Phase 2: Selection of I/O modules (part A)

In this article we discuss the selection of Input / Output Modules (part A) (index of design phases)


2A-Hardware selection of input / output modules

The selection of the type of modules should be based mainly on the technical requirements, on the one hand, and on the other in the economic cost of the complete solution that we are designing. On the other hand, we must take into account the list of approved manufacturers (the so-called “vendor list”).

Here are some of the points that should be considered in the design:

A) -Modules of digital inputs

modulesThe usual thing is to use 24 VDC modules and there are not many cases that justify using another voltage in the digital inputs. As examples that justify the use of a higher voltage, we can mention: i) we have cables of a very small section and very long that cause a non-allowable voltage drop (it is not usual for cables with sections of 1.5 or 2.5 mm2); ii) there are high electromagnetic noises.

Some advantages of using 24 VDC are as follows:

-It’s a low voltage not dangerous if it’s touched.
-It does not produce interferences and we can also mix digital inputs and 4-20 mA analog inputs in the same cable.
-Typically the cost of the module is lower and modules with high densities are available (number of inputs per module).
-It is best suited for high frequency inputs or explosion hazardous areas.
-We can connect sensors with output transistor (PNP or NPN) such as proximity sensors, Namur sensors, etc.

B) -Digital Output Modules

modulesThe most used output voltage is 24 VDC, but we can find technical requirements that advise another value such as 120VAC / 240 VAC / 125 VDC. On the other hand, it is necessary to take into account the voltages specified for the fields devices like solenoids, lamps, MCCs, etc. and whether there are intermediate relays or not.

It is usual that the technical specification defines the different field voltages but does not enter into what should be the voltage value of the digital outputs of the PLC.

The fundamental data to define the type of module is the power of the final element, i.e. the supply voltage and the current consumption in amperes. This information will also tell us whether or not to use intermediate relays.

In applications with high switching frequencies (e.g. in machine tools) another fundamental data is the maximum output frequency as well as the possible overvoltages that can be generated by deactivating outputs with inductive loads (such as contactors, etc.).

In what cases is it justified to use high voltages at the digital outputs when there are no intermediate relays?
-If the field elements are too far from the control cabinet.
-If there may be electromagnetic noises that advise it.
-If there are consumers with high powers.
-If we are in dirty environments or with suspended powders that affect the electrical contacts.

Another aspect to consider is the type of internal component of the module (relay, transistor, triac, etc.) Digital relay outputs have many limitations that need to be properly analyzed, although they may be appropriate if we want to use different voltages in the same module.

Some of these limitations are as follows:
-Low switching frequency.
-Possible temperature problems in applications with high density modules whose outputs remain active for long periods of time.
-Lower MTBF.

C) -Interposing Relays

The use of interposing relays is a common practice, in most cases due to the need to electrically isolate the control cabinet from field equipment or for other purely technical reasons such as:

-Field voltage is different.
-High consumption of the final elements.
-Use of the same output for the control of several field elements.
-Output with one or more voltage-free contacts.

It should also be considered that the use of interposing relays introduces another component that is susceptible to fail, and therefore affects to availability of the system. It is important to consider this in the design of redundant digital outputs.

D) -Range of the analog signals

The most used range, both in the inputs and in the analog outputs, is 4-20 mA.
Many of its advantages are derived from having raised the zero point to 4 mA, i.e. to have a live zero that allows to diagnose faults in the current loop, such as cable breakage. It also allows the transmission of HART digital data through the same cables and can be used for intrinsic safety signals as it allows the checking of the line.

In addition, current signals are generally more immune to electrical noise when compared to voltage signals (0-10 VDC, 1-5 VDC) and can operate over long distances (more than 1 km if the nominal power is 24 VDC). It is not recommended to work with voltage ranges over long distances and with intermediate junction boxes because there are voltage drops that make it impossible.

If we have to wire the 4-20 mA signal to several devices in the control cabinet we can do it with a precision resistance of 250 Ω that converts the signal to 1-5 VDC and it can be wired in parallel to several points.

Other types of analog inputs are temperature, RTD or thermocouples, but they should not be used if the distance to the PLC is large because of the error introduced by the resistance of the cables themselves. It is most advisable to use temperature transmitters, i.e. RTD or Thermocouple with a 4-20 mA converter.

E) -Resolution of analog signals

Normally a resolution of 12 bits is sufficient. By increasing 1 or 2 bits, the price of the module goes up significantly and is rarely justified. This is not usually a hot spot, but in any case you have to meet the specification.

F) -Density of the modules

In digital signals the most common densities are 16 and 32 channels, and in the analog ones it oscillates between 4 and 16. The selection of one type or another depends on several factors and there is not a general rule:

-How we are going to perform grouping and distribution of signals.
-Which is the redundancy architecture and how many simple and redundant signals there are.
-The number of signals of each type, as well as if there are “fail safe” signals, intrinsic safety signals, etc.

It is not always cheaper to use high-density modules, for example, with 32 or 64 I / O since it depends on the design we are making (if I/O distribution is made according to the location of the instruments and equipment in the plant, if there are redundant I/Os and/or logic 2 out of 3, if there are technical reasons in the construction and wiring of the electrical panel, etc.)

 Link to part B