PLC I/O Systems

Discrete inputs

A discrete input tells the program whether a circuit is closed. The field device, a float switch, a limit switch, a pressure switch, an auxiliary contact on a starter, closes a contact, and the module detects the voltage that the closed contact lets through. That voltage, called the wetting voltage, comes from a supply in the panel, and the module is designed for a specific one: 24 VDC in most new work, 120 VAC in older panels and in some motor control centers.

The words sinking and sourcing describe which way current flows through the module and are the most common cause of an input that never turns on. A sinking input expects the field device to connect it to the positive supply, and the module returns the current to common. A sourcing input supplies the positive itself and expects the field device to connect to common. Three-wire sensors with PNP or NPN outputs have to match. The module datasheet has a wiring diagram, and it is the drawing to follow, not the convention someone remembers.

Discrete outputs

Whatever the module, the output rarely drives the final load directly. A motor starter coil, a valve actuator, or a heater is switched through an interposing relay in the panel, so that the load current and the load voltage never reach the module, and so that a shorted coil takes out a two dollar relay rather than a sixteen point card. The interposing relay also converts the module voltage to the load voltage, which is how a 24 VDC transistor output starts a 120 VAC coil.

Analog inputs

An analog input converts a continuous signal into a number. The common signals are 4 to 20 mA current, 0 to 10 V, and the direct temperature inputs, RTD and thermocouple, which have their own module types. The module samples the signal, converts it with an analog to digital converter of a stated resolution, and presents the program with a count, which the scaling page turns into an engineering value.

Current inputs

A 250 ohm resistor, internal to the module or external, turns 4 to 20 mA into 1 to 5 V for the converter. Some channels supply loop power for two-wire transmitters; some only accept a signal. The 4-20 mA pages cover which is which and why connecting them wrong damages one of them.

Voltage inputs

Read the voltage directly. Sensitive to wire resistance and to noise, so they are used for short runs inside the panel and for devices that only offer a voltage output.

Single-ended and differential

A single-ended channel measures each signal against a shared common; a differential channel measures between its own two terminals. Differential inputs reject noise and ground potential differences that single-ended inputs read as signal. Where the field devices are far apart or grounded separately, differential is the safer choice.

Resolution

Twelve bits gives 4,096 counts across the range; sixteen bits gives 65,536. Twelve is enough for a level in a wet well; sixteen matters for a flow total or a chemical dose. The analog resolution calculator shows what each gives in engineering units.

Analog outputs

An analog output does the reverse, converting a number to 4 to 20 mA or 0 to 10 V for a drive speed reference, a valve positioner, or a chemical feed pump. The load it drives has a maximum resistance the output can push the current through, published on the datasheet, and a long run to a positioner can exceed it. The fault state of an analog output, hold or go to a value, is configured per channel and decides what the drive does when the processor stops.

Isolation and commons

Modules isolate the field side from the backplane, usually optically, so that a field fault cannot reach the processor. Between points on the same module the isolation varies: some modules have one common for all sixteen points, some have a common per group of four or eight, and some isolate every channel. The grouping decides which field devices can share a supply and which cannot, and it is the reason two devices on different supplies cannot be wired to the same eight-point group.

Diagnostics

Diagnostic modules report conditions that would otherwise take a meter and a trip to the field: an open wire on an input, an over-range or under-range on an analog channel, a blown output fuse, a short on an output, and a missing field supply. Each is available to the program as a status bit, and a program that alarms on them tells the operator that the level transmitter wire is broken rather than that the level is zero. The extra cost of a diagnostic module is paid back the first time it is needed.