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How to monitor parallel port data of win 3.1 system

We have an old PC running Windows 3.1 and a measurement software which communicates with an external device through the parallel port. How can we monitor the parallel port data send and received between the PC and the device. The device is a vision system not a printer!!
Most software based monitors will not work on the Win 3.1 system. Do any one here have any idea as how to go about doing this?
Aromal Lilly
Columbus, OH
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Message 1 of 23
Are you using any National Instruments hardware or software? It will be hard for us to help you in these forums if you are not using any of the products this forum discusses. You could also try Microsoft's support site, but since Microsoft also does not support 3.1 any more, I would say that searching the internet may be your best bet.
-Marshall R
National Instruments
Applications Engineer
One stop for all your NI-VISA Support

GPIB Support has a new homepage
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Hi Marshall... Right now we are not using any NI hardware... But if NI does have some hardware which we can install on a third PC to monitor the communication between the Win 3.10 PC and the device, then we would like to know more about it... Any help is greatly appreciated... Thanks.
Aromal Lilly
Columbus, OH
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Message 3 of 23


To be able to monitor your parallel port data you have to 'catch' the writes to the parallel port.

With Win3.1 (just like MsDos) you can write directly or indirectly to your ports. If a program writes directly to your parallel port you cannot monitor the parallel port data with a software monitor. If your program writes indirectly you can monitor the data if you can find a tool on the internet to do this.

I will look into my old software this week, but I have to digg very deep to get there Smiley Very Happy


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Hi KC... we were thinking more of a hardware solution if it is not so expensive as we don't wanna mess up the old PC... Will a DAQ device installed on a second PC be able to monitor the communication as well as pass through the signals between the Win 3.10 PC and the external device? Does anyone have any idea?
Aromal Lilly
Columbus, OH
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Message 5 of 23

Hello all.

I put a this thread:  asking for the same: Is there a way to get the parallel port data in another computer using a DAQ or another NI hardware?

Carlos Nava. 

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You should be able to use the existing printer port on your computer (unless you have one of the new computer that doesn't have the printer port) to receive the data that the old computer put out on its printer port.  You would need another printer port adapter if you want to send the data to the device.  You would need to write software to "catch" the data coming into your computer.  I did this (20 years ago) and it wasn't that bad at all.  Just make sure that you set the printer port mode be "bi-directional" in the BIOS.
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Hi nqn827... do you know how the cabling should be done... Right now the cable which run from the old PC to the device is a straight thru one... If we use a second PC's parallel port to monitor what kind of cable should we use?
Aromal Lilly
Columbus, OH
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Message 8 of 23

Hi Aro...  the cable has to be wired the way the software is written to capture the data.  The parallel printer port (Centronics mode) has 8-lines for data (access via DATA Port), 5-lines for status (access via STATUS Port), and 4-lines for control (access via CONTROL Port).  The 8-lines for data would be straight thru.  You can decide how to swap the status lines with the control lines whichever way your software is written.  What you need to keep in mind is the way that your Win 3.1 computer outputs a byte to the printer port (using Centronics mode):

1. Write a byte to the DATA Port.

2. Check to see if the printer is busy (from the STATUS Port).

3. Bring the Strobe line (from the CONTROL Port) low to tell the printer (and your software) that the data is valid on the DATA Port.

4. Bring the Strobe line high to conplete the hand-shaking.

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Message 9 of 23
Hi nqn827... Thanks for the info... I will try it out and see if it works...
Aromal Lilly
Columbus, OH
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Message 10 of 23