Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 79)

Asynchronous Link Protocols
Chapter 4
4-36
station alone may not work, since it could be that the only way to free up
space is to send a message to the master.
Line Monitoring
When monitoring half-duplex protocol in two-wire mode, you need to
monitor only one line. The example below shows a message sent by the
master and a reply sent by the slave in answer to a poll. Slave responses
are in boldface.
Message from Master to Slave:
DLE SOH STN DLE STX message DLE ETX BCC DLE ACK
Message Sent from Slave to Master in Response to Poll:
DLE ENQ STN BCC DLE STX message DLE ETX BCC DLE ACK
Poll with a DLE EOT in Response:
DLE ENQ STN BCC DLE EOT