please tell me how plc analog module scan during input scan. does plc scan them with in specified time? if it is so then analog module conversion should be fast enough to match plc scan speed. how analog module select Chanel and then store them in their internal register after A/D conversion and how plc scan this Chanel during I/O scan. who govern the multiplexer.

please reply about all this question and tell me about complete scanning process of plc for analog module and how fast their conversion time (Analog Module).

please reply

 

Since PLCs are not all made the same, the scanning can differ. For the PLC-5, the scan is 1)read the inputs, 2)execute the logic and then 3)write the outputs. An exception can be made by using immediate update instructions which go out and read a particular module out of sequence for particularly fast loop requirements. If the module was resident in the local chassis with the PLC-5, the data transfer and update could be much faster.

For the ControlLogix platform, the analog I/O is updated continuously and read as quickly as the processor can; the read is asynchronous with the logic. To be sure you are working with the same state of digital states for a complete scan, many programmers copy the module data at the beginning of a scan and use the copied data for logic (emulating the PLC-5 style of updates).

For analog loops in the ControlLogix, whatever you read while executing the PID instruction is the latest version of data available. The delay from the conversion to logic could be less than a mS or 10-20 mS depending on exactly when the logic starts, how fast a particular module is and how many different devices are on the communications network.

The old Modicon series would read one “chassis” of modules execute a segment of logic and then write the results to the same “chassis”, read the next group of modules and repeat until all were read processed. The placement of logic was critical to the operation. That mode is not seen anymore but was a method to get data quickly processed when processor speed was much slower.

So it depends on what unit you are using, how it is arranged, and if any special programming techniques are in use.

Russ

 

can you send me detail of analog module generalized hardware and its scanning process. dess analog module scanning is a part of input scan of plc scan time..please explain