How to read serial port (/dev/ttyS0)

seraph-seph · 10-21-2006, 05:34 PM

Hi everyone.

I'm having some troubles reading the serial port, I'm traying to communicate two machines (one with winxp and the other with slackware) when I set up the communication at 1200, 7, even parity and one(1) stop bits it works fine sending. The setup in linux is this

Code:

stty 1200 cs7 -parodd -cstopb < /dev/ttyS0

echo "testing" > /dev/ttyS0
I receive in a HyperTerminal (winXP) set up as 1200b, 7bits, parity even and one stop bit. testing

But when I try to read data using

Code:

cat /dev/ttyS0

And send data using an app in vbasic set up with the same parameters, it doesn't read anything, I figure out that as End of transmission is sending char 20 instead of char 13 and as requirement I can't change the way it transmits.

Does anybody knows how to read serial port using shell bash without waiting for the carriage return?

Thanks
Mario

osor · 10-21-2006, 07:01 PM

What if you try reading one byte at a time (e.g., while head -c 1 /dev/ttyS0; do : ;done)?

seraph-seph · 10-21-2006, 08:45 PM

Quote:

Originally Posted by osor

(e.g., while head -c 1 /dev/ttyS0; do : ;done)?

Seems logic, let me see if I understood, "head" would be the variable to print?

Mario

osor · 10-31-2006, 07:26 PM

Quote:

Originally Posted by seraph-seph

Seems logic, let me see if I understood, "head" would be the variable to print?

Mario

Sorry it's been so long...

Did you already figure this out?

head is not the variable. head is a UNIX command that usually prints the first few lines of a file (i.e., the head of the file). As this is intended for textfiles and looks for newlines, you use the -c flag, which, AFAIK, will print a number of characters (or bytes) at a time, disregarding EOF, EOL, EOT, or whatever. For any normal file, it is useless to print the first character repeatedly, but a serial device is (I think) like a FIFO, so it should work. So the entire (bourne shell) sequence

Code:

while head -c 1 /dev/ttyS0
do
	: # a dummy command that always returns true
done

does what you think "cat /dev/ttyS0" would do... that is, print the serial output (to stdout) as it comes in.

If you want to read it in a script, you could do a number of things (i.e., use parentheses/braces around the entire thing and use redirection or backtick quotes, or make the command a simple executable file on whose output you could use redirection or backticks).

seraph-seph · 11-01-2006, 06:55 AM

Hi again.

Yes, I figure it out, I tried this one but wasn't enough, what I did was to set control by hardware and keep reading it for a set time.

this is what I used.

Code:

stty 1200 ..... sane -echo -echok -icrnl -ixon -icanon -opost -onlcr time 3 min 0 < /dev/ttyS0
while [true]
do
     cat /dev/ttyS0
done

Where time and min where the cycle time to read the port.