The "scan time" question is a typical PLC comparison question. The way PLCs are architected, most often all of the I/O is wired to the I/O cards in the PLC chassis (we refer to this as "home run" wiring); it is similar, in ways, to a Pamux system. PLCs are scan-based systems. In other words, the PLC has a set "scan" sequence it follows which is typically like this:
- read all inputs
- solve all logic
- write all outputs
- host communication
An Opto 22 system is fundamentally different, so we don't have a "scan" rate. Our approach is typically more of a distributed intelligence approach where brains are mounted out near the field devices, and each brain has its own intelligence (like counting, pulse duration, thermocouple linearization, engineering unit conversion, etc.). It is not necessary to run wiring for all the field devices back to the controller. The strategy is not all sequential, and which logic gets executed depends on the state of the system at the time; we don't simply cycle through all of the logic on every "scan". Not all of the I/O is scanned by the controller at a set interval - it all depends on the logic of the strategy and which I/O points are necessary to solve the current logic; points are accessed when needed.
If you are interested in learning about the other differences between a PAC and PLC, please view Understanding Programmable Automation Controllers in Industrial Automation.