Select the PLC modules

The first step in selecting the PLC modules is to analyze the technical requirements of the application. In most cases this analysis will guide us in selecting the PLC and the I/Os. The next step will be to do different considerations that affect the total cost of the automation solution.

In this article we will focus only on the inputs / outputs modules and leave the rest of the hardware for another time.

1-Signal Range: voltage, current, temperature sensor. If we use different voltages it must be taken into account in the wiring of the control cabinet. For example, if the digital output voltage is 120 VAC, it is advisable not to mix with wires of 4-20 mA signals. On the other hand low voltage ranges are safer for the personnel.

2-Resolution of analog signals: Normally a resolution of 12 bits is sufficient (in this article the concept and the error introduced by the A / D converter is explained). Keep in mind that to raise the resolution 1 or 2 bits increases significantly the price of the module.

3-Electrical Isolation: There are different levels of isolation (channel to channel, among groups of channels, channel to earth, etc.). Standard modules do not have channel to channel isolation but isolation among groups is sufficient in most applications. Not mixing up the common and the ground is usually a good practice. Modules with channel to channel galvanic isolation are expensive so it is necessary to analyze this aspect carefully.

4-Diagnosis: Detection of internal faults in the module is an interesting plus but you have to pay it. If the downtime of the machine or process is important then be worth paying this extra cost to avoid or greatly reduce downtime.

5-SIL Certification: If your application requires a safety PLC we must use the corresponding modules whose cost is much higher. These modules incorporate many diagnostic functions and have a redundant internal structure with logic 1oo2D or 2oo3D (in this article we explain what is a safety PLC).

6-Extreme conditions: Manufacturers often have special modules to work in extreme conditions, such as high temperature. Its cost is much higher.

7-Classified Area: In hazardous zones, such as in certain areas of refineries, areas are classified into different categories. This implies having to design the safety PLC or DCS meeting some stringent technical requirements. Typically, in the case of inputs / outputs, they must be intrinsically safe, it implies either put intermediate barriers or use intrinsically safe modules available only for some PLC models.

8-Speed operation: In positioning applications we require modules that can work with fast and accurate movements. In digital signals, DC ranges allow more continuous switching speed and are more suitable for fast applications. Depending on the case we can use standard or special modules.

9-Redundant architecture: The concept of redundancy is wide and must be analyzed carefully. There are here two concepts: “Safety” and “Availability”. In the first one we normally use the logics 1oo1 (non redundant signal) and 1oo2 (redundant signal), in the second one we use 2oo2 and 2oo3. If we add a “D” (e.g. 1oo2D) it refers to the diagnostics of the signal failure.

10-Remote I/Os: Depending on the layout of instruments and equipment on site it can be interesting to use remote I/O racks close to the field elements. In such cases the usual aim is to save cabling. In critical applications (nuclear, airports, etc.) the target can be to have very high availability by using redundant I/Os in different racks separated several hundred of meters.

11-Extension of existing PLCs: In these cases we may have a physical limitation of space in the cabinet or in the room. This can influence the type of modules used.

12-Density of the modules: It is important in many cases, especially if the distribution of I/Os in the racks and modules is made in an orderly manner and according to certain criteria.

Once the analysis is made we will have few doubts about the type of modules to use that we can solve with other considerations affecting the cost. On the one hand we have the investment cost (the so-called CAPEX) and on the other the operational and maintenance costs (OPEX).