First, let's make sure that we have a common understanding of how long it takes to send or recieve a character at 9600 baud - that is in very rough terms about 1 ms per byte (character). Details include: number of "start" bits and the number of "stop" bits.

I don't know how many bytes the response contains or how the receiver decides that it has not received a "timely reponse" - I mean if the "reponse" takes say 8 characters just to send, then how is that judged against the allowed "20 character" (20 ms) response time? You may have a lot less than 20ms to start your response.

I can't test your code, but some parts do seem rather weird... what is this usleep() stuff in a blocking loop about? Why?

I think that this $PortObj method may be being misused? 

   $gotit = $PortObj->lookfor(100); #blocking read

[download]
I saw a recent question about this.
It appeared to me at the time, that this serial port module could be configured to report sucess after a particular stream of input characters were seen - that would appear to me to be inconsistent with this idea of "lookfor(100)", one hundred what?

What other stuff is running on this Windows machine?
Numbers like 116 ms are a looonnnng time, can you explain more how you calculated that? A process running under Normal priority in a blocking loop that essentially goes to the OS, should under usual circumstances be able to easily process your requirements.

In reply to Re: Timing issue while accessing serial port by Marshall
in thread Timing issue while accessing serial port by vrn

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