SCC HDLC Mode
MPC885 PowerQUICC Family Reference Manual, Rev. 2
23-20
Freescale Semiconductor
23.14.4 Delayed RTS Mode
Figure 23-14
shows local HDLC bus controllers using a standard transmission line and a local bus. The
controllers do not communicate with each other but with a station on the transmission line; yet the HDLC
bus protocol controls access to the transmission line.
Figure 23-14. HDLC Bus Transmission Line Configuration
Normally, RTS goes active at the beginning of the opening flag’s first bit. Setting PSMR[BRM] delays
RTS by one bit, which is useful when the HDLC bus connects multiple local stations to a transmission line.
If the transmission line driver has a one-bit delay, the delayed RTS can be used to enable the output of the
line driver. As a result, the electrical effects of collisions are isolated locally.
Figure 23-15
shows RTS
timing.
Figure 23-15. Delayed RTS Mode
Local HDLC Bus
HDLC Bus
Controller
RXD
CTS
TXD
A
HDLC Bus
Controller
RXD
CTS
TXD
B
RTS
+ 5 V
R
NOTES:
1. The TXD pins of slave devices should be configured to open-drain in the port C parallel I/O port.
2. The RTS pins of each HDLC bus controller are configured to delayed RTS mode.
RTS
Tx
Rx
EN
(1-Bit Delay)
Line Driver
TCLK
RTS active for
only 2 bit times
TXD
CTS
RTS
1st Bit
2nd Bit
3rd Bit
Collision
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