgoing and see if anything is getting out? Based on my read of the manual, I might want to look at WR on the 8251 and see if there's some kind of activity there when I'm hitting the 'OUT' key to output the A to serial out?

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    I assume that you've set the H8-5 jumpers up correctly for RS232 at P102. If you've got a 'scope, check to see if anything appears at pin2 of IC119--that's an opto.

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    I don't have any experience with H8 or PDP-8, but typically serial ports have line driver/receiver chips after the UARTs. on PC's they are commonly 1488 chips for the line driver (transmitter) and 1489 chips for the line receiver. It is common for one of these to burn out due to overvoltage (sacrificing itself to save your UART chip) but when that happens you need to replace it.

    Check to see if you have any 1488 or 1489 chips and replace them if you do.

    -ken

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    The H8-5 is very different--no IC drivers. Optoisoloator with discrete transistor logic.

    Take some time and research the issue. There's a schematic here that took me less time to search for than to type this message.

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    Quote Originally Posted by Chuck(G) View Post
    The H8-5 is very different--no IC drivers. Optoisoloator with discrete transistor logic.

    Take some time and research the issue. There's a schematic here that took me less time to search for than to type this message.
    I was looking at that.. but again, still not quite able to always understand what's going on. I've probed 8251's TxD pin 19 and it is registering something when I'm telling the computer to output.. so my problem is further along.. if I'm following correctly I need to look at IC122 next and see if it is going HIGH when something is put out from TxD.

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    IC122 goes high briefly when I hit the key to output data in the test program.. so I think that's working. Now it's up to Q105 or IC119 but that's where my ability to read schematics fails me. Not sure what's supposed to happen up there.

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    Okay, correct me if my logic is wrong here.

    If I'm reading the schematic right, Q105 is held low by R149. When output comes from IC122 pin 8, it causes a diode (?) in IC119 to open up, and that causes the collector on Q105 to go high (which I've tested with my probe, it does flicker high when I hit the OUT key on my test program to send data). However, the emitter is staying low.. if I understand transistors correctly, it should be going high, right?

    EDIT - I got that wrong.. I think.. it looks like IC119 pin 2 is high and goes low when data is sent to it. I hit the OUT key to output my test output, and that's what it does, the collector on Q105 then goes high. But yeah, nothing seems to happen on the emitter.
    Last edited by falter; May 14th, 2017 at 01:32 PM.

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    Check to see if your logic probe can handle voltages above 5V. RS-232 are usually +/- 10V or more.

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    Also... I checked R149 -- 1000ohm, so it's still good.

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    Quote Originally Posted by dave_m View Post
    Check to see if your logic probe can handle voltages above 5V. RS-232 are usually +/- 10V or more.
    Good point. I'm not sure.. I think I looked that up before but couldn't find the answer. I do know I can hook up to 8 or 10v for power..

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