Com端口队列延迟计量

Question

I have two programs (host and slave) communicating over a com port. In the simplest case, the host sends a command to the slave and waits for a response, then does it again. But this means that each side has to wait for the other for every transaction. So I use a queue so the second command can be sent before the first response comes back. This keeps things flowing faster.

But I need a way of metering the use of the queue so that there are never more than N command/response pairs in route at any time. So for example if N is 3, I will wait to send the fourth command until I get the first response back, etc. And it must keep track of which response goes with which command.

One thought I had is to tag each command with an integer modulo counter which is also returned with the response. This would ensure that the command and response are always paired correctly and I can do a modulo compare to be able to meter the commands always N ahead of the responses.

What I am wondering, is there a better way? Isn't this a somewhat common thing to do?

(I am using Python, but that is not important.)

Answer 1

Using a sequence number and modulo arithmetic is in fact quite a common way to both acknowledge messages received and tell the sender when it can send more messages - see eg http://en.wikipedia.org/wiki/Sliding_window_protocol . Unfortunately for you, the obvious example, TCP, is unusual in that it uses a sequence number based on byte counts, not message counts, but the principle is much the same - TCP just has an extra degree of flexibility.