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

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 <>

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