Question

This seems like a simple question, but it is difficult to search for. I need to interface with a device over the serial port. In the event my program (or another) does not finish writing a command to the device, how do I ensure the next run of the program can successfully send a command?

Example:

1. The foo program runs and begins writing “A_VERY_LONG_COMMAND”
2. The user terminates the program, but the program has only written, “A_VERY”
3. The user runs the program again, and the command is resent. Except, the device sees “A_VERYA_VERY_LONG_COMMAND,” which isn’t what we want.

Is there any way to make this more deterministic? Serial port programming feels very out-of-control due to issues like this.

Answer 1

The required method depends on the device.

• Serial ports have additional control signal lines as well as the serial data line; perhaps one of them will reset the device’s input. I’ve never done serial port programming but I think ioctl() handles this.
• There may be a single byte which will reset, e.g. some control character.
• There might be a timing-based signal, e.g. Hayes command set modems use “pause +++ pause”.
• It might just reset after not receiving a complete command after a fixed time.

It might be useful to know whether the device was originally intended to support interactive use (serial terminal), control by a program, or both.