CP341-Functions

SIMATIC S7-300/S7-400 Loadable Driver for Point-to-Point CPs: MODBUS protocol, RTU format, S7 is master

Part Number (for required dongle): 6ES7870-1AA01-0YA0

The last chapter of the manual contains programming examples for the CP 341 and CP 441-2.

Downloads:

Loadable Driver "Modbus Master (RTU)"

CP 341 Parametrization software: Setup_PtP_V5112.exe

Sample programs:

/tf/WW/en/Posts/34194

If your understanding of the Modbus standard are weak, or if you need to refresh your memory in order toplan how you will request data from the Modbus RTU slave you have selected, I suggest reading a very good (and concise) presentation of Modbus addressing and functions prepared by our friend smiffy; it has helped many here on the forum:
Modbus General Description

Once the data has been read from the slave it will be integrated as part of the S7 CPU's memory (in a DB) You can add tags in WinCC that will connect with these DB registers so you can display/log these data, assuming you already have a channel configured withg the PLC.

Hope this helps,
Daniel Chartier