This is one example of numerous similar errors that says my topology violates the rules. I have port out->port in, consistent data types. This seems to accord with the restrictions and examples in section 4.3.2 Port Connections in the SLIM language specification document. What could be wrong? /home/compasseval/Documents/.../xxx.slim:29: (G-9) Port Connection is invalid, because it violates the rules for possible connection topologies. data port pumpGrn.pressure -> isol.hyd_in; -- hydraulics pumpGrn is instance of device pump isol.hyd is instance of device valve device pump features pressure: out data port bool default false; end pump; device implementation pump.impl end pump.impl; device valve features cmd: in data port bool; hyd_in: in data port bool; hyd_out: out data port bool; end valve; device implementation valve.impl flows hyd_out:= hyd_in and cmd; end valve.impl; *Regards* Chris Wilkinson