Communications Programming Concepts

DLC8023 Link Sessions

The IEEE 802.3 Etheret data link control (DLC8023) device manager is initialized at an open link station (LS) as a combined station in asynchronous disconnect mode (ADM). As a secondary or combined station, DLC8023 is in a receive state, waiting for a command frame from the primary or combined station. Command frames accepted by the secondary or combined station at this time are:

SABME	Set asynchronous balanced mode extended.
XID	Exchange station identifications.
TEST	Test links.
UI	Unnumbered information - datagram.
DISC	Disconnect.

Any other command frame is ignored. Once a SABME command is received, the station is ready for normal data transfer, and the following frames are also accepted:

I	Provides information.
RR	Indicates a receive ready.
RNR	Indicates a receive not ready.
REJ	Indicates a reject.

As a primary or combined station, the DLC8023 device manager can perform ADM XID or ADM TEST exchanges, send datagrams, or connect the remote to asynchronous balanced mode extended (ABME).

XID exchanges allow the primary or combined station to send out its station-specific identification to the secondary or combined station and accept a response. Once an XID response is received, attached information fields are then sent to the user for further action.

TEST exchanges allow the primary or combined station to send out a buffer of information to be echoed by the secondary or combined station. This transfer of information tests the integrity of the link.

Initiation of the normal data exchange mode, ABME, prompts the primary or combined station to send a SABME command to the secondary or combined station. Upon successful delivery, the connection is said to be contacted; the user is notified. Information frames can now be sent and received between the linked stations.

Link Session Termination

The IEEE 802.3 Etheret data link control (DLC8023) device manager is stopped by the user or by the remote station in the following ways:

The user issues a close link station command to the DLC8023 device manager. The command initiates a disconnect (DISC) packet sequence to the primary or combined station.
The user directs the link to stop automatically after a specified period of inactivity. This is useful in detecting a loss of connection in the middle of a session.
The remote station terminates the link by sending a DISC command packet as a primary combined station.

Note: Abnormal termination is a result of certain protocol violations or resource outages.