Motorola MPC8260 - Manual (Page 625)

MOTOROLA Chapter 21. SCC HDLC Mode 21-17
Part IV. Communications Processor Module
21.14 HDLC Bus Mode with Collision Detection
The HDLC controller includes an option for hardware collision detection and
retransmission on an open-drain connected HDLC bus, referred to as HDLC bus mode.
Most HDLC-based controllers provide only point-to-point communications; however, the
HDLC bus enhancement allows implementation of an HDLC-based LAN and other point-
to-multipoint conÞgurations. The HDLC bus is based on techniques used in the CCITT
ISDN I.430 and ANSI T1.605 standards for D-channel point-to-multipoint operation over
the S/T interface. However, the HDLC bus does not fully comply with I.430 or T1.605 and
cannot replace devices that implement these protocols. Instead, it is more suited to non-
ISDN LAN and point-to-multipoint conÞgurations.
Review the basic features of the I.430 and T1.605 before learning about the HDLC bus. The
I.430 and T1.605 deÞne a way to connect eight terminals over the D-channel of the S/T
ISDN bus. The layer 2 protocol is a variant of HDLC, called LAPD. However, at layer 1, a
method is provided to allow the eight terminals to send frames to the switch through the
physical S/T bus.
To determine whether a channel is clear, the S/T interface device looks at an echo bit on the
line designed to echo the last bit sent on the D channel. Depending on the class of terminal
and the context, an S/T interface device waits for 7Ð10 ones on the echo bit before letting
the LAPD frame begin transmission, after which the S/T interface monitors transmitted
data. As long as the echo bit matches the sent data, transmission continues. If the echo bit
is ever 0 when the transmit bit is 1, a collision occurs between terminals; the station(s) that
sent a zero stops transmitting. The station that sent a 1 continues as normal.
The I.430 and T1.605 standards provide a physical layer protocol that allows multiple
terminals to share one physical connection. These protocols handle collisions efÞciently
because one station can always complete its transmission, at which point, it lowers its own
priority to give other devices fair access to the physical connection.
The HDLC bus differs from the I.430 and T1.605 standards as follows:
¥ The HDLC bus uses a separate input signal rather than the echo bit to monitor data;
the transmitted data is simply connected to the CTS input.
¥ The HDLC bus is a synchronous, digital open-drain connection for short-distance
conÞgurations, rather than the more complex S/T interface.
¥ Any HDLC-based frame protocol can be used at layer 2, not just LAPD.
¥ HDLC bus devices wait 8Ð10 rather than 7Ð10 bit times before transmitting. (HDLC
bus has only one class.)
The collision-detection mechanism supports only:
¥ NRZ-encoded data
¥ A common synchronous clock for all receivers and transmitters
¥ Non-inverted data (GSMR[RINV, TINV] = 0)
¥ Open-drain connection with no external transceivers