Modbus and the SLC/MicroLogix family controllers
A brief summary of Modbus and the SLC-500/MicroLogix family controllers:
The SLC-5/03, SLC-5/04, and SLC-5/04 modular controllers all support Modbus RTU Master on their RS-232 serial ports.
The MicroLogix 1100, 1200, 1400, and 1500 controllers support both Modbus RTU Master and Modbus RTU Slave on their RS-232 serial ports.
The MicroLogix 1100 and 1400 controllers also support Modbus RTU on their RS-485 serial ports, which are wired to the same connector as the RS-232 port.
MicroLogix 1400 Series B controllers support Modbus/TCP protocol as a peer-to-peer messaging protocol.
I have not included firmware revisions that support these features; all MicroLogix controllers can be updated in the field at no charge, but SLC-500 controllers require a purchased firmware kit.
If the OP responds with details about what he wants (master or slave, SLC or MicroLogix) we can point him to the correct user manual.