# OPTA MODBUS RTU problem

**URL:** <https://forum.arduino.cc/t/opta-modbus-rtu-problem/1402303>\
**Category:** Opta\
**Created:** [August 14, 2025, 6:30pm UTC](https://forum.arduino.cc/t/opta-modbus-rtu-problem/1402303 "2025-08-14T18:30:15Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![merki1](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/merki1/32/1126705_2.png) [@merki1](https://forum.arduino.cc/u/merki1)\
**Post date:** [August 14, 2025, 6:30pm UTC](https://forum.arduino.cc/t/opta-modbus-rtu-problem/1402303/1 "2025-08-14T18:30:15Z")

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Hello,

I have a huge problem (for me) with run the Modbus communication on the OPTA ADVANCED.

For 99% I remember it works fine for a last month, but when I run it yesterday there is a problem (no comm).

I tried many things - resetting OPTA, testing via few modbus testers, connecting resistor on the end, also connecting GND wire.

The major plus (I think) that it seems RS 485 is working properly - I try to send string from OPTA to Serial Monitor (Putty) and this work good.

But with Modbus I tried: connecting between OPTA and ESP 8266 (in two configs i.e. OPTA as master/slave), trying to sniff it, nothing works. What’s wrong with me? 😉

Code with configuration: OPTA programmed in PLC IDE as Master, ESP as slave in Arduino IDE:

> **[TEST.rar](https://wetransfer.com/downloads/2f3e61bf5707e5b36f603ed21019a51320250814182746/b7b1e32542df3124f8f7f49b2a22c02820250814182839/8fb3dd?t_exp=1755455266&t_lsid=2b6d0b7f-166b-42c7-bc68-38ef17f6e13d&t_network=email&t_rid=ZW1haWx8NjdhNWRkZGE2ZTQ3YzczOTVlYzMxYjE1&t_s=download_link&t_ts=1755196119&utm_campaign=TRN_TDL_01&utm_source=sendgrid&utm_medium=email&trk=TRN_TDL_01)**
>
> 2 files sent via WeTransfer, the simplest way to send your files around the world

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**Author:** ![system](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/system/32/1140315_2.png) [@system](https://forum.arduino.cc/u/system)\
**Post date:** [February 10, 2026, 6:31pm UTC](https://forum.arduino.cc/t/opta-modbus-rtu-problem/1402303/2 "2026-02-10T18:31:05Z")

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