Thoughts on serial communications

Maybe these issues have been solved in the past. I don’t think there is necessarily a very good solution, though.

The problem is that serial communications is imperfect. Characters get messed up in transit between two devices. When they do, that in turn messes up carefully thought out protocols, like “Capture characters in a buffer until you see the linend; then process the buffer as a single message.”

It’s a small problem if a message character gets corrupted. The message probably won’t make sense; that can be handled with an error message and some recovery process.

But what if the linend itself gets corrupted? Then your program won’t see it as the conclusion of a message, so it will continue capturing characters into the buffer. Then what? Two messages are now corrupted. Still no big deal. But then you have an increased risk of buffer overflow. ( You are checking to avoid buffer overflow, right? ) How to handle that? Flush the buffer and hope the next message is better? But the next message will most likely be broken, ’cause you probably flushed the buffer halfway through it, discarding the first part.

Finally, what do you do if the source goes away? There might be a partial message in the buffer. Should you have a timeout, after which you conclude that the source has crashed, and you need to flush the buffer so the next message that arrives, when the source has recovered, will not also be clobbered?

Think of these possibilities. Writing embedded software is not necessarily as simple as we’d like. But if we consider all the possible ways something can go wrong, and deal with them carefully, we can make software work well and robustly.

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