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Thread: Amstrad PC5086: troubleshooting serial port

  1. #1

    Default Amstrad PC5086: troubleshooting serial port

    Hi there!
    I have some issues with the serial port of an Amstrad PC5086. When tested with a null-modem cable, the port receive data correctly, but cannot send data. A further test with some leds connected to the pins revealed that the TX, RTS and DTR signals are always on, no matter how they are set by software. The pins turns on as soon as the pc is turned on, so this seems to indicate an hardware problem rather than a software one. Any suggestion on the possible causes and on how to proceed further?

  2. #2
    Join Date
    May 2006
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    Melbourne, Australia
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    Quote Originally Posted by davide78 View Post
    Hi there!
    Welcome to these forums.

    Quote Originally Posted by davide78 View Post
    When tested with a null-modem cable, ...
    What pins are connected to what?

    Quote Originally Posted by davide78 View Post
    A further test with some leds connected to the pins revealed that the TX, RTS and DTR signals are always on, no matter how they are set by software.
    What do you mean by "on" ?

    RS232 output signals have two levels; a positive voltage or a negative voltage. There is always a voltage.

    Quote Originally Posted by davide78 View Post
    Any suggestion on the possible causes and on how to proceed further?
    Try some hardware testing software that tests RS232 serial ports. For the port to be fully tested, a loopback connector will need to be fitted to it.

    CheckIt is such software, one of its tests being 'Serial Ports'. Connect the loopback connector to the port, then when CheckIt asks if an external loopback connector is attached, answer with 'Y'.

    Details of the loopback connector required by CheckIt can be found on the Internet.

  3. #3

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    Quote Originally Posted by modem7 View Post
    What pins are connected to what?
    I used a Startech USB to Null Modem adapter. According to the manual, this is the pinout of the DB9 serial:
    Code:
    Pin Signal
    1   DCD
    2   TxD
    3   RxD
    4   DTR
    5   GND
    6   DSR
    7   RTS
    8   CTS
    9   RI
    It works no problem on a more modern machine: Macbook on USB side, Win98 on serial side.

    Quote Originally Posted by modem7 View Post
    What do you mean by "on" ?
    RS232 output signals have two levels; a positive voltage or a negative voltage. There is always a voltage.
    Didn't know that: I assumed zero voltage or positive voltage. In any case, I mean positive voltage.

    Quote Originally Posted by modem7 View Post
    Try some hardware testing software that tests RS232 serial ports. For the port to be fully tested, a loopback connector will need to be fitted to it.
    CheckIt is such software, one of its tests being 'Serial Ports'. Connect the loopback connector to the port, then when CheckIt asks if an external loopback connector is attached, answer with 'Y'.
    The loopback test fails in CheckIt:
    Code:
        Loopback Plug...............................................Present
    
        Test data register..........................................FAILED ***
            [Ext] Expected 00h, received no character.
    
        Test interrupt enable.......................................Passed
        Test interrupt id...........................................Passed
        Test line control...........................................FAILED ***
            [Ext] Transmitter wouldn't go ready.
    
        Test modem control..........................................Passed
        Test line status............................................Passed
        Test modem status...........................................FAILED ***
            [Ext] Expected 00h, received F0h.
    
        Test different baud rates...................................FAILED ***
            [Ext] Baud Rate 300: Expected 54h, received no character.
    With no loopback connector, and No answer to the question, the test pass.

  4. #4
    Join Date
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    Quote Originally Posted by davide78 View Post
    ... with the serial port of an Amstrad PC5086.
    I found a photo online of the rear of an Amstrad PC5086.
    So a 9-pin RS232 port, which appears to be part of the motherboard.

    Quote Originally Posted by davide78 View Post
    Didn't know that: I assumed zero voltage or positive voltage. In any case, I mean positive voltage.
    The TxD pin (pin 3 on the 9-pin connector) normally sits at the negative voltage.

    Quote Originally Posted by davide78 View Post
    The loopback test fails in CheckIt:
    With no loopback connector, and No answer to the question, the test pass.
    An explanation of internal and external loopback is at [here].

    Some alternative software (for DOS only), serial port specific, is at [here]. Based on what you saw with CheckIt, that software is expected to pass on its 'internal loopback' test and fail on its 'external loopback' test.

  5. #5

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    Quote Originally Posted by modem7 View Post
    An explanation of internal and external loopback is at [here].

    Some alternative software (for DOS only), serial port specific, is at [here]. Based on what you saw with CheckIt, that software is expected to pass on its 'internal loopback' test and fail on its 'external loopback' test.
    Indeed, the software fails the internal loopback test and fail the external loopback test. By my understanding of the explanation. this indicate that the UART behaves correctly, and that the fault may be in the level converters. The PC5086 uses one MC1488 and two MC1489 as level converters. I tested the MC1488 with a multimeter: the outputs are stuck at +9V, inputs at +5V.

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