I will start with question 2)b What are timeouts in general?
A timeout is used to keep a portion of the program from waiting forever. Look at the case of a serial read. In your serial loop you try to read one byte. If the remote device has failed or the cable has been disconnected, the VISA Read has no way of knowing that so it will wait indefinitely for that byte. Because of dataflow the loop will not complete its iteration until that byte is read. So, you cannot stop the loop and anything which depends on that loop like the enqueued data will also be delayed. The timeout, in effect, is a way for the function to say, "I am tired of waiting for the byte which is past due and may not be coming at all."
Ok That clarifies what timeouts are. I thought it was something to do with the inability to read the data even though it is available. I wonder why my assumption went there.
Using timeouts to set the loop timing is a secondary effect and will not be accurate if data arrives before the timeout. This is because the Read (or Dequeue or whatever) returns immediately when the expected data is present. The timeout will set that maximum time to wait but not the minimum.
Understoood! But, why is it that if I set VISA READ to a small value timeout like 50 ms, Loop 1 is runs at 50ms but with many many problems: 1) I get the error -1073807339 all the time and sometimes it flip flops between error and no error 2) When I set VISA READ timeout to 1 second (1000), I see that loop 1 runs at 200ms and it directs loop2 to run at 200ms, in this case, I don't see the error -1073807339 once communication starts.
1) The timeout on VISA configure sets the timeout for all VISA functions on that port which may need to wait for data, primarily the Read function. The primary purpose of serial port timeouts is to allow the program to recover from disconnected cables or non-responsive external devices.
OK!!
The default timeout for serial ports is 10 seconds (not milliseconds!). The VISA timeout has nothing to do with the loop 1 timing. If it is taking 200 ms, that most likely is due to the time it takes the remote device to respond. I think we determined that it only takes a little over 10 ms to transmit the data (plus a few more ms to send the ACK). To get that much extra delay in the loop something else must be happening.
THe VISA Configure Timeout defaults to 10000, so can I not set 200 for it to mean ms? Just curious.!! 🙂
I think you are right in that the device responds that late. At this stage, I have set Q and Notifier timeout to 0ms and calculating the iteration time reveals 200ms which should be solely due to VISA functions. In this case, VISA timeout is set to 1 sec as explained before.
2) Timeout on the Dequeue function is the maximum time the function will wait for an element to appear in the queue. If an element is already present when the fucntionis called, it returns the element immediately and timed out? is False. If no element is in the queue when Dequeue is called, the function waits until an element appears or the timeout is reached, whichever comes first. If data shows up at 9.99 ms, the element will be at the output and timed out? is False. If no data shows up before the timeout, then the data output has the default value for the datatype and timed out? is True
IN my model, I am making sure there is an element all the time. DeQ keeps up with enQ or lags by a few elements. Hence, I hypothesize that I should never get timeouts. But, somehow, I see some timeouts and this is mainly in the fast case scenerio and not when loops run at 200ms
Is it that my model ties up VISA to a Q that there is so much complication? I somehow feel like my Loop1 and Loop2 are somehow in sequence because of the Q. I never had problems when I was using wireless communication using property nodes to send data from loop1 to loop2.
OK Maybe this is what I have been missing out. "If no data shows up before the timeout, then the data output has the default value for the datatype and timed out? is True" Where is this default value coming from? Is it the constant cluster connected to the Q to represent datatype? The specific reason to avoid this is why I use shift registers, but if the deQ is setting to default values due to timeout, then feeding that to shift register makes no difference. Maybe I should use the timeout TRUE to ignore the defaults.
3) The VISA Configure timeout can only be set in integer seconds. To get shorter timeouts you must use the property node. The cluster at the Dequeue will be exactly what the cluster at the Enqueue was unless a timeout occurred in which case the value will be default. This is why it is necessary to check the timed out? output on the Dequeue function.
Get it now. I think I understand why I see those default values when I want what values I sent.
4) Yes!
Sorry, that is not very helpful. This is why I suggested a timing diagram or flow chart of some type to try to define what should happen and when.
5) Probably nothing. It may make you feel like you have the timing under control, but it mostly adds anaother layer of complexity.
In Summary, I want my state machine to run at 10ms cycle and dont care how fast or slow Loop1 runs. But somehow, Loop1 controls Loop2 speed and that is causing problems.
