The function block ModbusMaster operates on an I / O stream (Parameter Fillet of type FILEP) that must be returned by a function Sysfopen. To operate in master mode it is necessary to perform a TCP connect towards the slave module, currently it is not possible to perform a TCP connect. The systems SlimLine currently they can only operate as a TCP server and not as a client.
So you can make the systems talk SlimLine via the ModbusMaster FB using a serial connection but not TCP. But if your problem is to make two or more systems talk to each other SlimLine via Ethernet you can use the FB UDPDataTxfer which uses the UDP protocol, Topic (1), (2).