TcpClient managing a real device

Has anyone used TcpClient to access (manage, control, etc.) a real device (not a SW server) on the same LAN? I was able to connect to the device but every command defined for the device was rejected. I made sure the command was sent exactly as what's required. I use socket stream IO for my application and a TcpListener as the simulator when the real device (HW) wasn't ready. Things worked well with the simulator. Can anyone experienced in such applications shed some light? Thanks.