[Python] Serial & reset of the device

A few suggestions:

1) You could try strace'ing the TCL, and then strace'ing the Python, and
comparing - especially if you're comfortable with low-level I/O in C, but
not only. Hunt for the open() call to the relevant device file, and other
system calls near it. Even if you aren't that cozy with C, you could try
putting the strace results on a website or ftp server, and posting links to
them here.

2) Check and double check and triple check your line discipline parameters:
bits per second, parity, word size, start/stop bits.

3) It's also possible the cable is bad (or merely inappropriate), especially
if the TCL is accessing such a device over a different cable. There is no
"one true serial (RS-232) cable standard" - RS-232 is a rather large
collection of ways of doing communications, and they are frequently not
compatible with one another.

4) One of the more likely causes of a reset of a device that is accessed
serially, is sending it a break signal. There are official ways of sending
a break, and accidental ways. Sometimes the two boil down to the same thing
behind the scenes, but look very different in the code. A break signal is a
longish sequence of unframed 0 bits (IE, with no start/stop bits separating
one byte from another). An accidental way of sending a break, is to set
your bits per second too low, and then sending a 0 byte - because those 8 0
bits at the too-low speed will look like a long series of unframed 0's from
the perspective of the reader process that's running at the correct speed.
So I guess the gist of point #4 (this point), is "make sure your bps is set
correctly" and "check the device doc to see if a break signal causes a
reset"

5) If the protocol layered over the serial communication is ASCII-based, you
could try connecting to the device using something like minicom, to make
sure the device is behaving as expected. If the device is speaking a binary
protocol, this is unlikely to help much

HTH

I'm not sure what that means. The RS-232 standard does not have the
concept of "reset". What is it that triggers a device reset?

While not a "reset" per-se, it might be triggered by the RTS/CTS,
DSR/DTR, or carrier-detect pins depending on the configuration.
Without the code and with minimal pySerial experience, I don't
know whether opening a serial-port in pySerial automatically
lights up one of those aux. lines and unsignals it when the
connection is closed. If the device expects a "power on" signal
on one of those pins, I'd start by looking to see if pySerial's
.close() drops the signal on those pins and if it offers a way to
keep the signal high while releasing the port. Otherwise, you
may have to open once, do all your work and only close the port
when you're done (and the device can be reset)

-tkc

On Unix, the serial port device driver generally turns DTR and RTS off
when a port is closed and turns them on when it's opened. I don't
know what Windows does. A quick glance through the pyserial sources
shows that it turns on DTR and RTS when a port is opened, and does
nothing with them when a port is closed.

If you need RTS/DTR to stay in a known state, then open the port, set
them to the state you want them, and keep the port open.