Hi Jerry,
yes I wrote a user space RS485 'driver' for a RS232/RS485 converter and a
single (!) RS485 device (Philips KS20). It is a very simple routine:
fd=open(Device,O_RDWR|O_NOCTTY); //open serial device
tcgetattr(fd,&Oldtio); // save old attributes
tcflush(fd,TCIOFLUSH); // empty 'trashbin'
tcsetattr(fd,TCSANOW,&Newtio);
aLSR=0; aMCR=TIOCM_RTS;
if(ioctl(fd,TIOCMBIS,&aMCR)==-1) // Turn on RTS
fprintf(stderr," (RTS=1 FEHLER)");
res=write(fd,vString,KS20CommLength); // KS20 command length = 17
while(1) // wait for empty TSR
{
++u;
if(ioctl(fd,TIOCSERGETLSR,&aLSR)==-1) {
fprintf(stderr," (GETLSR FEHLER!)");
break;
}
if(aLSR) break; // TSR is empty
usleep(3000UL); // sleep for 3 ms (1/BAUDRATE*3)
}
if(ioctl(fd,TIOCMBIC,&aMCR)==-1) // Turn off RTS
fprintf(stderr," (RTS=0 FEHLER)");
res=read(fd,retString,100);
tcsetattr(fd,TCSANOW,&Oldtio);
close(fd);
This is cut'n'paste (partially) of my routine.
The KS20 answers after it gets a command with definit length. I experienced
a delay of 3 loops = around 9 milliseconds. But perhaps you may tune the
while(1)-loop and the nice-level of the process. Beware that the
TIOCGETLSR does _not_ return the whole LSR, but a flag if the transmit shift
register (TSR) is empty.
hth,
Michael
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