string[] data = new string[] { "13", "0" };modbusTCPCom1.Write("40001", data);
if your SwapBytes and SwapWords are correct, it should just be:modbusTCPCom1.Write("L40001", "13");An important note is that version 3.99v has a bug in the Modbus drivers. You need to either upgrade or apply the driver patch.