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Thread: Heathkit H8 not sending anything out via serial

  1. #11
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    Either use a scope or use a multimeter to see if you can read fluctuations on the output. You did double-check the jumper configuration on P102, right?

  2. #12
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    Quote Originally Posted by falter View Post
    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..
    Most Logic Probes were made to measure 5V ttl logic or similar 5Vdc families. I gather some newer ones could handle 3v and higher Cmos family logic.

    But I have not had any experience using a TTL logic probe on the 12v signals associated with RS232.

    I would hazard a guess that if your logic probe is a normal TTL family one, then using it on RS232 or RS485 signal level, could easily destroy your probe.

    I asked "Mother Google" and got the following information. So as far as I read that if your have a newer logic probe that handles CMOS as well then likely you can use it on the RS232 12v level signals.

    Last edited by inotarobot; May 14th, 2017 at 03:30 PM.

  3. #13
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    Quote Originally Posted by falter View Post
    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.
    I did a quick cut and tuck of the circuit to add here, so others that may be reading this post, can then add their 2 cents worth to help u.
    Last edited by inotarobot; May 14th, 2017 at 04:08 PM.

  4. #14
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    Alright, let's look at this thing.

    For RS232, pins 6,7 and 10 are connected to -18V through a 680 ohm resistor
    pin 8 is connected to +18 through what amounts to a 2880 ohm resistor.

    So, when the computer has something to send, it wiggles pin 2, through Q105, which is driven by, what I assume is a TTL signal, the LED side of the opto is driven through a 150 ohm resistor to +5. So it blinks when you're sending bits.

    The phototransistor turns on when the opto LED is illuminated. So far so good?

    The RS232 to terminal is connected between pin 4 (ground) and pin 8. Don't let Q104 throw you--it's connected as a current limiter--for practical purposes, you can view it as a simple resistor.

    So normally, when PNP transistor Q103 isn't conducting, pin 8 is pulled by resistors 144 and 145 to +18 (assuming P102 is correctly configured). So when isn't Q103 conducting? When the optotransistor is illuminated by the LED, it pulls the base of Q103 positive and Q103 is biased off.

    When the opto isn't illuminated, then the base of Q103 is pulled negative through Q104, the JFET current limiter. and pin 8 is pulled more negatively--since pin 8 is pulled positive through a fairly high resistance, when Q103 conducts, it pulls pin 8 toward -18V very strongly and you read a negative voltage on pin 8.

    Simple!

  5. #15
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    Quote Originally Posted by Chuck(G) View Post
    Alright, let's look at this thing.

    For RS232, pins 6,7 and 10 are connected to -18V through a 680 ohm resistor

    Hi Chuck, I do disagree with this part of your reply. Its ONLY 7 to 10 ( NOT pins 6,7 and 10 ) Connected for RS232 .. where as you connect 6 to 7 for TTY.

    You connect 6 to 4 for RS232 and TTY

    And for rs232, there is also the connection from Pin 8 to the free end of R145 the 2200 ohm, this giving voltage to pin 5 of the opto IC119

    Also there should be NO link across 680 ohm R151. that's only for TTY
    Last edited by inotarobot; May 14th, 2017 at 05:17 PM.

  6. #16
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    Fair enough, but it doesn't bear on the discussion. We're looking at the transmit section, so you can disregard anything below pin 7.

    So the explanation still holds.

    Now, If I were falter, I'd start looking at the output of the optotransistor, IC 119, between pins 4 and 5. You should see some changes if sending is occurring. A 'scope would be ideal, but a voltmeter would also work.

    A logic probe is going to be difficult because of the logic levels and swings.

  7. #17
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    Sorry got busy with work.

    I still haven't really had time to figure out this USB scope I have.. but I do have a VMM.. am I literally putting the probes on pins 4 and 5 and watching for activity when I tell the computer to send out?

    Also.. I got one of these things:

    20170524_211000.jpg

    I tested it with various serial terminals I have... works great.. can see all activity, etc. But when I plug into the H8 -- zero. No lights, nothing. No activity. I'm wondering, can one of those function with just TxD, RxD and ground (that's what the H8 is wired for).

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    After months of letting it sit, I pulled out the H8 again to see what more I could learn about the serial output.. and to try to figure out if I really do have a serial output problem or possibly a RAM problem.

    As mentioned in my last post, the breakout box doesn't reveal any activity when I enter the test program in the H8-5 manual and try to send out data. If I try to send text from terminal to h8, and then try to read it in, the machine sees nothing.

    However, interestingly, if I loop the h8's output back to itself and read in, it does see something. It's not the correct data, but it does change. If I remove it and go back to PC and read in again, the data changes back to nothing again. So there is *some* functionality here, I think. I've tried switching IC 119 and 121 but the results don't change. I don't think those ICs are at fault.

    As this machine has two RAM boards, I have been thinking about pulling one out and just trying to get serial output going on the base RAM. I might also try loading something more complex, like Benton BASIC, to see if it's trying to output anything there. Stay tuned.

  9. #19
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    Chuck asked a sensible question way back - but I can't see a definitive response. Is the card configured for RS232 operation?

    Look on P102 and make sure the RS-232 links are IN and the TTY links are OUT.

    I assume somewhere you have a P102 to 25-way D cable? If so, how is this cable wired?

    Dave

  10. #20
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    Hi Dave.. yes. I cant recall specific wires/colors/locations at the moment but I did do a thorough check back when I was last investigating. I confirmed the rs232 links were set as well as baud rate (9600). I also confirmed the ribbon connector that runs to the back of the machine was terminated properly according to what the manual called for in the little molex connector that you'd plug your H9 or other terminal into. My machine came with a pigtail that plugs into that and turns into a female db25, and the only pins configured in it are the ones for rx, tx and signal ground. I'll recheck anyway as it has been a while.. but I have been over it a few times in the past and never had occasion to doubt it was rs232 configured.

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