Hello.
There is a Modbus network Master CPU 1217C + CM1241 and 10 connected slave devices.
Can I send simultaneously request all 10 devices? Or, need to send request the 1st device, wait for an answer and then the 2nd, etc...

Hello,

you can also look for Broadcast message but in this case you don't have response from Slaves.

Hi Shapovalov.

What type of power meter do you intend to communicate with? Since large modbus program with many devices tend to be quite complex, I have made a library that simplify the complexity. The library may also include a block for the specific devices you are using. 

Check out https://github.com/rexhip/rexhip , look at the «Station blocks» folder and see if your device is there. I am available if you have any questions.

Kind regards.