The RS232 "Station Manager" port on the CPU364 is for firmware upgrades. It is not user-configurable. None of the 90-30 CPU models with ethernet have a configurable serial port. If you need both ethernet and Modbus slave capabilities you'll need another module. As diat150 mentioned, you can add a CMM311 module which will give you two ports, either of which can be configured as a Modbus slave. If you're not using the ethernet port, you could replace the (obsolete) CPU364 with a CPU363 which has two configurable serial ports.