Internal relays in a PLC: memory bits and relay outputs

This guide explains both meanings, because the first version of this post used "internal relay" for the relay outputs built into the PLC. You will see how to wire and program a relay output on an M-Duino or an Ardbox, and how to use an internal relay to latch a motor start in an Arduino program.

Internal relay or relay output: what is the difference?

In ladder logic, an internal relay is a coil that is not wired to an output. Other rungs read it with normally open or normally closed contacts. It keeps an intermediate state: a motor that must keep running after the start button is released, a step of a sequence, an alarm that is waiting for acknowledgement.

Relay outputs on Industrial Shields PLCs

The Arduino based PLCs of Industrial Shields with relays, such as the Ardbox Relay and the M-Duino 19R, use one type of relay. Each relay output switches:

• Up to 5 A at 250 Vac
• Up to 3 A at 30 Vdc

These values are for resistive loads. For inductive loads such as contactor coils, solenoid valves or motors, add a suppressor across the load (an RC snubber in AC, a flyback diode in DC) and leave a margin on the current.

How to wire a relay output

The contact of the relay has no polarity. It works like a switch between two terminals: connect the supply of the load to one terminal and the load to the other one. The relay can switch an AC or a DC circuit, separate from the supply of the PLC.

How to switch a relay output in the Arduino IDE

Install the Industrial Shields boards in the Arduino IDE and select your PLC model. Each relay has a name defined by the boards package:

HIGH energises the relay and closes its contact. LOW releases it and opens the contact. This sketch switches R0_1 of an M-Duino 19R every second and writes the state on the serial monitor:

// Relay output R0_1 of an M-Duino 19R: on for 1 s, off for 1 s
// On an Ardbox Relay, use R1 instead of R0_1

void setup() {
  Serial.begin(9600);
  pinMode(R0_1, OUTPUT);
}

void loop() {
  digitalWrite(R0_1, HIGH);   // relay energised, NO contact closed
  Serial.println("R0_1 closed");
  delay(1000);
  digitalWrite(R0_1, LOW);    // relay released, NO contact open
  Serial.println("R0_1 open");
  delay(1000);
}

Internal relays in an Arduino program: a start and stop latch

A classic use of an internal relay is the seal-in circuit: a start button that is only pressed for a moment, and a motor that keeps running until the stop button is pressed. In ladder logic the rung is (Start OR Run) AND Stop = Run. In an Arduino sketch, the internal relay motorRun is a bool variable:

// Internal relay (memory bit) that latches a motor start
// M-Duino 19R: start button on I0_0 (NO), stop button on I0_1 (NC)
// The physical relay output R0_1 switches the motor contactor coil

bool motorRun = false;        // internal relay: a bit in memory, no hardware

void setup() {
  pinMode(I0_0, INPUT);
  pinMode(I0_1, INPUT);
  pinMode(R0_1, OUTPUT);
}

void loop() {
  bool startPressed = digitalRead(I0_0) == HIGH;
  bool stopOk       = digitalRead(I0_1) == HIGH;   // NC: LOW when pressed or wire cut

  // Same logic as a seal-in rung: (Start OR motorRun) AND Stop
  motorRun = (startPressed || motorRun) && stopOk;

  digitalWrite(R0_1, motorRun ? HIGH : LOW);
}

The stop button is wired normally closed: if its wire breaks, the input goes low and the motor stops, as with a hard-wired stop circuit. Use a safety relay or a safety PLC for an emergency stop: an internal relay in a standard PLC program is not a safety function.

FAQ about internal relays in a PLC

What is an internal relay in a PLC?

A memory bit that the program uses like a relay coil and its contacts. It has no physical output. It stores an intermediate state, such as a latch or a step of a sequence.

Is an internal relay the same as an auxiliary relay or a marker?

Yes. Auxiliary relay, marker, flag, memory bit and M bit are names for the same idea. Each brand has its own memory area and addresses for them.

How many internal relays does a PLC have?

It depends on the PLC memory. In an Arduino based PLC or in CODESYS you declare them as BOOL variables, so the limit is the free memory, not a fixed list.

What voltage and current can the relay outputs switch?

On Industrial Shields PLCs, up to 5 A at 250 Vac or 3 A at 30 Vdc with a resistive load. Check the datasheet of your model for inductive loads.

Can I use internal relays with OpenPLC?

Yes. In OpenPLC, as in CODESYS, you program ladder logic with coils that are not linked to a physical output. See the OpenPLC guide for industrial PLCs.

More about PLC programming

• How to install Industrial Shields boards in the Arduino IDE
• Choosing an industrial controller
• OpenPLC guide for industrial PLCs
• Industrial applications: 50 real examples by sector

See the Ardbox Relay, the M-Duino 19R and all the Arduino based PLCs.