Z8523016VEG Datasheet, PDF (23/118 Pages) Zilog, Inc. – Enhanced Serial Communications Controller

Z80230/Z85230/L
Product Specification
18
NRZ, NRZI, or FM coding may be used in any 1X mode. The parity options available in
ASYNCHRONOUS mode are also available in SYNCHRONOUS mode. However, parity
checking is not normally used for SDLC because CRC checking is more robust.
SDLC LOOP Mode
The ESCC supports SDLC LOOP mode as well as normal SDLC. In SDLC LOOP mode,
a primary controller station manages the message traffic flow on the loop and any number
of secondary stations. In SDLC LOOP mode, the ESCC performs the functions of a sec-
ondary station. An ESCC operation in regular SDLC mode may act as a controller (see
Figure 10). SDLC LOOP mode is selected by setting WR10 bit 1 to 1.
Controller
Secondary #1
Secondary #4
Secondary #2
Secondary #3
Figure 10. SDLC LOOP mode
A secondary station in an SDLC LOOP mode always monitors the messages sent around
the loop and passes these messages to the rest of the loop, retransmitting them with a one-
bit time delay. The secondary station places its own message in the loop only at specific
times. The controller indicates that the secondary stations can transmit messages by send-
ing a special character, called EOP, around the loop. The EOP character has a bit pattern
11111110, the same pattern as an Abort character in normal HDLC. This bit pattern is
unique and easily recognized, because of the zero insertion in the message.
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 action changes the
EOP into a flag sequence. The secondary station now places its message on the loop and
terminates the message with an EOP. Any secondary stations further down the loop with
messages to transmit appends their messages to the message of the first secondary station
using the same process. Secondary stations without any messages to transmit merely echo
the incoming message. All secondary stations are prohibited from placing messages on the
loop except upon recognizing an EOP. In SDLC LOOP mode, NRZ, NRZI or FM coding
can be used.
PS005308-0609
Functional Description