# MODBUS bug report

**URL:** <https://forum.universal-robots.com/t/modbus-bug-report/27960>\
**Category:** Report a Bug\
**Created:** [April 15, 2023, 2:16pm UTC](https://forum.universal-robots.com/t/modbus-bug-report/27960 "2023-04-15T14:16:01Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![mvrooij](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.universal-robots.com/mvrooij/32/2151_2.png) [@mvrooij](https://forum.universal-robots.com/u/mvrooij)\
**Post date:** [April 15, 2023, 2:16pm UTC](https://forum.universal-robots.com/t/modbus-bug-report/27960/1 "2023-04-15T14:16:01Z")

</div>

Steps to reproduce:  
Let the robot connect via MODBUS to a PC, were the PC act as a MODBUS server

Expected Behavior:  
Exchange of data as if the PC is just any other MODBUS device

Actual Behavior:  
Robot crashes violently claiming CAN bus communication between robot and safety board has been lost

Workaround Suggestion:  
Disconnect misbehaving MODBUS servers

Robot Serial Number:  
2018300970

Affected Version(s):  
Tested on UR10, CB3 script version 3.15.9

Details:

I wrote a MODBUS server program in order to have a PC communicate with the robot. Due to a bug in my software, instead of giving a single reply to a single request, I accidentally returned an infinite stream of bytes, causing the robot to crash. Please note, that the infinite stream of bytes was not random junk, it was actually a valid response packet, just repeated indefinably.