Question

I am sending/receiving data over a serial line in Linux and I would like to find the delay between characters.

Modbus uses a 3.5 character delay to detect message frame boundaries. If there is more than a 1.5 character delay, the message frame is declared incomplete.

I’m writing a quick program in C which is basically

fd = open(MODEMDEVICE, O_RDWR | O_NOCTTY | O_NONBLOCK);
// setup newtio
....
tcsetattr(fd, TCSANOW, &newtio);
for(;;) {
    res = read(fs, buf, 1);
    if (res > 0) {
        // store time in milliseconds?
        //do stuff
    }
}

Is there some way of measuring the time here? Or do I need to look at retrieving data from the serial line in a different way?

I’ve also tried hooking into SIGIO to get a signal whenever there is data but I seem to get data 8 bytes at a time.

(yes, I know there exist some modbus libraries but I want to use this in other applications)

Answer 1

MODbus is like a lot of old protocols and really hates modern hardware.

The reason you’re getting 8 bytes at a time is :
Your PC has a (at least) 16 byte serial FIFO on receive and transmit, in the hardware. Most are 64byte or bigger.

It is possible to tell the uart device to time out and issue a received interrupt after a number of char times.

The Trigger Level is adjustable, but the low-level driver sets it “smartly”. try low-latency mode using setserial)
You can fiddle with the code in the serial driver if you must. Google it (mature content warning) it is not pretty.

so the routine is as pseudocode

int actual=read (packet, timeout of 1.5 chars)

look at actual # of received bytes

if less than a packet, has issues, discard.

not great.