There are several ways to perform Modbus RTU Master on RS-485 from a CompactLogix controller, and they follow the usual linear complexity vs. expense equation.
1. CompactLogix serial port, Modbus RTU ladder code (from Samplecode website) with 1761-NET-AIC or similar RS232/RS485 isolator.
2. 22-SCM-232 DF1/Modbus RTU converter attached to CompactLogix serial port, with DF1 MSG instructions.
3. DeviceNet -> Modbus RTU or EtherNet/IP-> Modbus RTU converter box from Prosoft, HMS Fieldbus, Realtime Automation, Western Reserve Controls, etc.
4. MVI69-MCM Modbus RTU module from Prosoft.
If you are building just one unit and are inexperienced with Rockwell gear, the Prosoft MVI69-MCM module is the easiest to set up and the easiest to troubleshoot.