Solicited and Unsolicited Data Transmissions

A message or data packet that a slave device (such as an outstation) sends to the master device (such as a server) in response to a request for that information is known as a solicited data transmission.

Some devices also support the transmission or handling of unsolicited messages. An unsolicited message is a message or data packet that a slave device sends to the master device without the master device sending a request for that information.

Half-duplex communications limit transmissions to solicited messages only.