Editing Synchronous Data Link Control (section)

==== 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}})