Editing Synchronous Data Link Control (section)

=== SDLC features not in HDLC ===
Two U frames in SDLC which do not exist in HDLC are:
* BCN (Beacon): When a secondary loses carrier (stops receiving any signal) from the primary, it begins transmitting a stream of "beacon" responses, identifying the location of the communication failure.  This is particularly useful in SDLC loop mode.
* CFGR (Configure for test) command and response: The CFGR command contains a 1-byte payload which identifies some special diagnostic operation to be performed by the secondary.{{R|GA27-3093-3|p=47–49}}  The least significant bit indicates that the diagnostic mode should start (1) or stop (0).  A payload byte of 0 stops all diagnostic modes. The secondary echoes the byte in its response.
** 0: Stop all diagnostic modes.
** 2 (off)/3 (on): Beacon test.  Disable all output, causing the next recipient to lose carrier (and begin beaconing).
** 4 (off)/5 (on): Monitor mode.  Disable all frame generation, becoming silent, but do not stop carrier or loop mode operation.<!--
6/7 not assigned-->
** 8 (off)/9 (on): Wrap mode.  Enter local loopback, connecting the secondary's input to its own output for the duration of the test.
** 10 (off)/11 (on): Self-test.  Perform local diagnostics.  CFGR response is delayed until the diagnostics complete, at which time the response is 10 (self-test failed) or 11 (self-test successful).
** 12 (off)/13 (on): Modified link test.  Rather than echoing TEST commands verbatim, generate a TEST response consisting of a number of copies of the first byte of the TEST command.
Several U frames are almost entirely unused in HDLC, existing primarily for SDLC compatibility:
* Initialization mode, and the associated RIM and SIM U frames, are so vaguely defined in HDLC as to be useless, but are used by some peripherals in SDLC.
* Unnumbered poll (UP) is almost never used in HDLC, its function having been superseded by asynchronous response mode.  UP is an exception to the usual rule in normal response mode that a secondary must receive the poll flag before transmitting; while a secondary ''must'' respond to any frame with the poll bit set, it ''may'' respond to a UP frame with the poll bit clear if it has data to transmit.  If the lower-level communication channel is capable of avoiding collisions (as it is in loop mode), UP ''to the broadcast address'' allows multiple secondaries to respond without having to poll them individually.
The TEST U frame was not included in early HDLC standards, but was added later.

==== Loop mode ====
A special mode of SDLC operation which is supported by e.g. the [[Zilog SCC]] but was not incorporated into HDLC is SDLC loop mode.{{R|GA27-3093-3|p=42–49,58–59}}  In this mode, a primary and a number of secondaries are connected in a unidirectional [[ring network]], with each one's output connected to the next's input. Each secondary is responsible for copying all frames which arrive at its input so that they reach the rest of the ring and eventually return to the primary. Except for this copying, a secondary operates in half-duplex mode; it only transmits when the protocol guarantees it will receive no input.

When a secondary is powered off, a [[relay]] connects its input directly to its output.  When powering on, a secondary waits for an opportune moment and then goes "on-loop" inserting itself into the data stream with a one-bit delay.  A similar opportunity is used to go "off-loop" as part of a clean shutdown.

In SDLC loop mode, frames arrive in a group, ending (after the final flag) with an all-ones idle signal.  The first seven 1-bits of this (the pattern 01111111) constitute a "go-ahead" sequence (also called EOP, end of poll) giving a secondary permission to transmit.  A secondary which wishes to transmit uses its 1-bit delay to convert the final 1 bit in this sequence to a 0 bit, making it a flag character, and then transmits its own frames.  After its own final flag, it transmits an all-ones idle signal, which will serve as a go-ahead for the next station on the loop.

The group starts with commands from the primary, and each secondary appends its responses.  When the primary receives the go-ahead idle sequence, it knows that the secondaries are finished and it may transmit more commands.

The beacon (BCN) response is designed to help locate breaks in the loop.  A secondary which does not see any incoming traffic for a long time begins sending "beacon" response frames, telling the primary that the link between that secondary and its predecessor is broken.

Because the primary also receives a copy of the commands it sent, which are indistinguishable from responses, it appends a special "turnaround" frame at the end of its commands to separate them from the responses.  Any unique sequence which will not be interpreted by the secondaries will do, but the conventional one is a single all-zero byte.{{R|GA27-3093-3|p=44}}  This is a "runt frame" with an address of 0 (reserved, unused) and no control field or frame check sequence.  (Secondaries capable of full-duplex operation also interpret this as a "shut-off sequence", forcing them to abort transmission.{{R|GA27-3093-3|p=45}})