Modbus gateway, I can read but can't write

Anybus Support Anybus Gateways
#1

I am using an AB7000-C Modbus communicator to help my PLC with the workload of communicating with 13 VFD Drives. Whilst I can read data via the drives without a problem I am hitting a snag when I try to write. If I understand the manual correctly I need to write my (setpoint) data to a memory area in the Gateway, starting at 0x200 or 512 decimal. When I try to write a value from the PLC though (either using function code 6 or 16) I am getting an exception thrown (0x02) in my PLC and the data does not apear in the Anybus.

Am I correct in assuming that I should be able to write data anywhere in the “Out Area” of my anybus?

Also in my subnetwork monitor there is a red collision block at the first data area (left top) - I am not sure if this is related.

I have tried to write data directly to my drives via the PLC and this works, I also can write from the Anybus to my drive if I manually configure a telegram.

Any guidance would be greatly apreciated.

#2

Are you trying to write to the Anybus using Modbus? The Anybus Communicator is a Modbus Master, so you can’t write to it. It needs to read and write from the slave devices.

If you have a PLC that is a Modbus master, you need a Modbus slave like the AB7850. You could use the AB7000 if you can put the PLC in slave mode and read and write the registers with the Anybus.

#4

Hi Kyle, I am confused now, but let me explain:

I have a PLC that is a modbus master - it is very inefficient when it comes to polling many slaves so I purchased the Anybus device. The idea being that the Anybus would poll the slaves and then present one big data table to my PLC.
This scheme actually works, my PLC polls the anybus via the port on top of the device, that needs setting up via DIP switches - and my slaves are being polled for their data using the configuration I setup.
My only issue now is that my PLC needs to write data to the slaves as well; and I believe this must be possible with this module by writing to a memory area. Unfortunately this does not seem to work and I wonder where I am going wrong.

To recap, the Anybus is both a mobus master (polling 13 slaves) and a modbus slave for my PLC (presenting one large data table of all the information collected from the slaves). I hope this clarifies things; if not please let me know!
thanks

#5

OK, now I’m confused. The AB7000 is a Profibus to serial/modbus device and the port on the front by the dip switches is a Profibus port.

Are you sure you don’t have the AB7010?

#6

Hi Kyle,

I have the AB7010 (does the walk of shame).
I also have fixed my writing issue; I was too quick of the mark and assumed the write registers were 0x200(hex) - but after going through the manual I found the relevant table which indicated that my data write telegrams should write to slave address 1025 onwards.

So all is well and I am communicating quite hapily now. The only snag that i have is that my link between the master PLC and Anybus will not communicate at 57600, although 19200 works fine. Any ideas as to why that could be? The cable is less than 1meter long, should I have to consider end of line resistors?

#7

If you are using RS485, terminating resistors may fix that issue.