Field devices are wired to a PLC (Programmable Logic Controller) using various types of wiring methods and communication protocols, depending on the specific application requirements and the type of field device being used. Here's an overview of how field devices are typically wired to a PLC:

1. Digital Input (DI) Wiring:

   • Digital input devices, such as pushbuttons, limit switches, proximity sensors, and switches, are wired to the PLC's digital input modules.
   • Each digital input device is connected to a specific input terminal on the PLC's input module.
   • The wiring typically consists of two conductors (usually a signal wire and a common or ground wire) for each input device, terminating at the appropriate input terminals on the PLC module.
   • The input signals are typically in the form of discrete ON/OFF signals, which are sensed by the PLC's input modules and processed accordingly.

2. Digital Output (DO) Wiring:

   • Digital output devices, such as solenoid valves, relays, motor starters, and indicator lights, are wired to the PLC's digital output modules.
   • Each digital output device is connected to a specific output terminal on the PLC's output module.
   • The wiring consists of two conductors (usually a signal wire and a common or ground wire) for each output device, terminating at the appropriate output terminals on the PLC module.
   • The PLC sends discrete ON/OFF signals to the output modules, which activate or deactivate the connected output devices accordingly.

3. Analog Input (AI) Wiring:

   • Analog input devices, such as temperature sensors, pressure transducers, flow meters, and level sensors, are wired to the PLC's analog input modules.
   • Analog input signals are typically transmitted as voltage or current signals, which are converted by the input modules into digital values that the PLC can process.
   • The wiring typically consists of two or three conductors (signal, common, and sometimes shield) for each analog input device, terminating at the appropriate input terminals on the PLC module.
   • The PLC's analog input modules often provide options for signal conditioning, such as scaling, filtering, and linearization, to ensure accurate measurement and processing of analog signals.

4. Analog Output (AO) Wiring:

   • Analog output devices, such as control valves, variable frequency drives (VFDs), and proportional control valves, are wired to the PLC's analog output modules.
   • Analog output signals are typically transmitted as voltage or current signals from the PLC's output modules to the connected output devices.
   • The wiring consists of two or three conductors (signal, common, and sometimes shield) for each analog output device, terminating at the appropriate output terminals on the PLC module.
   • The PLC's analog output modules often provide options for signal conditioning, such as voltage or current output and adjustable output ranges, to meet the requirements of the connected output devices.

5. Communication Networks:

   • In addition to wired connections, field devices can also be connected to the PLC using communication networks, such as Ethernet, PROFIBUS, PROFINET, Modbus, DeviceNet, or AS-Interface.
   • Communication networks allow for more flexible and efficient integration of field devices into the control system, enabling data exchange, remote monitoring, and control over longer distances and multiple devices.

Overall, field devices are wired to a PLC according to the specific requirements of the application, taking into account factors such as signal type, wiring distance, environmental conditions, and communication protocols. Proper wiring and installation are essential to ensure reliable operation and accurate control of the automation system.