# How safe are multiple systems writing to the same PLC location?

**URL:** <https://forum.inductiveautomation.com/t/how-safe-are-multiple-systems-writing-to-the-same-plc-location/117044>\
**Category:** General Discussion\
**Created:** [September 10, 2026, 4:17pm UTC](https://forum.inductiveautomation.com/t/how-safe-are-multiple-systems-writing-to-the-same-plc-location/117044 "2026-09-10T16:17:24Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![peter](https://avatars.discourse-cdn.com/v4/letter/p/278dde/32.png) [@peter](https://forum.inductiveautomation.com/u/peter)\
**Post date:** [September 10, 2026, 4:17pm UTC](https://forum.inductiveautomation.com/t/how-safe-are-multiple-systems-writing-to-the-same-plc-location/117044/1 "2026-09-10T16:17:24Z")

</div>

Suppose you have 1 PLC, and 2 totally independent HMIs (and in physically separate locations). And you want to set a value in the PLC from either of the HMIs.

How bad would it be to simply have a button on each HMI that writes a value via OPC UA to the exact same location in the PLC (so last write wins)?

(Note that there is no way that there would be a person in front of both HMIs at the same time simultaneously mashing down on the buttons.)

My gut feeling says that this architecture would be problematic, but I don't know if that's true or not.

---

<div class="post-metadata">

**Author:** ![pturmel](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pturmel/32/3295_2.png) [@pturmel](https://forum.inductiveautomation.com/u/pturmel)\
**Post date:** [September 10, 2026, 4:23pm UTC](https://forum.inductiveautomation.com/t/how-safe-are-multiple-systems-writing-to-the-same-plc-location/117044/2 "2026-09-10T16:23:41Z")

</div>

Lots of systems do this. In and of itself, it is not unsafe. It is most likely to be non-problematic if it is only user-initiated.

---

<div class="post-metadata">

**Author:** ![peter](https://avatars.discourse-cdn.com/v4/letter/p/278dde/32.png) [@peter](https://forum.inductiveautomation.com/u/peter)\
**Post date:** [September 10, 2026, 4:24pm UTC](https://forum.inductiveautomation.com/t/how-safe-are-multiple-systems-writing-to-the-same-plc-location/117044/3 "2026-09-10T16:24:27Z")

</div>

Thanks for the confirmation.

---

<div class="post-metadata">

**Author:** ![pturmel](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pturmel/32/3295_2.png) [@pturmel](https://forum.inductiveautomation.com/u/pturmel)\
**Post date:** [September 10, 2026, 4:27pm UTC](https://forum.inductiveautomation.com/t/how-safe-are-multiple-systems-writing-to-the-same-plc-location/117044/4 "2026-09-10T16:27:51Z")

</div>

Let me clarify: HMIs using **momentary** buttons (aka jog buttons, press & release) **will** clash. My statement above assumes you use action buttons that just send on press.

If the HMIs **must** use momentary buttons, the two HMIs must use separate bits, and PLC must combine them appropriately in its logic.

---

<div class="post-metadata">

**Author:** ![peter](https://avatars.discourse-cdn.com/v4/letter/p/278dde/32.png) [@peter](https://forum.inductiveautomation.com/u/peter)\
**Post date:** [September 10, 2026, 4:31pm UTC](https://forum.inductiveautomation.com/t/how-safe-are-multiple-systems-writing-to-the-same-plc-location/117044/5 "2026-09-10T16:31:58Z")

</div>

This will be for action buttons. I agree that jog buttons would be Bad News™.