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RTS Signal in Serial Ports using Win32

Posted on 1998-11-11
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Last Modified: 2013-12-03
I have an app. where I have to write some characters
to a serial port upon receipt of the RTS signal from
another computer. I am using the WriteFile() function
to write this file name to my serial port. I am assuming
handling of the RTS is automatically taken care of by
the API (maybe upon CreateFile()?)How do I detect the presence of an RTS signal and then use WriteFile() to write the characters. Thanks.
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Question by:mparavas
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by:mparavas
ID: 1415993
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stefanr earned 100 total points
ID: 1415994
I am assuming that you are connected to the other computer by a "null-modem" cable, i.e. the RTS of the other computer is connected to the CTS of the computer where application is running (DTR and DSR is also cross-connected). Then when the other computer raises its RTS signal, the CTS signal becomes raised on your computer.
Use WaitCommEvent to check if the CTS have changed. Then use GetCommModemStatus to check the actual value of the CTS signal.
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by:mparavas
ID: 1415995
Thanks for the information. Yes, I am using a NULL modem. Is it
true that when I use the Win32 API, RTS and CTS are always
there, since the assertion is taken care of by the API?
Exactly when/how are they asserted. Is it during CreateFile()? Thanks.
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Expert Comment

by:stefanr
ID: 1415996
No, I don't believe that you can expect that. It seems that the state of the signals depends upon what the previous programs (if any) using the communication port had on them when closing. That previous state is restored when you call CreateFile. You should use GetCommState or BuildCommDCB, followed by SetCommState to set the baud rate, etc., and EscapeCommFunction to set the state of DTR and RTS. To retrieve the status of the signals you use GetCommModemStatus.
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