To operate in the real world, the programmable logic controller (PLC) needs to connect to a variety of household electrical equipment. The PLC is a versatile product that has many hardware options to accommodate the large number of different electrical interfaces out there. The PLC is modular in design so that one can mix and match the input/output (I/O) hardware requirements to meet the needs of the project.

Discrete I/O Modules

Discrete I/O modules come in both AC/DC voltage levels. When voltage is applied to a discrete input module, the PLC registers that as an ‘On’ condition. No voltage on the discrete input module is an ‘Off’ condition. When the PLC needs to activate an electrical device, it will turn ‘On’ one of its outputs with an AC or DC voltage depending on the discrete output module installed. Some discrete I/O modules come with a combination of inputs and outputs usually with the same voltage levels. Another useful discrete I/O module is the relay output type. What is nice about this type of discrete I/O module is that it allows the PLC to interface with other electrical equipment that source their own voltage. The AC voltage levels range from 90-240VAC and the DC voltage levels range from 5-24VDC.

Analog I/O Modules

Analog I/O modules come in both a current and voltage level types. The current level is 0-20mA and the voltage level is 0-5V or 0-10V. When current or voltage is applied to an analog input module, the PLC registers that as a 0-100% of maximum input range. When the PLC needs to adjust a variable electrical device, it will modulate one of its outputs with a current or voltage depending on the analog output module installed. Some analog I/O modules come with a combination of inputs and outputs. This combination is ideal for analog loop control, which the PLC have built-in PID algorithms to do this type of control. A typical example of analog loop control is in a heating system. One has to measure the temperature of the room (analog input) and adjust (PID algorithm) the heating valve (analog output) to maintain a certain comfortable room temperature. There are RTD and Thermocouple modules available to measure all sorts of temperatures.

Communication Modules

Communication modules come in Data and Ethernet types. The data communication module allows for PLC programming via PC or handheld programmer. Also, the data communication module connects to other control systems via RS232 protocol and operator interfaces like touchpanels. The Ethernet communication module enables peer-to-peer networking capability. Other types of communication modules are Modbus RTU and Profibus Slave, which allow for network integration with other electrical control systems. Specialty modules like the Basic coprocessor module are used with intelligent devices like bar code readers.