Introduction to Industrial Automation

Figure 6.8  Typical form of a digital output module of a modular PLC.

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  Introduction to Industrial Automation

6.4   Digital Input Modules

Digital inputs modules are designed to receive discrete signals from the input devices and convert 

them appropriately for further processing, while electrically isolating them before being trans-

ferred to the memory of the PLC. These signals are electrical voltages that can have different 

values, and can be either continuous (DC) or alternating (AC). For this reason, the input module 

performs a conversion of the voltage level and a modulation of the input signals, so that it becomes 

compatible with the operating voltage (5 V DC) of the microprocessor and the other electronic 

components. Subsequently, in order to isolate the high power levels that characterize industrial 

input devices, from the low power (logic) signals at the microprocessor, the modulated signals are 

directed to the electrical isolation unit. Subsequently, each shaped and electrically isolated signal 

is multiplexed with other similar signals produced by the same input module in order to be trans-

ferred serially to the microprocessor. For the case of small PLCs, when we have a parallel transfer 

of the inputs’ status, no multiplexing unit is utilized. All the processing steps for the input signals 

are presented in Figure 6.10. Each digital input module also features a simple visual indication of 

the corresponding circuit status for each of the inputs (ON or OFF), so that the user is aware of 

the operational mode of the input devices.

Figure 6.9  An electronic card (PLC module) with digital outputs for stepper motor driving