Motorola PowerQUICC II MPC8280 Series Reference Manual page 784

HDLC Bus Mode with Collision Detection
After 5 bytes and CRC have been sent, the Tx buffer is closed;
the Rx buffer is closed after a frame is received. Frames larger
than 256 bytes cause a busy (out-of-buffers) condition because
only one RxBD is prepared.
22.13.2 SCC HDLC Programming Example #2
The following sequence initializes an HDLC channel that uses the DPLL in a Manchester
encoding. Provide a clock which is 16× the chosen bit rate of CLK3. Then connect CLK3
to the HDLC transmitter and receiver. (A baud rate generator could be used instead.)
Configure SCC2 to use RTS2, CTS2, and CD2.
1. Follow steps 1–22 in example #1 above.
2. Write 0x004A_A400 to GSMR_L2 to make carrier sense always active, a 16-bit
preamble of '01' patterns, 16× operation of the DPLL and Manchester encoding for
the receiver and transmitter, and HDLC mode. CTS and CD should be configured
to control transmission and reception. Do not set GSMR[ENT, ENR].
3. Write 0x0000 to PSMR2 to use one opening and one closing flag and 16-bit
CCITT-CRC and to reject multiple frames in the FIFO.
4. Write 0x004A_A430 to GSMR_L2 to enable the SCC2 transmitter and receiver.
This additional write to GSMR_L2 ensures that ENT and ENR are enabled last.
22.14 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 configurations. 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 configurations.
Review the basic features of the I.430 and T1.605 before learning about the HDLC bus. The
I.430 and T1.605 define 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
22-18
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