How can I monitor data on a serial port in Linux?

Question

I'm debugging communications with a serial device, and I need to see all the data flowing both directions.

It seems like this should be easy on Linux, where the serial port is represented by a file. Is there some way that I can do a sort of "bi-directional tee", where I tell my program to connect to a pipe that copies the data to a file and also shuffles it to/from the actual serial port device?

I think I might even know how to write such a beast, but it seems non-trivial, especially to get all of the ioctls passed through for port configuration, etc.

Has anyone already built such a thing? It seems too useful (for people debugging serial device drivers) not to exist already.

Answer 1

I have found pyserial to be quite usable, so if you're into python it shouldn't be too hard to write such a thing.

http://pyserial.wiki.sourceforge.net/pySerial

Answer 2

strace is very useful for this. You have a visualisation of all ioctl, with the corresponding structure decoded. The following options seems particularly useful in your case :

-e read=set

Perform a full hexadecimal and ASCII dump of all the data read from file descriptors listed in the specified set. For example, to see all input activity on file descriptors 3 and 5 use -e read=3,5. Note that this is independent from the normal tracing of the read(2) system call which is controlled by the option -e trace=read.

-e write=set

Perform a full hexadecimal and ASCII dump of all the data written to file descriptors listed in the specified set. For example, to see all output activity on file descriptors 3 and 5 use -e write=3,5. Note that this is independent from the normal tracing of the write(2) system call which is controlled by the option -e trace=write.

Answer 3

A simple method would be to write an application which opened
the master side of a pty and the tty under test. You would then
pass your tty application the slave side of the pty as the 'tty device'.

You would have to monitor the pty attributes with tcgetattr() on the pty
master and call tcsetattr() on the real tty, if the attributes changed.

The rest would be a simple select() on both fd's copying data bi-directionally and copying it to a log.