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Parallel port relay problem

Discussion in 'General Electronics Discussion' started by Rob324, Nov 17, 2012.

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  1. Rob324


    Nov 17, 2012

    I use an older PC for some automation tasks with a relay board like this: link.
    This basically works well, but I have a problem. When I shut down the PC but leave the monitor switched on, the relay also stays switched on. If I turn off the monitor, the relay will turn off.

    The problem is that the LPT port should turn off with the PC regardless the energy state of the monitor. That works on my other computers.
    The motherboard is an ASUS P5LD2-VM, the monitor is a CRT.
  2. shrtrnd


    Jan 15, 2010
    Not a lot to work with here.
    You may have a relay board control problem, but since it's an older unit, I'll ask:
    Does the Control Board have a battery back-up for the memory, and does the
    battery need to be replaced?
    Another question is if this problem just started, or has it been like this for a while.
    It sounds like your relay board is getting it's power somehow routed through the power
    to the monitor. (Or from the same source).
    Is this happening after you rewired something, or is it a problem that just started?
  3. Rob324


    Nov 17, 2012
    Thanks for the reply.

    The problem is not new, it began when I started to use this PC a few months ago.
    Previously, I used a different computer with the same relay board and monitor and it worked fine.
    The battery on the motherboard was replaced, this should not be a problem. The relay board
    gets its power from a separate power supply, the whole thing is isolated from the computer
    with an optocoupler, as shown in the schematics.

    I rather think that this is a motherboard problem. I have tried to change the parallel port,
    power management and VGA settings in the BIOS. (In my previous post I forgot to mention
    that I use the integrated VGA.) I have tried to reset the BIOS too, but nothing really helped.
    Last edited: Nov 19, 2012
  4. CocaCola


    Apr 7, 2012
    First off a BIG word of advice on doing this, spend $5 on a parallel port card instead of risking any damages to your motherboard, even if you have it isolated now this will add another layer of security... Second not all parallel ports are created the same, in fact a vast majority of the motherboard bundled ones are just horrible and don't hold any specifications especially in output current and voltage levels... This is the second reason to get a plug in card port vs the bundled one as most of the cards hold much better specifications...

    Back 10 or 20 years ago as the parallel port started to be phased out this was common knowledge as all the sudden a lot of parallel port devices that were not your run of the mill 'computer printers' started to act up... This included a lot of hobbyist stuff like parallel port programmers, the parallel port was/is a dying interface and thus the motherboard companies simply didn't put much into them anymore as 99% of the people will not use them and 99% of the rest of that 1% will use them for printers that are generally a lot more tolerant of lower specs...
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Or $5 on a USB to parallel converter.

    They're even less standard, but if your circuit can pretend to be a printer you may be OK.
  6. gorgon


    Jun 6, 2011
    Have you tried to use a different monitor? Could be that the monitor is backfeeding some voltage.
    Will this happen even if the parallel output is turned off when you turn off the PC?
  7. Timescope


    Aug 30, 2012
    Try biosagentplus to check the BIOS. I have a similar problem with my Legend desktop after a friend used it to test a Visual Basic based robot control program : the data pins of the LPT port remain H, the mouse stays on when the computer is switched off and there is no sound. I tried all the usual recovery methods, system restore, disk check, sfc /scannow, uninstalled the ports and restarted etc but evidently, the BIOS is corrupted.

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