Link Configurations
Link configurations can be categorized as being either:
- Unbalanced, which consists of one primary terminal, and one or more secondary terminals.
- Balanced, which consists of two peer terminals.
The three link configurations are:
- Normal Response Mode (NRM) is an unbalanced configuration in which only the primary terminal may initiate data transfer. The secondary terminal transmits data only in response to commands from the primary terminal. The primary terminal polls the secondary terminal(s) to determine whether they have data to transmit, and then selects one to transmit.
- Asynchronous Response Mode (ARM) is an unbalanced configuration in which secondary terminals may transmit without permission from the primary terminal. However, the primary terminal still retains responsibility for line initialization, error recovery, and logical disconnect.
- Asynchronous Balanced Mode (ABM) is a balanced configuration in which either station may initiate the transmission.
An additional link configuration is Disconnected mode. This is the mode that a secondary station is in before it is initialized by the primary, or when it is explicitly disconnected. In this mode, the secondary responds to almost every frame other than a mode set command with a "Disconnected mode" response. The purpose of this mode is to allow the primary to reliably detect a secondary being powered off or otherwise reset..
Read more about this topic: High-Level Data Link Control
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