# What is the function of a PLC's MRES Switch?

**URL:** <https://forumautomation.com/t/what-is-the-function-of-a-plcs-mres-switch/11833>\
**Category:** Siemens\
**Tags:** plc\
**Created:** [February 29, 2024, 2:32pm UTC](https://forumautomation.com/t/what-is-the-function-of-a-plcs-mres-switch/11833 "2024-02-29T14:32:27Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![Rabert](https://forumautomation.com/user_avatar/forumautomation.com/rabert/32/6784_2.png) [@Rabert](https://forumautomation.com/u/Rabert)\
**Post date:** [February 29, 2024, 2:32pm UTC](https://forumautomation.com/t/what-is-the-function-of-a-plcs-mres-switch/11833/1 "2024-02-29T14:32:27Z")

</div>

## What is MRES Switch?

- A PLC’s “MRES” switch denotes “Memory Reset.”
- By using this switch, the PLC’s memory can be reset to its default (or) uninitialized state, erasing any stored programs or data.
- This feature is helpful when programming or troubleshooting PLCs because it lets you restart or undo any accidental modifications to the data or program that was stored in memory.
- This switch will remove all stored data, so it’s essential to use caution when using it. If done incorrectly, this could result in the loss of crucial data or programs.
- MRES switch is frequently seen on Siemens PLCs such as the S7-300 series.
- This particular mode is used to reset memory on the PLC’s physical switch located on the CPU (Central Processing Unit).

 ![1](https://cdn.forumautomation.com/original/2X/0/0dd743dc93aeaf5dbe05329f379a6cc8977a0e65.jpeg)

## MRES Function

- A PLC memory reset is triggered by the MRES switch, which is engaged by keeping it in the MRES position for a predetermined amount of time.
- Through this reset procedure, the PLC’s work memory is cleared, encompassing:
  - Program for the user (function blocks, ladder logic, etc.)
  - Temporary information (values kept while a program executes)

- However, it has no significance on if this memory is configured in the software, certain data values are kept there even after resets or power cycles.
- The MRES reset has no effect on the contents of an external memory card used by the PLC.

## Applications of MRES Switch

- MRES is frequently used as a stage in the troubleshooting process to eliminate possible program mistakes or unusual PLC performance.
- The MRES reset may be used to force a restart of the PLC in cases when it is stuck or unresponsive, possibly fixing the problem.

In conclusion, the MRES switch on a PLC is an effective tool for memory reset, but in order to prevent unexpected effects, it should be used carefully and with understanding of its implications.