Motorola MPC860 PowerQUICC, User Manual

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MOTOROLA Chapter 24. SCC HDLC Mode 24-17
Part V. The Communications Processor Module
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
Figure 24-11 shows the most common HDLC bus LAN conÞguration, a multimaster
conÞguration. A station can transfer data to or from any other LAN station. Transmissions
are half-duplex, which is typical in LANs.