Communication via Modbus to bus coupler (Beckhoff)

Hi guys.

I´m trying to communicate with a Beckhoff bus coupler BK9000.
It´s a ethernet based coupler and reading inputs works.
But I´m not able to write an output.
Does anybody got this working ?

Thanks in advanced,

According to this page, the BK9000 supports modbus-tcp.

In addition to ADS the Bus Coupler supports Open Modbus (Modbus TCP), a simple, widespread master/slave protocol based on TCP/IP.

So install modbus nodes and communicate to it.

As already said, reading is possible, but not writing.

Are you formatting the write data correctly?
How many registers are you trying to write in one go?
Are you writing to writable registers?
What node (node-red-contrib-xxxxx) are you using?
Do you have a demo (non-working) flow you can post?

I´m getting this fault: "Modbus exception 4: Slave device failure (device reports internal error)"

I´m having a BK9000, 1 digital input module and 1 digital output module.
For the output module I´m using:
Unit-ID: 1
address: 2048
There´s no difference if I´m using boolean oder numbers to set the output.

When you read 2048 and 2049, do the bits reflect the inputs and output cards status correctly?

As 2048 == 0x800 - is this not trying to write to the input card?

Have you tried writing a number between 0 & 65535 to address 2049 (hex 0x801)?

Actually, looking at manual, you might need to use FC 5 and bits


Try FC5 and a value of 1 (or true) to address 2048

1 Like

FC6 for a digital out device ? shouldn't that be FC5 or FC15 ?


Input addresses are between 0 ans 255.
Output addresses are between 2048 and 2303.
First input module is address 0, input feedback signals are 1, 2, 4 and 8.
If input 1 and 3 are true, the feedback is 5.

Doesn´t matter if I´m using FC5, 6, 15 or 16.
I´m always getting the fault message.

As the manual is not 100% clear, I would contact the support guys & ask for exact info. Explain what IO you have (and their arrangement) - ask what FC and what values to write to set / reset a DO.

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