So the last week I have spent some time converting from the good old modbus library to the recent built-in modbus client in PLCnext Engineer 2025 (currently using 2025.9).
I’m not sure how to explain this, but there seem to be a strange issue with the register variables. If I understand correctly, once you enter a function code and name a register range, there should pop up a new port in the Data List and GDS Port list.
This new port you have to map to a correct data type (which is not clearly stated, but after a while I found that a FC3 holding register group from for example address 0 to 20 needed an array of words from 1 to 21 to be able to map it.
The issue (I’m getting to the point now), is that sometimes these ports fail to pop up in the lists.
I then have to delete the entire modbus server instance in the Plant node of Modbus TCP, and re-write all the register ranges and map them. It’s a real bummer.
So, after many days of successful testing with no issues, i needed to restart my computer - and when I start working in PLC next engineer, it complains that all the modbus register mappings are invalid:
Alright, I think, let’s just disconnect the ports and reconnect them!
Before: (and this was working fine for many days):
I then proceed to disconnect the GDS ports:
Poof, and the register ports are just gone from the Data List and the GDS Port List even though the registers are still there. And because there is no CSV export/import function for the registers (PLEASE ADD!) I have to manually delete and re-insert all the 11 registers. Which is OK the first time, and increasingly not ok when you have to do it several times.
My project is going to have 7-8 modbus clients, and i really dont want to make this a 9-4 job.
Please look into this. Or explain to me what i have misunderstood