I was having terrible trouble with a serial peripheral attached via a USB to serial device, which I eventually tracked down to "odd" characters turning up in the serial stream my C program was sending to it.
It turns out that Jessie (and possibly other distros) ships with something called ModemManager which scampers round anything which looks like a serial port and prods it with the classic modem "hello" sequence, namely AT followed by carriage return.
It seems to do this a few times a few seconds apart after system startup, then shuts up (making it very difficult to detect what is happening).
To me, this is unhelpful - it has taken me almost half a day to work out this is what was going on
. - thank goodness for tcpdump + usbmon giving me the ability to sniff the USB bus
The cure is simple issue the following command as root:
Code: Select all
systemctl disable ModemManager.service