Our C program control a device (connecting to RS232 port) through SW RS232 driver. We have Serial Port Monitor echo everything sent by the program.
In program, string 'SSS100' is sent to device consistently, but after a period time, no more 'SSS100' shows but '?' appears during each sending period. We don't know where '?' comes from and this charactor is not surposed to be sent by RS232 port.
What the problem seems to be? HW? SW? or RS232 driver?