Re: "Spy'ing" characters sent thru serial port ?

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Howdy.

At 7/6/2005 17:06 +0200, Paul Rolland wrote:
We have a machine connected to a modem using the serial port, and from time to time, the modem complains the machine sent him a full 2K buffer (in fact, 2047 bytes) which were already sent.

We've been investigating at the application level, using strace to monitor what is sent to the serial port, and at no time such a buffer is sent.

This problem is occurring on a random basis, and attempts to reproduce it in a test environment failed to date.

Is it possible to '(log|copy|...)' the chars that are sent on the serial port to some other place (without altering too much the performance of the machine, we are running the port a 9600bps), at the lowest level ?

I don't have any software answers, but it sounds like the modem is an external type connected by RS232 cable to a serial port. RS-232 is pretty simple at the hardware level and you should be able to create a "Y" cable that "sniffs" the transmit from the computer to modem line without interrupting the dialogue between the serial port and modem. Just connect the transmit and signal ground lines to the receive and signal ground lines of a serial terminal or another computer with some terminal software (or even "cat </dev/serial1" or similar) listening to the serial port. Back in Ye Olde Days (1980s) we used to diagnose modem problems like that all the time.

--
Jeff Woods <[email protected]>
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