Hello, 

Mehdii

Hello doctor. I have a question..

I have a power meter that has an RS485 output and needs to be connected to the sc 300 plc and shows the amount of current and power on the monitor, but I have no information in this regard. Thank you for your help

for first you need tell us details about your S7-300 PLC type (order number and version) and Siemens SW you have. What other modules your S7-300 has? If you want collect data from device via RS485 line you need have some communication module attached to your CPU with correct interface type. What type of module also depends to communication protocol that Energy Meter use, in this case you need have details about this comm protocol. Check documentation to your Meter. After details you know, you can then better select correct comm module for your CPU.