Zilog Z16C35 - Manual (Page 86)

ISCC
User Manual
UM011002-0808
80
The receiver searches for synchronization when it is in Hunt mode. In this mode the
receiver is idle except that it is searching the data stream for a flag match.
When the receiver detects a flag match it achieves synchronization and interprets the fol-
lowing byte as the address field.
The SYNC/HUNT bit in RR0 reports the Hunt Status and an interrupt can be generated
upon transitions between the Hunt state and the Sync state.
The ISCC will drive the /SYNC pin Low to signal that the flag has been received.
4.4.3 SDLC LOOP MODE
The ISCC supports SDLC Loop mode in addition to normal SDLC. SDLC Loop mode is
very similar to normal SDLC but is usually used in applications where a point-to-point
network is not appropriate (for example, Point-of-Sale terminals). In an SDLC Loop there
is a primary controller that manages the message traffic flow on the loop and any number
of secondary stations. In SDLC Loop mode, the ISCC operating in regular SDLC mode
can act as the primary controller.
A secondary station in an SDLC Loop is always listening to the messages being sent
around the loop, and in fact must pass these messages to the rest of the loop by retransmit-
ting them with a one-bit-time delay.
The secondary station can place its own message on the loop only at specific times. The
controller signals that secondary stations may transmit messages by sending a special
character, called an EOP (End of Poll), around the loop. The EOP character is the bit pat-
tern 11111110.
When a secondary station has a message to transmit and recognizes an EOP on the line, it
changes the last binary 1 of the EOP to a 0 before transmission. This has the effect of turn-
ing the EOP into a flag pattern. The secondary station now places its message on the loop
and terminates its message with an EOP. Any secondary stations further down the loop
with messages to transmit can append their messages to the message of the first secondary
station by the same process.
All secondary stations without messages to send merely echo the incoming messages and
are prohibited from placing messages on the loop, except upon recognizing an EOP.
SDLC Loop mode is quite similar to normal SDLC mode except that two additional con-
trol bits are used. Writing a 1 to the Loop Mode bit in WR10 configures the ISCC for
Loop mode. Writing a 1 to the Go Active on Poll bit in the same register normally causes
the ISCC to change the next EOP into a flag and then begin transmitting on loop. How-
ever, when the ISCC first goes on loop it uses the first EOP as a signal to insert the one-bit
Notes:
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