A programmable logic controller or PLC is a general-purpose controller, applicable to many different types of process control applications. There are different I/O capabilities in PLC modules and there are also different types of I/O modules.
Monolithic and Modular I/Os:
Monolithic(brick) is fixed I/O module connected to the PLC device which has fixed number I/O ports. Each input lines have corresponding output lines and they are fixed.
The Modular (rack-based) PLC can be replaced and shifted from the PLC rack. In case of any fault and maintenance purpose, the cards can be removed and reconnect properly without replacing the entire PLC. Specific I/O cards can be chosen for custom applications.
In some PLCs there is a facility to change the rack even at the running time, without removing the power supply. There are analoge I/O cards and discrete I/O cards and remote interface I/O in the rack.
If there is a deficiency of slots in some PLCs there are remote I/O slot to connect one PLC with another PLC.
There will be Remote I/O interface rack. The programmed instruction from the Host remote I/O interface sents to the slave PLC after processing the output is sent back to the I/O interface of the Host through the Remote interface of the of the slave. Thus extra racks can be attained through this method.
Discrete I/Os has only two states of operation ON/OFF. Process switches, pushbutton switches, limit switches, and proximity switches are all examples of discrete sensing devices. The picture below shows an discrete input connection:
When the hand switch is turned ON, current passes through the circuit which turns ON a LED light. The light is sensed by a photosensitive device such as a phototransistor inside the module, which in turn activates a bit (a single element of digital data) inside the PLC’s memory. Each input channel has its own optocoupler, writing to its own unique memory register bit inside the PLC’s memory.
Using photosensitive devices in the switching action protects the PLC processor from an external current circuit. It isolates the circuit from peak or excess current in the external circuitry.
Now lets look at the Discrete output module:
The output is given out from a PLC using a LED and when the light is emitted and there is a photo sensing device at the other end. Which senses the light signal and turns ON the circuit. Alternatively, small electromechanical relays may be used in lieu of opto-isolating semiconductor switching elements such as transistors (DC) or TRIACs (AC).
Each output channel has its own optocoupler, driven by its own unique memory register bit inside the PLC’s memory. Discrete output cards for PLCs also typically have 4, 8, 16, or 32 channels.
Modern PLC technology, though, is powerful enough to support the measurement, processing, and output of analogue (continuously variable) signals. All PLC devices are digital devices, for connecting continuous AC current to the PLC. The AC is to be converted to digital. Inside every PLC analog to digital converters (ADC) are used to convert analog input value to digital value. And used DACs Digital-to-Analog Converters to convert digital internal command to analog output. Mostly analog inputs will be 4-20mA in range.
Analog I/O is commonly available for modular PLCs for many different analog signal types, including:
• Voltage (0 to 10 volt, 0 to 5 volt)
• Current (0 to 20 mA, 4 to 20 mA)
• Thermocouple (millivoltage)
• RTD (millivoltage)
• Strain gauge (millivoltage)