# Modbus Serial - 750-652 - Slave detection/packet count

**URL:** <https://www.wago.community/t/modbus-serial-750-652-slave-detection-packet-count/1870>\
**Category:** CODESYS\
**Tags:** pfc200, serial, codesys\
**Created:** [May 22, 2025, 9:44am UTC](https://www.wago.community/t/modbus-serial-750-652-slave-detection-packet-count/1870 "2025-05-22T09:44:48Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Catch22.FMX](https://avatars.discourse-cdn.com/v4/letter/c/8edcca/32.png) [@Catch22.FMX](https://www.wago.community/u/Catch22.FMX)\
**Post date:** [May 22, 2025, 9:44am UTC](https://www.wago.community/t/modbus-serial-750-652-slave-detection-packet-count/1870/1 "2025-05-22T09:44:48Z")

</div>

Hi all,

I have had some issues trying to implement some form of a packet/frame count on a Modbus Serial Master setup with your 750-652 card, there is one slave on the bus.

I wanted to use the packet count to detect when the slave is active/alive, and when it is not, I can wipe the data registers to zero (currently they stay with last value, incorrectly).

I cannot see, with my knowledge a simple variable or declaration within the FbMbMasterSerial FB that I am using from the WagoAppPlcModbus library that would help me achieve this, or maybe I am missing something.

Any ideas folks? Much appreciated.

---

<div class="post-metadata">

**Author:** ![WagoIcard](https://sea2.discourse-cdn.com/flex016/user_avatar/www.wago.community/wagoicard/32/1508_2.png) [@WagoIcard](https://www.wago.community/u/WagoIcard)\
**Post date:** [May 26, 2025, 1:11am UTC](https://www.wago.community/t/modbus-serial-750-652-slave-detection-packet-count/1870/2 "2025-05-26T01:11:07Z")

</div>

Hello,  
If your 652 is used as a Modbus Master you should be able to get the Timeout error in case of a disconnected slave / not active one. With this error you could left the data as it or reset everything to whatever state you like.