Introduction to Industrial Automation

6.7.1   Fast Input Response Modules

Fast input response modules are utilized when the PLC needs to detect pulses of a very short dura-

tion. If the scanning time of a PLC is for example of 1 ms, then the frequency of state change for 

a discrete input cannot be more than 1 KHz, and in the case that the scanning time is 100 ms, 

which is another typical value, then the frequency of the input cannot be more than 10 Hz. Thus, 

the PLC is not able to detect pulses of a frequency e.g., 50 KHz or 500 KHz. The fast input 

response module contains a very fast microprocessor that it is able to count pulses of a very fast 

speed, independently of the scan cycle of the PLC. This measurement is stored in the module’s 

memory, and is transmitted to the main processing unit during the exact moment of sampling of 

the rest of the inputs, which means once in every general scanning loop. These modules are very 

useful in applications that use fast counting, fast rotation of drums, position decoders, etc.