Processing Inputs - Programmable Logic controllers

A PLC is continuously running through its program and updating it as a result of the input signals. Each such loop is termed a cycle. PLCs could be operated by each input being examined as it occurred in the program, its effect on the program determined, and the output correspondingly changed. This mode of operation is termed continuous updating.

Because there is time spent interrogating each input in turn with continuous updating, the time taken to examine several hundred input/output points can become comparatively long.

To allow more rapid execution of a program, a specific area of RAM is used as a buffer store between the control logic and the input/output unit. Each input/output has an address in this memory. At the start of each program cycle the CPU scans all the inputs and copies their status into the input/output addresses in RAM. As the program is executed, the stored input data is read, as required, from RAM and the logic operations are carried out. The resulting output signals are stored in the reserved input/output section of RAM. At the end of each program cycle all the outputs are transferred from RAM to the appropriate output channels. The outputs then retain their status until the next updating. This method of operation is termed mass I/O copying. The sequence can be summarized as follows:

  1. Scan all the inputs and copy into RAM.
  2. Fetch, decode, and execute all program instructions in sequence, copying output instructions to RAM.
  3. Update all outputs.
  4. Repeat the sequence.

Processing Inputs

The time taken to complete a cycle of scanning inputs and updating outputs according to the program instructions, that is, the cycle time, though relatively quick, is not instantaneous and means that the inputs are not watched all the time, but instead that samples of their states are taken periodically. A typical cycle time is on the order of 10 to 50 ms.This means that the inputs and outputs are updated every 10 to 50 ms and thus there can be a delay of this order in the system reaction. It also means that if a very brief input cycle appears at the wrong moment in the cycle, it could be missed. In general, any input must be present for longer of digital impulses that can be detected? The maximum frequency will be if one pulse occurs every 40 ms, that is, a frequency of 1/0.04 ¼ 25 Hz.

The cycle or scanning time for a PLC, i.e. its response speed, is determined by:

  1. The CPU used.
  2. The size of the program to be scanned.
  3. The number of inputs/outputs to be read.
  4. The system functions that are in use; the greater the number, the slower the scanning time.

As an illustration, the Mitsubishi compact PLC, MELSEC FX3U), has a quoted program cycle time of 0.065 ms per logical instruction. Thus the more complex the program, the longer the cycle time will be.


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