AB7000-C modbus master configuration doubt

Hi to everyone,
I got an AB7000-C back-up from a client, asking to add some modbus command for the same slave.
Currently, the module it’s working properly, in fact i’m able to read the valus from my S7 400 (as a master profibus), but there is something i don’t understand.
For all the response arriving from the modbus slave, it’s been reserved an amount of memory bigger than the necessary. for example:
first query:
01-03-1c-e9-00-0f (15 registers, from reg 7401, it means 30 bytes).
first response
byte count 3c (60 bytes)
Register value
Data length 3c (60 bytes)
Data location 00 (from the first byte of the internal memory.

I can assume that, ok the master reads 30 bytes, but stores them into 60, so from 0 to 29 there are my values and from 30 to 59 are empty.
So, why if i try to replace the same configuration at the workshop it doesnt work?
if i try to store 30 bytes into 60 like the original doesn’t work, if i put 30 bytes into the same amount then works.
I dont understand what i’m missing.
Thanks for any advice.

You have to make sure that the size in the Anybus matches the size in the PLC. Are you following this document?

PROFIBUS_Slave_Step7_2.1.pdf (3.2 MB)

Hy kyle,
Size between Anybus and PLC is ok, my doubt is between Anybus and modbus slave, because the bytes configured in the response are 60 while in the command are requested 30. If you request 15 registers, the modbus slave will reply with 30 bytes, so why the response is configured for 60? how could be work this configuration?
ABB (4).cfg (16.0 KB)

It because that’s how they configured it. I have no idea why. Feel free to fix it.


Hi Kile,
There’s still something i don’t understand, because the back-up i’ve posted is that one sent me from the client for integration, and currently it’s working, then, in the worshop, i’ve mounted an s7 cpu, an AB7000C and a modbus slave simulator, and i tried to reply the first command/response in the same way, 30 bytes requested and 60 configured for the response, result, communication from anybus to modbus always in fault, i got no error, only if response is configured with the same size of the command (as well as from anybus to s7 cpu).
The only reason that could explain it, is that the modbus slave installed in the plant, reply with 60 bytes a 15 registres request, but to be honest, i’ve never seen somethig like that.
Thanks aniway for your support.

I don’t understand it either. It’s sending a request to read 15 registers, but the response is set up for 60 bytes. The next one is 8 registers read and it’s 32 bytes in the response, and the next is 2 registers and 8 bytes. However, the ones that are for 1 register are set up for 2 bytes. It makes no sense to me. I would recommend asking the client about it.