Greetings scottsull,
As my distinguished colleague Operaghost has already mentioned, with the PLC-5 processor family, the transfer of analog data between the processor and the analog modules, is taken care of by Block Transfer instructions which you must program into the ladder logic.
Let’s look at the Block Transfer Read for a minute. The same concepts apply to the Block Transfer Write also.
The sad truth is, when the processor scans the BTR with true logic, the actual execution of the BTR is not NECESSARILY going to happen BEFORE the scan continues on to the next rung. Think about this: We’re literally talking about TRANSFERRING (moving) a big BLOCK of data. Actually accomplishing that transfer of data might conceivably take up a considerable amount of time - time which would tend to slow down our processor as he tries to continue scanning the ladder logic program. And so - at least in the PLC-5 family - the processor puts this “request” for a transfer of a block of data into a “queue” or a “buffer” - or in other words, into a line-up of “communication-type” requests. These requests will then be accomplished as soon as practical by a “communications handler” inside the processor - but NOT necessarily before the scan moves on to the next rung.
So the truth is that we do NOT know exactly when the actual transfer of analog data will take place. It’s a totally random thing. The transfer MIGHT happen quickly - while the processor is right there on the Block Transfer rung. Then again, it MIGHT happen several rungs later during the same ladder scan sequence. Then again, it MIGHT happen at some random point during a subsequent scan sequence - and even while the processor is executing a completely separate ladder (subroutine) file. Your guess is as good as mine.
If you want to further research this effect, the keyword “asynchronous” should help get you started.