Freescale Semiconductor MPC8260 - page 745

SCC HDLC Mode
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 22-17
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 efficiently 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 configurations,
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 8–10 rather than 7–10 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 22-10 shows the most common HDLC bus LAN configuration, a multimaster configuration. A
station can transfer data to or from any other LAN station. Transmissions are half-duplex, which is typical
in LANs.
Figure 22-10. Typical HDLC Bus Multimaster Configuration
HDLC Bus
Controller
RXD CTS
TXD
A
RCLK/TCLK
HDLC Bus
Controller
RXD CTS
TXD
B
RCLK/TCLK
HDLC Bus
Controller
RXD CTS
TXD
C
RCLK/TCLK
Clock
HDLC Bus LAN
+ 3.3 V
R
Master MasterMaster
NOTES:
1. Transceivers may be used to extend the LAN size.
2. The TXD pins of slave devices should be configured to open-drain in the port C parallel I/O port.
3. Clock is a common RCLK/TCLK for all stations.