Sending and receiving repeated commands to a serial instrument with LabVIEW - while-loop

Question

I'm writing a program in LabVIEW 2014 in order to control a linear actuator. The program is very simple, it sets a speed and then runs the subVIs to move the actuator back and forth.
There is a case structure inside a while loop so it would stop when a desired number or iterations is reached. The problem is that the iteration count of the while loop occurs faster than the execution of the program inside the case structure, and therefore the program stops before all the cycles of movement have been completed.
send pulses subVI:
activate subVI:
I tried different time delays in different parts of the code, but none of that worked. I think that the issue is that the while loop iterations run faster than the code of the case structure and somehow I need to slow it down. Or maybe I'm wrong and it is a complete different thing.
Here is the link of the actuator documentation:
https://jp.optosigma.com/html/en_jp/software/motorize/manual_en/SRC-101_InstructionManual_Ver1_1_EN.pdf

Answer 1

Welcome to the fun and infuriating world of interfacing to serial instruments.
Each iteration of a LabVIEW loop can only complete once all the code inside the loop structure has completed, so it's not possible that 'the while loop iterations run faster than the code of the case structure'. There's nothing explicitly wrong with any of your code, but evidently it isn't doing what you expected it to. The way to approach developing an instrument driver is always to start with the simplest case (e.g. one single movement of your actuator), get that working, and build up from there.
The documentation for an instrument's serial interface is rarely perfect and yours is no exception, but it does tell us that
every command is acknowledged by a response, and
you should not send a new command until you have received the response from the previous command.
Your code to send commands and receive the response looks OK. A VISA Read operation will read bytes from the computer's serial buffer until either the number of bytes to read is reached, or a byte matching the termination char is read, or the timeout expires. The manual implies that the instrument's responses are followed by the CR and LF characters, and the default configuration of the serial port in LabVIEW is to terminate each read when an LF is received, so you shouldn't need a time delay between each write and the following read; the instrument's response will be received into the buffer by the OS, then your code will read it out and return it as soon as it hits the LF.
What isn't completely clear from the manual is how the instrument responds to the activation command, G: - does it
Return the acknowledgement immediately, then execute the movement: you can check whether the movement is finished using the status command !:, or
Execute the movement, then return the acknowledgement to show that it's finished.
I think it's 1. but that's the first thing I would check. Unless all your movements are completed in less than 500 ms then I think this is what is wrong here: your program receives the acknowledgement then moves straight on to send the next command, but the actuator is still moving and not ready. In this case you have two options:
add a time delay after the read, calculated to be long enough for the actuator move to finish - this would be easiest, but potentially unreliable
in a While loop after you have got the acknowledgement of the G: command, send the !: command and check the response until you get R for 'ready'. (Remember that the acknowledgement string you receive will also have the CRLF on the end.) Use a time delay in this loop so you don't bombard the instrument with status checks - maybe something like 200 to 1000 ms would be suitable.
If it's case 2. then you would also have two options:
configure your serial port with a read timeout long enough to cover the longest move operation, then the read operation will just block until the acknowledgement is received - again this is the quick and dirty way, or
configure a short timeout, say 1000 ms, and place the read in a While loop that repeats until the acknowledgement is received or too many timeouts have occurred. Note that a timeout is considered an error, so you will have to turn off automatic error handling for the VI and instead test the error wire out of the VISA Read, discard the timeout error and handle any other error yourself.
Just as a general tip, whenever you pass an error wire into a loop and out again, I would use a shift register. That way if one iteration generates an error, the next iteration will see that error and fail immediately, so (for example) if communication fails you don't have to wait for the read timeouts to expire multiple times before your code can exit.
You'll probably have to do some experimenting and referring to LabVIEW help to get this fully working but hopefully this is enough to get you going.