A PLC I/O card is the module that connects a programmable logic controller to the physical world. Inputs bring field signals into the PLC; outputs let the PLC drive field devices. Without I/O cards a PLC is just a processor with nothing to read or control. This guide explains discrete and analog I/O, wiring, and how that data reaches SCADA.
PLC I/O Card in one line: A PLC I/O card is a plug-in module that connects a PLC's processor to field devices, converting real-world signals from sensors and switches into data the PLC can read (inputs) and turning the PLC's logic into signals that drive actuators (outputs).
I/O cards come in two broad families. Discrete (digital) I/O handles on/off signals: a discrete input reads the state of a switch, limit, or contact, while a discrete output switches a device on or off, driving a relay, solenoid, or contactor coil. Analog I/O handles continuous values: an analog input measures signals like a 4-20 mA current or a 0-10 V voltage from a transmitter and converts it to a number, while an analog output sends a variable signal to a valve positioner or drive speed reference.
Cards are specified by point count and signal type, for example a 16-point 24 VDC discrete input card or an 8-channel 4-20 mA analog input card. They plug into a rack or clip onto a bus, and the PLC processor reads and writes them each scan. Field wiring lands on the card's terminals directly or on separate terminal blocks tied to the card, keeping the field cabling organized and serviceable.
At a wellsite or compressor skid, the PLC's I/O cards are where every measurement and command physically connects. Analog input channels read tubing and casing pressure, flow, and temperature transmitters; discrete inputs read ESD pushbuttons and level switches; discrete and analog outputs drive shutdown valves, choke actuators, and injection pump speeds. The count and mix of I/O effectively defines what the controller can see and do at that site.
A SCADA platform does not talk to the I/O cards directly. It polls the PLC over a protocol such as Modbus, EtherNet/IP, or OPC UA, and the values in the PLC's registers, which came from those I/O cards, are what SCADA reads. So a Merobix dashboard showing wellhead pressure is ultimately displaying an analog input channel, scaled and served up by the PLC. Understanding the I/O map is often the first step in commissioning tags in any monitoring system.
Discrete (digital) I/O handles two-state on/off signals, such as a running switch or a solenoid command. Analog I/O handles continuous values, such as a 4-20 mA pressure signal or a variable valve position. A single PLC usually carries a mix of both to cover switches and measurements.
The most common industrial analog input is the 4-20 mA current loop, favored for noise immunity and fault detection. Cards also accept voltage signals like 0-10 V or 1-5 V, and specialty inputs for RTDs and thermocouples. The card converts the signal to a digital count that the PLC scales into engineering units.
SCADA reads the PLC, not the I/O cards directly. Field signals enter through I/O cards, the PLC stores them in registers or tags, and SCADA polls those over a protocol such as Modbus, EtherNet/IP, or OPC UA. The value on a SCADA screen traces back to a specific I/O channel on a specific card.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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