To implement Modbus RTU master functions on a S7-400 (assuming you want 2 distinct RS-485 networks to connect your 72 modbus slaves):
- a CP441-2 (6ES7 441-2AA0-0AA0)
- 2 x RS-485 sub-modules (6ES7 903-1AA00-0AA0)
- a Modbus master (6ES7 870-1AA00-0XA0)
Look at the following manual for more details and programming xamples:
There's a CP441-2 for a S7-400. It can work as Modbus RTU master. However you will need an additional config package and drivers for a Modbus master functionality as well as a dongle.
I have no idea about Modbus, but according to Siemens manual for Modbus master it might be slight problem to connect 72 slaves with CP441-2. They say that possible config is 2 x 32 slaves per CP441-2.
I remember there used to be some problem with redundancy in the serial modbus communication with S7-400. It was not native build-in to the system. You had to do some programming jugglery.
Has this problem been addressed by Siemens and fixed? Would appreciate feedback.