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COM port difficulties

Posted on 2011-10-30
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Last Modified: 2012-05-12
I have a problem with communicating with an Industrial type PLC from my desktop PC ( ASUS M4a77T motherbd, Phenom processor, I can provide more details if necessary ).  The program on the PC gives no reason for failure to communicate apart from an error message asking one to check cabling,etc.  Connecting to an older Dell Latitude with the same cable and everything works fine.  

Can anyone suggest what might be causing the problem.  I've checked device manager and set the same settings as the laptop, viz :-  9600,E,1.  I have run winmsd and the port is there and looks OK. The BIOS settings are 3F8 and IRQ 4 which matches with what everything else is configured as.  

I am not sure where else to look or what to try to debug the problem.
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Question by:AndrewHB
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14 Comments
 

Expert Comment

by:kloyar
ID: 37054372
try going into the bios and verify that the LPT port is set as bi-directional.
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Author Comment

by:AndrewHB
ID: 37054413
Thanks for prompt reply, checked bios and parallel port mode options are :-  Normal/EPP/ECP/EPP+ECP

 No option to select bidirectional.  Will the config of the LPT effect the serial port?

LPT is set to IRQ 7 and 378 hex address.  

Hmmmm?
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Expert Comment

by:fjkaykr11
ID: 37054719
what is the parallel port set to now?
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Expert Comment

by:_
ID: 37055019
>> ...verify that the LPT port is set as bi-directional

3F8 and IRQ 4 is usually for the COM 1 serial port, not the LPT (parallel port).

>> ...Will the config of the LPT effect the serial port?

no, LPT should have no effect on the serial ports.


It sounds like it might be a "timing" issue between the 'old' device and this new chipset (not waiting long enough for the device to ACK).
Or it just plain don't like it.

It's been awhile since I messed with COM ports, but I remember there used to be similar problems around the time they went from 486s to Pentiums.

You can see if going to Device Manager > Ports > Com 1 > Properties, and try changing Hardware Control from NONE to XON/XOFF or HARDWARE, helps.
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Assisted Solution

by:kloyar
kloyar earned 300 total points
ID: 37055552
i ran into a performance problem several years ago. when upgraded from 486 to Pentium, the communication wasn't fast enough. the solution was to manually pause the application just after its launch. is it possible for u to do ?
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Author Comment

by:AndrewHB
ID: 37055736
I have tried increasing the timeouts but that doesn't help. I can't actual pause the application software in the PC.  
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Author Comment

by:AndrewHB
ID: 37055853
I have tried Xon/Xoff and hardware settings but makes no difference.  Still get an error message saying check cabling and communication settings.

I  am wondering if the remarks about upgrading from 486 to Pentium are relavent.  I think these difficulties go back to when I upgraded a motherboard previous to the current one.

Think I might try a query to ASUS they were very helpful when I had a problem upgrading the bios recently.

Thanks for all the ideas.  I will post if I solve the problem.  
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Expert Comment

by:kloyar
ID: 37056108
did u try to update the bios ? is your bios up to date ?
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Author Comment

by:AndrewHB
ID: 37056223
Bios is upto date with latest upgrade from ASUS
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Accepted Solution

by:
_ earned 450 total points
ID: 37060326
>> ...remarks about upgrading from 486 to Pentium are relavent

Is as much as sometimes old devices don't play well with new hardware.

Since it works with an older Dell Latitude, then it needs to be checked if the port is bad, or has been "tweaked" to work with the new chipset and now the old device doesn't work correctly with it.

>> ...try a query to ASUS

That's a good idea.
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Author Closing Comment

by:AndrewHB
ID: 37073382
I have solved my original problem but do not regard it as an elegant solution.

ASUS were not very helpful on this occasion although they tried.

I really should like to know why the serial port on the motherboard doesn't work.
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Expert Comment

by:_
ID: 37074306
>> ...like to know why the serial port on the motherboard doesn't work

That could take some time and effort. But if you want to try, I can give you some starting places.

First I would make sure the port is good. If you have an old serial mouse and a Live Linux CD (or a W98, W2K CD.  XP should also work, but not sure about Vista/W7), that should do for a quick test.
An external dialup modem can also be used, but take a little more to set up.
Ditto for a serial loopback plug. You can either make one or buy one, then run a diag program.

If that is good, then:

- try different cables. This one might not be making good contact with all the mobo pins. You never know.

- start trying different "newer" mobo chipsets as you get the chance (ie: plug it into a bunch of different systems, and keep track of which work and which don't). Might be something about the AMD 770 North Bridge or AMD SB710 South Bridge (don't remember which one deals with Com at the moment), so try another one of these also. Maybe a different Manufacturer.

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Author Comment

by:AndrewHB
ID: 37075071
Serial mouse test is a good idea and I will see if I have one in the "junk" box.  The priority was to get a working link as time was not available to chase a proper solution.  

I don't want to start changing chipsets on a new mobo.  I have other, older PCs that I can use to check the cable, plug combination.  

Spare time to do these checks is the problem.  I needed to get a working RS232 connection and that I have done.  Maybe later I will dig in further to see what I can find out.

Thanks for your interest.
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Expert Comment

by:_
ID: 37077634
Have fun.  ; )
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