23 - High-speed counters

Abstract

This chapter introduces high-speed counter that are used when it is necessary to count a high-frequency input signal, such as those coming from an encoder disk. These high-frequency signals can be counted by feeding the signals into the Inputs XO-XS and using the high-speed 32-bit Counters C235-C255. It discusses that within the FX range of PLCs there are four types of high-speed counters: Single phase with user start/reset, Single phase with assigned start/reset, two-phase bi-directional, and A/B phase. If all of the high-speed inputs, X0-X5, have been assigned to be used with high-speed counters and any of these inputs goes positive, the main program scan will be interrupted while the corresponding high-speed counter is incremented/decremented. Therefore, all of the above inputs operate together at their maximum frequencies. While using high-speed counters, it is not possible to use standard comparison instructions to determine when such counters reach a particular value. This is because the counters operate using interrupts, whereas the standard comparison instructions are executed during the normal scan of the program.