# PMS/System Communication Protocol

The PMS/System bidirectional (through half-duplex) link uses the ENQ /ACK/STX-text-ETX/ACK protocol. Message flow diagrams in this section follow a consistent notation. The arrow indicates the direction of the message, from sender to receiver. The label aligned with the tail of the arrow indicates the type of message being sent. The order of the arrows will indicate the sequence of messages exchanged by the entities. Therefore, the PMS to system transmission sequence is

The transmission of the message is complete.

A similar sequence occurs for the system to PMS transmission.

There are timing restrictions imposed on the transmission sequences:

The maximum time to await for the ACK after a STX + msg + ETX transmission is 3 seconds.