Why are Analog modules used in PLC more expensive ?

As with every technological advancement, each device now evolves to cater to a specific application range like small scale, large scale, analog, and digital to name a few. The Programmable Logic Controller (PLC) has adapted to this technological advancement by branching out into different types that meet a specific purpose.

If the physical structure of PLC is kept in mind, then it is classified into 2 broad categories, the Fixed I/O PLC and the Modular PLC.

The fixed I/O (Input/Output) PLC has it's I/O's fixed and integrated into the microcontroller itself rendering it to be unexpandable.

On the other hand, Modular PLC as indicated by its name allows expansion of the system through modules enabling the usage of additional I/O's.

These modules can be of 2 types, Analog and Digital module.

The Digital modules only deal with digital signals (HIGH or LOW) and are used to control devices that only have two states such as a start or a stop.

However, the Analog modules deal with analog signals (variable signals) like temperature or rate of flow and are used for operating loads variable in nature like operating a valve to a certain desired position.

If we see the price comparison between Analog & Digital modules, there exists a stark difference between the two. The Siemens S7-1200 8-Analog input module stands at 291 EUR, while the 8-Digital input module stands at 87 EUR.

This stark price difference is due to the following reasons:

• DAC & ADC

The Analog input modules require Analog to Digital converter (ADC) for the controller of the PLC to correctly read the input values which can be as small as ±10 mV, from bridge transducers, as large as ±10 V, from actuator controllers, or 4-to-20 mA currents, in industrial process control systems.

Similarly, the Analog output module requires Digital to Analog converter (DAC) for converting the instruction from the PLCs controller (in digital form) to operate control actuators, valves, and motors in industrial environments.

Moreover, the resolution of I/O modules and the conversion time directly affect the accuracy of the modules and their price.

• Isolation

Unlike Digital I/O modules which read and operates devices having only two states and do not a converter of any sort, Analog I/O modules incorporates DAC & ADC due to which it is neccessary to isolate the ADCs, DACs, and signal-conditioning circuitry on the field side of the system from the controller(PLC) on the digital side so that there is no direct conduction path but data and power can still be exchanged.

This isolation is obtained through optocouplers or electromagnetic isolators which is used to protect circuitry and human beings, and improve noise rejection performance.

The two main reasons of expensive price of Analog modules, Isolation and Converters