Types of Controllers Inputs/Outputs

The controller is one of the most important parts of the control system, because the controller controls the process. For that purpose, the controller needs to get the information and signals from the process, and after processing them it sends another information back to the process. Depending on the complexity of the process there can be more or less process variables as input/output signals. The simplest process has one signal input (from process to controller) and signal output (from controller to process). Anyway, no matter how much input/output variables are included into process control they must be send to and back from the controller. Some controllers have a few inputs and outputs on them, and in case of need for more inputs/outputs then the I/O modules are connected to the controller. The connection between the controller and the I/O modules can be various from controller to controller, depending on the manufacturer. For example, they can be connected with Interbus and use the well-defined protocol for communication.

The number of inputs/outputs in one module is various. Usually, there are modules with 8, 16, 24 or 32 inputs/outputs, and one module have only inputs or only outputs. The type of the I/O terminals can be continuous (analog) or discrete (digital).

The analog I/O terminal operate with current or voltage signals. The industry standard defined for current analog signal is from 0 to 20 mA, or from 4 to 20 mA (so called live-zero) and for voltage analog signal is from 0 to 10 V. These signals if are output they are generated in controller and send to the process, usually to some actuator (for example, electronic valve which is opened/closed that much depending on the signal level, and puts the amount of energy in the process according to the signal level). From the other side, when analog signal is input for the controller, it bring some information from the process (for example, an ultra sound sensor or an inductive sensor which detect the current position of some process parameter). All these analog signals are live only out of the controller. When the analog signal comes to the input terminal, it is converted into appropriate digital signal with A/D converter, and then it is send into the controller for further processing. Also, the results from the calculations of the controller that need to be send to the process as analog signals are converted with D/A converter.

The digital I/O terminals are quite simple. These terminals provide discrete input/output signals. They can have two states, 0 or 1. As in the digital electronics, we can implement these signals with voltage levels, high voltage level on the terminal means 1 and low voltage level means 0 (this is relative and depends on the logic that we use, so this levels are positive logic, but if we use negative logic, then high voltage level is 0, and low voltage level is 1). Usually, the high voltage level is defined by the voltage of the power supply of the controller and I/O modules. In automation industry standard power supplies have voltage level of 24 V DC, which is used most, and in some cases the voltage level of 110 V DC. So, when we have the voltage level of 24 V on the terminal that means that this terminal have digital signal with value 1, and the other case when the voltage is 0 V, then we have digital signal with value 0. When controller sends logic 1 on one of its output terminals, that means it provide 24 V voltage on that terminal and this voltage will activate a relay which is connected to this terminal, and the normally open contact of the relay will activate, for instance, a power contactor that feeds energy into the process. On the other hand, some relay that is activated by some state in the process (for example, open valve activate the relay, closed valve deactivate the relay) with its normally open contact will bring 24 V voltage level on the input terminal on the module (this voltage on the input tells to the controller that the valve is opened).

Relays are very frequently used as the I/O devices. The switched contacts are usually implemented as changeover sets, NO/NC (normally open/normally closed) contacts. Sometimes the output terminals are implemented as relays. But, when the controlled process is relatively fast, the binary output will have to switch very frequently. In this cases, a mechanical component would wear out fairly fast. So, In order to achieve sufficiently accurate results in the control, manufacturers supplies the controllers with solid-state relays and logic outputs to switch, for example, 230V AC or provide a 24V output.