Editing High-Level Data Link Control (section)

====Defined in other standards====
{{Further|Synchronous Data Link Control#SLDC features not in HDLC}}
There are several U frames which are not part of HDLC, but defined in other related standards.

; Nonreserved (NR0, NR1, NR2, NR3)
: The "nonreserved" commands and responses are guaranteed by the HDLC standard to be available for other uses.
; Ack connectionless (AC0, AC1)
: These are defined in the [[IEEE 802.2]] logical link control standard.
; Configure (CFGR)
: This command was defined in SDLC for debugging.  It had a 1-byte payload which specified a non-standard test mode for the secondary.  Even numbers disabled the mode, while odd numbers enabled it.  A payload of 0 disabled all test modes.  The secondary normally acknowledged a configure command by echoing it in response.
; Beacon (BCN) response
: This response was defined in SDLC to indicate a communications failure.  A secondary which received no frames at all for a long time would begin sending a stream of beacon responses, allowing a unidirectional fault to be located.  Note that ISO/IEC 13239 assigns UIH the same encoding as BCN.