Using Serial Devices in FreeBSD / How to set a terminal baud rate

Posted by Benjamin Close on January 8, 2010 under FreeBSD, UniSA | Read the First Comment

Recently I was working on a php command line program that required access to a serial port.

Initially developed under Linux the program was then shifted to it’s permanent location on a FreeBSD server. This is where we first started having problems. Initially we discovered the server didn’t have a native serial port. We fixed this using a USB to serial converter/dongle (FTDI Chipset). This was fine as FreeBSD has the ufdti kernel module. Upon loading the module new devices appears in /dev

crw-rw----  1 uucp  dialer    0, 157 Oct  6 08:39 /dev/cuaU0
crw-rw----  1 uucp  dialer    0, 158 Oct  6 08:39 /dev/cuaU0.init
crw-rw----  1 uucp  dialer    0, 159 Oct  6 08:39 /dev/cuaU0.lock
crw-rw-rw-  1 root  wheel     0, 154 Jan  8 10:50 /dev/ttyU0
crw-------  1 root  wheel     0, 155 Oct  6 08:39 /dev/ttyU0.init
crw-------  1 root  wheel     0, 156 Oct  6 08:39 /dev/ttyU0.lock

We attempted to connect to our device using screen (screen /dev/ttyU0 115200) and everything worked as expected. We could send AT commands to the device all ok.
We then stopped screen and ran our php program. It ended up hanging on a fgets call to the serial port. This is really strange we though.
Next we queried the port to find out what baud rate it was set at:

>stty -f /dev/ttyu0
speed 9600 baud;
lflags: echoe echoke echoctl
oflags: tab0
cflags: cs8 -parenb

Strange we thought as we’d just connected with screen at 115200. Under linux we use screen to set the baud rate, all other programs accessing the port use the port at 115200. So what had set it back to 9600 baud?
We tried to use stty to set the speed:

>stty -f /dev/ttyU0 speed 115200
>stty -f /dev/ttyu0
speed 9600 baud;
lflags: echoe echoke echoctl
oflags: tab0
cflags: cs8 -parenb

What on earth was happening? We set the speed to 115200 but directly quering the port again indicated it was still at 9600 baud? At this point we were perplexed.
Eventually we found the solution. The newer FreeBSD terminal drivers provide the *.init devices, in this case /dev/ttyU0.init . These devices indicate the terminal settings to be applied to the terminal when the device is closed. Whilst Linux leaves the device in the same state the last program put the port into, FreeBSD restores the terminals state to what ever is specified in the init file. So a quick command:

> stty -f /dev/ttyU0.init -icanon -isig -echo echoe echok echoke echoctl -icrnl -ixany -imaxbel ignpar -opost -onlcr -oxtabs cs8 -parenb -hupcl clocal

And then to check:

> stty -f /dev/ttyU0
speed 115200 baud;
lflags: -icanon -isig -echo echoe echok echoke echoctl
iflags: -icrnl -ixany -imaxbel ignpar
oflags: -opost -onlcr -oxtabs
cflags: cs8 -parenb -hupcl clocal

Excellent. The terminal was now configured exactly how we wanted. We ran the program and it worked like a charm!



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