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Serial Communications Emulation with Software

Posted on 2013-05-28
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Last Modified: 2013-06-13
I need to emulate a Symbol PDT Gun communicating with an application over a serial line.  I have monitored the communication between the devices and found the the device sends 0x"32", 0x"32", 0x"2d", 0x"ff", 0x"ff" before it receives a response from the application.  When I send this character stream from a program on a different computer, I get no response.  Any suggestions on what I may be missing?  The normal device wiring configuration has CTS wired to RTS (pins 7 to 8) and DTR is grounded (pins 4 to 5).
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Question by:ceepaulw
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6 Comments
 
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Expert Comment

by:Dave Baldwin
ID: 39202753
DTR may be grounded on your device but that would be very unusual for a computer because it shorts on the line driving it on the other end.  For computer to computer, try the "RS232 null modem with full handshaking" on this page: http://www.lammertbies.nl/comm/cable/RS-232.html
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Author Comment

by:ceepaulw
ID: 39206473
Thank you, that was a lot to read.  Maybe I've gotten ahead of myself.  I have a computer talking with a bar coded reader.  The device is defined as a printer in the computer.  I want to replace the bar  code reader with a computer program.  I have intercepted the data transfer and have replaced it in a program but when I send the first packet, there is no response. I am trying to set up CTS and RTS as well as DTR combination with no luck.  What I tried to describe above was what the original wiring had-it had a DE9 connector on one end and a DB25 on the other.

How can I figure out how to interface with the hardware on the computer as some sort of hardware interaction must be taking place Out-of-Band.  One person suggested that I need a circuit board to accomplish this.

Thanks.
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LVL 84

Expert Comment

by:Dave Baldwin
ID: 39206575
I don't know what you mean by "Out-of-Band".  I do know that Windows serial port drivers implement handshaking to control both sending and receiving.  In Device Manager, you can right-click on your 'Communications Port' and select Properties.  On the Port Setting tab, you can set the default values for that port.  That would be for setting up another computer to talk to the one that the bar code reader was attached to.

Now if you are talking about replacing the bar code reader with a program on the Same computer, then you have to intercept the calls to that serial port which is much more difficult.
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Accepted Solution

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ceepaulw earned 0 total points
ID: 39231524
I found a program "com0com" that enabled me to easily try different combinations of dtr, dsr, etc.  Once the right signals were wired or shorted, the computer responded to the data from the other computer.  By out-of-band I was referring to signalling not visible in the data recorder.
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Author Closing Comment

by:ceepaulw
ID: 39243846
I spent considerable time to also search for solutions in other areas.  One of these panned out.
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LVL 84

Expert Comment

by:Dave Baldwin
ID: 39245294
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