When an advanced driver sends a message to an outstation, it performs a calculation to determine how long the message should take to arrive at the outstation. If a reply is required, the driver also calculates how long it should wait for a reply. The calculations are used to determine the timeouts for messages. If a message has not been sent or received within the calculated timeout period, the driver deems the message to have failed.
Advanced drivers also perform timeout calculations for unsolicited messages (messages that the outstation sends to the driver). An advanced driver calculates a timeout that is used when the outstation sends a partial message—the timeout defines how long the driver will wait for the remainder of the message. If the remainder of the message is not delivered within the timeout limit, the driver deems the unsolicited message to have failed.
The advanced drivers calculate the timeouts for messages as follows:
For a message that is sent from the driver to the outstation and for which the driver expects a reply, the calculation is:
Transmission time of message + Transmission time of largest possible reply + Standard reply delay + Turnaround time = Timeout.
For a message that is sent from the driver to the outstation and for which the driver does not expect a reply, the calculation is:
Transmission time of message + 1 second = Timeout.
For an unsolicited message (a message that is sent from the outstation to the driver without being requested by the driver), the calculation is:
Transmission time of remainder of message + Unsolicited message timeout = Timeout.
Where:
- The transmission times are calculated by dividing the size of the message by the line speed.
- The turnaround time is an estimate (calculated by the driver) of the amount of time the outstation will take to begin transmitting the reply after receiving the complete message.
- The timeouts are rounded up to the nearest whole millisecond.