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IO (PLC)

This analog signal varies between a range of values, while the digital signal can have only one of two discrete values.

IO, short for 'Input/Output', refers to the communication between PLCs and other devices.

Digital vs Analog Signals

Digital

Digital signals have one of two discrete values (1 or 0, On or Off). Examples include switches, solenoids and relays.

Analog

Analog signals can have any value within a range of values (0 to 10 VDC, 4 to 20 mA). Examples include flow sensors, RTDs, and flow regulators.

In this circuit closing the limit switch completes the circuit, providing a digital input signal to channel 3.

Input

A PLC input is a signal from a peripheral device which is measured by the PLC and used in the control of the system. The PLC may directly connect to the sensor, such as a thermocouple or relay, or it may take an input from another device which in turn controls a sensor, such as a flow controller. Analog signal formats are commonly 0-10 VDC and 4-20mA, while digital signals are 24VDC.

Digital Input Examples

• Enable button
• Exhaust OK relay output
• Door Up sensor (Acid/Clean)

Analog Input Examples

• Heater thermocouples
• Tank Level Float Sensor (Acid/ILSS)
• CDIW pressure

In this circuit a DC source (such as a PLC digital output) energizes a relay coil, which in turn closes the contact and applies AC power to the motor. THis allows a PLC DO to control an AC load.

Output

A PLC output is a signal sent to a peripheral device from the PLC, used to cause a change in the peripheral. The PLC often directly drives this with its own 24 VDC power; for larger loads, the PLC's power is used to drive a relay which in turn switches power to the peripheral.

Digital Output Examples

• O3 Exhaust Valve
• HDIW Heater Contactor
• SSRs

Analog Output Examples

• CDIW Flow Regulator
• Pump Speed

Troubleshooting

Understanding the types of inputs and outputs allows an engineer to half-split between PLCs and devices, independently of PLC status lights. For example: given an observed issue of a relay failing to energize, swapping the PLC DO wires with another relay can determine if the relay is faulty or if the PLC DO is not properly activating.

This is especially important on inputs, as swapping an AI and a DI may produce misleading indications. DI jumpering (directly jumpering between connections on the PLC DI) is also a useful check for components which don't have visual indication of their status, such as panel switches.

Various PWM duty cycles (on and off times) provide an equivalent average analog value of power to components, allowing a PLC DO to provide analog control.

Analog vs Digital Control of Analog Components

Some system components such as heaters are analog in nature but are controlled digitally via pulse width modulation. In this case, the PLC and IPC control the power output to the heater (and therefore the heat added to the system) through digital outputs, switching the heater on and off at a frequency determined by settings and based on thermocouple inputs to achieve the desired result. This is often simpler and easier to precisely control compared to the equivalent analog control circutiry.