Rockwell MODBUS TCP AOI

[Tim Ganz]

I have a PH meter that I am trying to bring its data into 1756-L81. I have downloaded the Rockwell MODBUS AOI kit, but I am not sure if I need to set my PLC as master or slave when doing MODBUS TCP?

Does anyone have any tips or things to look out for when using these AOIs? I am only trying to read the meters readings into the PLC not writing anything to it.

 

[nicherbert]

I have not used those AOIs, but MODBUS functions as a Master/Slave setup (similar to Client/Server). The master (client) is in control and will initiate requests to a slave (server) device. For your case, you want to select Master as you want to request data from the PH Meter (slave). Everything else from there should be straightforward (hopefully). This looks like a good video detailing what needs to be done to configure it. Studio 5000 Modbus TCP Client Allen Bradley Controllogix Compactlogix

 

[Ken Moore]

The PLC would be the Master/Client and the field device would be Slave/Server. Master/Slave has gone away due to political considerations. Now most vendors use Client/Server. The Client request data, and the Server "serves" it up.
I have used the AOI a few times. When you only need a small amount of data, it works okay. If you have many devices or a lot off data, I would recommend a dedicated Modbus module. I usually place it in a dedicated timed task of 50ms.

 

[LegacyLee]

If you run into the dreaded "Overlap/Overload problem, you can separate reads/writes into separate AOIs. They're a crude beast at best and get frustrating when using for more than a few devices. They take a bunch of memory and are essentially scripted which makes them slow, non-deterministic memory hogs. I'm not impressed by the Modbus AOI but I do use it.

 

[Tim Ganz]

I have 2 devices but each has a lot of data points and 1 second poll for the temp part the ph part can be slow. Should I do 2 read AOI's and 2 write AOI's then?

 

[Contr_Conn]

If your data points are contiguous, then you can read ad write them in one transaction, up to 120 per transaction. The AOI can have 5 transactions, but they will execute consecutively, one after another.
It takes about 8 scans to execute a transaction. For 50ms task, it will take 400ms to finish a transaction, for 20 ms task -160ms.
1 second poll rate is default, you can change it, but keep transaction execution time in mind.
Using multiple AOIs is OK on the Logix side, you are limited to 32 sockets, each client AOI uses one. However, each AOI will use its own TCP connection to the device, and I know that not all devices will allow multiple TCP connections from the same IP address. Most will allow one per IP and some will do one total, because devices usually have limited memory and processing power to handle TCP stack.