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Hello everyone

I started using Unitronics not long ago and I do not have much experience with their hardware/software nor Modbus protocol.

I am using a V570 + optidrive E3 which I have to slower its speed with the program based on an Encoder.

the problem I am having is that sometimes the PLC wont send the instruction for the driver to change its speed. i have noticed from a lot of reasons that it happens because the data wont be transfered.

I am using modbus with RS485 which is already right.

i would like to know if you guys would have any advice or probable solution for it. Could i solve it by changing the baudrate (which is 57,6k).


thank you all!

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  • MVP 2023


As for me - best solution for this type of application (CUT to LENGTH or object positioning )  : 

- send different fixed setpoint configured in VFD menu (i use two setpoint FAST and SLOW) via Modbus (for safety i allow user change this settings only when VFD is disabled  and appication is in STOP MODE) ;

- select one of this  setpoints and RUN/STOP condition via PLC output and VFD logic input .


Modbus connection may be lost if EMI from  VFD present. Correct grounding, shielding and wiring needed for smooth connection.

Slow Modbus speed is solution but sometimes due to connection error information is not come to VFD also.


Sometimes i configure FAST STOP INPUT in VFD and use it for SAFETY if encoder overtravel error possible.

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  • MVP 2023

It looks to me like you're being killed by SB 7 in the first ladder rung (Modbus subroutine). If the entire sequence doesn't complete in 100ms then you restart it with the SB 7 pulse. Why not do away with that rung and restart the sequence by Setting MB 3 in rung 7 instead of Resetting MB 24? If that specific logic is important for some reason I'm unaware of, try using SB 3 (or better, direct contact of SB 13 [you should also be using SB 15 instead of SB 7 in your current configuration]), or best, time the entire sequence and replace SB 7 with a self-resetting timer that has plenty of time for the entire sequence.

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