Branch on the status of the transmit function. Depends on the protocol.
ModBus: Not used - See COMBranchScriptStatus
UserASCII:
0: Idle - nothing happening, nothing waiting, free to transmit
1: Transmitting (including RTS leadin/out delays)
2: Transmit lockout timer active
3: Completed a Master poll (e.g. ModBus) and timing window for a response.
4. Timing TxResponseTime