May 26

Written by: CPKSoft Engineering Support
5/26/2008 8:27 PM

The "timeout threshold", the "CommTimeout property" or the "timeout value", which are all the same thing, is an amount of time that you configure and that the driver uses to know when it is time to abandon a communication if no response was received. If a timeout value is too big, an HMITalk object could remain waiting for an answer for a long time. The only reason a timeout exists is to prevent the driver to stay waiting forever. So it is important to define how long you want to wait in order to be sure that the device will not respond anyway after that time.

The timeout is a software setting at the driver side and it does not affect the device behaviour, but the driver behaviour. It does have nothing to do with some physical or electrical thing. It is just a maximum time period and the driver checks what happens first: a reply arrival or the timeout period expiration.

A timeout cannot be too short either. If you set a timeout to 1 millisecond, it will never be possible to send a message and receive a response within that short time. So the timeout will have to be set to a value longer than that. If you define the timeout to 60000 milliseconds, you will have to wait one minute before the driver discards the current request and is ready for the next request. A right value will be something between 1 and 60000 ms and the typical setting is 2000 ms, this is, 2 seconds.

The minimum value for a timeout should be at least the time it takes for a device to return a complete valid answer plus a few milliseconds. If a communication log file shows that a valid communication takes 60 ms to be completed (from tx to rx), then the timeout will have to be greater than that. If you set the timeout to a value smaller than 60, you will always receive a timeout error because the timeout period expiration will happen before the reply was received. In this case, the timeout should be at least of 120 ms, which is two times the time required to communicate when the device responds well. It must be greater because you must be tolerant to some unknown factors that could cause minimal delays and make the conversation time to fluctuate.
 
As an analogy, imagine when you make a phone call. How many rings do you wait until you say: "Ok, there is no body there, lets hang up". Do you wait for 50 rings?. Too much. specially if you have too many other calls to make. Do you wait for 1 ring and then you hang up?. Too short and probably you will never have the chance to speak to anybody. What we define with the CommTimeout property is "how many rings do we have to wait".If you know that the average person answers the phone at the 3rd or 4th ring, you will probably wait up to 6 rings before hanging up. You will not wait strictly for 3 rings because probably some times you will lose the call just when the other person was about to pick up the phone.

There is no problem to set a timeout that is longer or even much longer than the time actually involved in a valid communication. Continuing with the phone rings  analogy, think of HMITalk as a secretary who has many phone numbers to call. She starts calling one by one, and waits up to 6 rings for each phone to answer. She expects people to answer within the 3 first rings. If everybody answers within the 3 rings, she will never have to wait for the 6 rings. This shows that the timeout value is only the "maximum number of rings" that she will wait. But if everything is OK, this value is "never used" because that limit is never reached.

Now imagine that only some of the phone numbers that she has to call are not available. Sometimes they answer, sometimes not. What does she do when she attempts with those numbers?. Instead of going immediately to try with the next number, she might retry 2 or 3 times with each failed number. The person she was trying to reach could have been busy at that moment, so it is worth a retry. At the HMITalk level, this can be donde by configuring the ScanRetries property.
 
Now, how long would it take to this secretary to complete all the phone calls? Does it depend on her "finger dialing skills"?. Would you say that she is a "slow person" if she takes too much time to complete the cycle?. Is it her fault that nobody answers at random patterns to certain numbers in an unpredictable way?. What would you do? Change the secretary?. Increase the number of rings she waits for?. Or check why there are some numbers that are not responding as quickly as they should?. The problem is probably not on the secretary side. Focus on the adapter, the radio link, the noise conditions of your line, etc.

Tags: