Input/output modules are electronic boards that provide the link between the programmable controller's CPU and the system's field devices. They facilitate the exchange of information, whether for data acquisition or for the control of the machines involved in the process.
The most commonly used input field devices are: switches, limit switches, thermostats, pushbuttons, temperature sensors, among others.
The output field devices The most commonly used are: main contactors, indicator lamps, and speed regulators.
The input modules transform the incoming signals to levels acceptable to the CPU. Using an optical coupler, the input modules electrically isolate the internal circuitry, protecting it from dangerously high voltages, electrical noise, and stray signals. Finally, they filter the signals from the various sensors located on the machines.
The output modules allow voltage to reach the output devices. Using an optical coupler and a pulse relay, the electronic circuits of the controller are isolated, and commands are transmitted to the control actuators.
Types of input and output modules
Because there is a wide variety of external devices (sensors and actuators), we found different types of input and output modules, each of which serves to handle a certain type of signal (discrete or analog) at a certain voltage or current value in DC or AC.
Discrete Input Modules
These electronic cards are used as a link or interface between external devices, also called sensors, and the PLC CPU.
These sensors are responsible for reading system data, which in this case is only discrete. They also have the characteristic of communicating two logical states: activated or deactivated, or in other words, allowing or blocking the passage of the digital signal (1 or 0). The sensors can be manual (pushbuttons, switches, selectors, etc.) or automatic (limit switches, inductive or capacitive proximity detectors, level switches, etc.).
The following figure shows the equivalent and elementary electrical circuits of the discrete input modules for DC and AC respectively. Both types of interface operate on the same principle, except that the AC versions include a pre-rectification stage.
Discrete input modules of the Simatic-S5 family (Courtesy of Siemens)
Discrete Output Modules
Similar to discrete input modules, these modules are used as an interface between the programmable controller's CPU and external devices (actuators), requiring the transmission of only two logic states: on or off. Actuators connected to these interfaces can include contactors, relays, indicator lights, solenoid valves, displays, annunciators, and so on.
Transistor-Type Discrete Output Modulesr
Its operating principle is based on transistors, which means a completely solid-state construction with characteristics for working in direct current (DC) with a long lifespan and low current level.
Equivalent circuit of a discrete DC output interface (Type
transistor)
Discrete Output Modules Type Triac
These interfaces operate by switching a Triac, are also solid state, and are used to handle alternating current signals.
Discrete Relay Type Output Modules
Unlike previous models, these modules consist of electronic devices and a micro electromagnetic switching relay. Their operating range allows them to function with both AC and DC and at different voltage levels, offering the advantage of handling higher currents but with the drawback of a short lifespan due to wear on the moving contacts.
During operation, these modules are characterized, compared to solid-state modules, by the recognizable sound of the switching contacts emitted by the micro-relays.
Analog Input Modules
Analog input modules are electronic boards whose function is to digitize analog signals so they can be processed by the CPU. These continuously varying analog signals can be magnitudes of temperature, pressure, voltage, current, etc.
Depending on their design, these modules can be connected to a specific number of analog sensors. These connection terminals (2) are known as channels. Cards with 4, 8, 16, and 32 analog input channels are available.
It is important to note that any analog magnitude that is to be processed through the analog input modules must be represented by a voltage, current, or resistance signal; this work is done by the sensor itself or through a transducer (a device that transforms any physical, chemical, and biological parameter into an electrical magnitude).
These modules are distinguished by the type of signal they receive, which can be voltage (mV) or current (mA)
Those that fall within certain standardized ranges. The most common are:
Current signal: 0-20 mA, 4-20 mA, ±10 mA
Voltage signal: 0-10V, 0-5V, 0-2V, ±10V
The advantage of working with current signals and not with voltage signals is that the problems of electrical noise and voltage drop do not occur.
Analog Output Modules
These modules are used when it is desired to transmit continuously varying voltage or current signals to analog actuators.
Its operating principle can be considered as a reverse process to that of analog input modules.
Analog output signals are of two types: current signals and voltage signals. Standardized values include:
Current signal: 0-20mA, 4-20mA, ± 20 mA
Voltage signal: 0-10V, ± 10V
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Hello Jose Carlos, I congratulate you on your excellent contribution.
I will take the liberty of asking you about a certain doubt I have,
I want to automate an industrial pasteurization process where RTD sensors are currently connected to temperature controllers.
I plan to connect RTDs to temperature transmitters, and these in turn to an analog input module of a PLC. My question is whether this is redundant, since I could connect the RTDs directly to the PLC modules and thus save the cost of the transmitter.
There are specific modules for receiving RTD signals directly, but otherwise you do need the transmitter. If you have standard analog input modules, whether you need the transmitter depends on what you have.
Thank you so much for your contribution!