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====Mode setting==== The various modes are described in {{slink||Link configurations}}. Briefly, there are two non-operational modes (initialization mode and disconnected mode) and three operational modes (normal response, asynchronous response, and asynchronous balanced modes) with 3-bit or 7-bit (extended) sequence numbers. ; Disconnected mode (DM) response : When the secondary is disconnected (the default state on power-up), it sends this generic response to any poll (command frame with the poll flag set) except an acceptable mode setting command. It may alternatively give a FRMR response to an unacceptable mode set command. ; Unnumbered acknowledge (UA) response : This is the secondary's response to an acceptable mode set command, indicating that it is now in the requested mode. ; Set ... mode (SNRM, SARM, SABM) command : Place the secondary in the specified mode, with 3-bit sequence numbers (1-byte control field). The secondary acknowledges with UA. If the secondary does not implement the mode, it responds with DM or FRMR. ; Set ... mode extended (SNRME, SARME, SABME) command : Place the secondary in the specified mode, with 7-bit sequence numbers (2-byte control field). ; Set mode (SM) command : Generic mode set, new in ISO/IEC 13239, using an information field to select parameters. ISO/IEC 13239 added many additional options to HDLC, including 15- and 31-bit sequence numbers, which can only be selected with this command. ; Disconnect (DISC) command : This command causes the secondary to acknowledge with UA and disconnect (enter disconnected mode). Any unacknowledged frames are lost. ; Request disconnect (RD) response : This response requests the primary to send a DISC command. The primary should do so promptly, but may delay long enough to ensure all pending frames are acknowledged. ; Set initialization mode (SIM) command : This rarely-implemented command is used to perform some secondary-specific initialization, such as downloading [[firmware]]. What happens in initialization mode is not otherwise specified in the HDLC standard. ; Request initialization mode (RIM) response : This requests the primary to send SIM and initialize the secondary. It sent in lieu of DM if the secondary requires initialization.
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