Various Causes for PLC going into Stop Mode

In this article, we will discuss the causes of PLC stop mode from its run mode or program mode. Allen Bradley PLC has 3 modes. Modes are Run mode, Program Mode, and Stop mode. Similarly, different makes of Programmable Logic Controller (PLC) have different modes.

Causes of PLC Stop Mode

Many reasons are there for PLC to go into stop mode. The main causes of PLC stop mode are as follows.

  • Hardware Fault
  • Software Error
  • Hardware Switch on CPU
  • Stop Digital Bit
  • Memory Mismatch
  • External Command from Network
  • Downloading of Program

Let us see the reasons in detail for which a PLC can go into stop mode.

causes of plc stop mode

1. Hardware Fault

Hardware fault occurs because of any hardware failure in the PLC. The hardware here does not mean the Analog Input, Analog Output, Digital Input, or Digital Output module. The hardware here is referred to as the PLC controller or the Power Supply Unit.

If both the controllers go faulty, then the PLC goes into the stop mode as there is no PLC scan cycle running. Also if power to the PLC is disrupted, then also the PLC’s mode is stopped mode. When power resumes, the PLC also starts working normally. 

In short, when there is a major hardware fault or any power disruption to the PLC, the PLC will go into stop mode.

2. Software Error

In software error, actions like bugs in the program, watchdog timer error, overflow error, underflow error, any unhandled exceptions, or an infinite calculation can lead to PLC going into stop mode.

The watchdog timer supervises the software operation of PLC. When the watchdog timer completes its timing on not receiving any input from the program for a defined period, then the watchdog timer generates a bit. This bit generated by the watchdog timer can lead to PLC stop mode if configured in the PLC. Also, infinite calculations like any number getting divided by zero or addition/subtraction / multiple / division going into infinite and generating overflow or underflow continuously also lead to PLC stop mode.

3. Hardware Switch on CPU

A hardware switch in the PLC controller is present. There may be 3 or 4 selection modes using this hardware switch. One of these modes is the stop mode. Whenever the hardware switch is kept in stop mode, the PLC will go into stop mode and will remain in stop mode until the mode is changed to some other mode.

Many times we try to put PLC in Remote Run mode from Run mode or vice versa as per the requirement. This is done to force some Inputs or Outputs. So at this time, we need to take care while changing the hardware switch position. Because a slightly more applied force on the hardware switch will change its position to stop mode. This will stop the PLC from executing the current program and go into stop mode. This will trip the running plant or process. 

4. Stop Digital Bit

We can set some bits in PLC which make PLC go into stop mode. There are various kinds of situations in which we need to make PLC go into stop mode. Situations like any hardware fault, communication break with remote IO chassis, or communication lost with other systems like Machine Condition Monitoring System or even communication break with the network also can be configured as a bit. This bit can turn on and off as per our requirement. The bit’s status can be used to put PLC into stop mode as per requirement.

5. Memory Mismatch

Some of the PLCs have a manual function to save the last changes in the programming. If we forget to save it, then PLC will go into the stop mode after power reboots.

6. External Command from Network

Most of the PLCs have features to run through web browsers. It is possible to operate the PLC from a remote just by entering the IP address on the internet browser. The PLC can go into the run mode to stop mode if a user gives commands to stop it.

7. Downloading of Program

During full downloading of the program PLC can go into stop mode.

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