PLC Digital Input and Digital Output Modules

A data point that has only two states on and off is called a “discrete” data point. Discrete sensing devices include process switches, push-button switches, limit switches, and proximity switches. A PLC must receive a signal from a discrete sensor through a discrete input channel in order to be aware of its state.

Each discrete input module has (usually) a set of light-emitting diodes (LEDs) that are activated when the accompanying sensing device is powered up.

Each LED shines on a photo-sensitive device inside the module, such as a photo-transistor, which activates a bit (a single element of digital data) in the PLC’s memory.

This optocoupler configuration makes each input channel of a PLC rather tough, capable of shielding the PLC’s delicate computer hardware from transient voltage “spikes” and other damaging electrical phenomena:

The componentry common for a single input channel on a discrete input module (“card”) is represented in the internal schematic diagram shown above.

Each input channel has its own optocoupler, which writes to a separate memory register bit in the PLC. PLC discrete input cards typically contain four, eight, sixteen, or thirty-two channels.

Discrete control devices include indicator lamps, solenoid valves, and motor starters (assemblies made up of contractors and overload protection devices). A discrete output channel connects a PLC to any number of discrete final control devices in the same way as discrete inputs do (Note1).

The internal PLC circuitry drives an LED, which then activates some type of photosensitive switching device. Discrete output modules typically use the same form of opto-isolation to allow the PLC’s computer circuitry to send electrical power to loads: the internal PLC circuitry drives an LED, which then activates some type of photosensitive switching device.

Small electromechanical relays can also be utilized instead of opto-isolating semiconductor switching devices like transistors (DC) or transistors (T).

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Note 1: In industry jargon, I/O “channels” are often referred to as “points.” As a result, a “32-point input card” is an input circuit having 32 channels for receiving signals from on/off switches and sensors.

The schematic design for a discrete output module, like the one for a discrete input module previously displayed, illustrates the componentry usual for a single channel on that card.

Each output channel has its own optocoupler, which is controlled by a separate memory register bit in the PLC. PLC discrete output cards typically contain four, eight, sixteen, or twenty-two channels.

The distinction between current-sourcing and current-sinking devices is a key topic to grasp when working with DC discrete I/O. The words “sourcing” and “sinking” relate to the flow of current into or out of a device’s control wire (as shown by traditional flow notation) (Note2).

A device that is sourcing current is one that is sending (conventional flow) current out of its control terminal to another device(s), whereas a device that is sinking current is one that is absorbing (conventional flow) current into its control terminal.

Note 2: When I say “control wire,” I’m referring to the single conductor that connects an I/O card channel to a field device, as opposed to conductors that are directly connected to each other.

For example, in the diagram below, a PLC output channel is supplying current to an indicator bulb, which is sinking current to the ground:

These concepts only make sense when considering electric current in terms of conventional flow, in which the positive terminal of the DC power supply is considered as the current’s “source,” with current flowing “down” to ground (the negative terminal of the DC power supply).

One element in every circuit created by a PLC’s output channel driving a discrete control device or a discrete sensing device driving a PLC’s input channel must be sourcing current while the other sinks current.

Blowing and sucking is how one of my engineering colleagues describes sourcing and sinking. A gadget that “blows” current from one device to another. Any sort of PLC I/O module will suffice if the discrete device connecting to the PLC is not polarity-sensitive.

A mechanical limit switch connected to a sinking PLC input and a sourcing PLC input, for example, is shown in the pictures below:

The sinking card’s common terminal and the source card’s common terminal have different polarity and labelling.

The input channel terminal on the “sinking” card is positive, whereas the common (“Com”) terminal is negative. The input channel terminal on the “source” card is negative, but the common (“VDC”) terminal is positive.

Electronic proximity sensors with transistor outputs, for example, are polarity-sensitive discrete sensing devices.

Only a “sinking” PLC input channel can interface with a “sourcing” proximity switch, and vice versa:

The “sourcing” device sends current out of its signal terminal in all instances, but the “sinking” device receives current.

Here are two photos of a DC (sinking) discrete input module for an Allen-Bradley model SLC 500 PLC, one with the plastic cover closed over the connection terminals and the other with the plastic cover open to view the terminals.

The purpose of each screw terminal is indicated on the inside of the cover by a legend: eight input channels (numbered 0 through 7) and two redundant “DC Com” terminals.

A series of LED indicators visually representing the status of each bit is a standard feature found on almost every PLC discrete I/O module (discrete channel). The LEDs on the SLC 500 module show as a cluster of eight numbered squares towards the module’s top.

The labelling of discrete output terminals on a different brand of PLC (a Koyo model DL06) is slightly different:

Each “common” terminal in this situation is shared by only four output channels. There are four separate “common” terminals on this PLC, which has sixteen total output channels. While this may appear odd (why not have a single “common” terminal for all sixteen output channels?) It allows different DC power supply to service different sets of output channels more easily.

Because the polarity of AC reverses periodically, electrical polarity is not an issue with AC discrete I/O. However, whether the “common” terminal on a discrete PLC module connects to the neutral (grounded) or hot (ungrounded) AC power conductor remains a question.

The next image depicts a discrete AC output module for an Allen-Bradley SLC 500 PLC that uses TRIACs instead of transistors as power switching devices, as is common with DC discrete output modules.

This eight-channel module has two sets of TRIACs for switching power to AC loads, with each set of four TRIACs getting AC power from a “hot” terminal (VAC 1 or VAC 2) and the other side of the load device connected to the AC power source’s “neutral” (grounded) conductor.

Fortunately, every PLC’s maker provides a hardware reference handbook that includes diagrams. Before attaching devices to a PLC’s I/O points, always refer to these schematics!