Zilog Z16C35 User Manual
Page 263
Application Note
Using SCC with Z8000 in SDLC Protocol
12-8
RECEIVE OPERATION
(Continued)
If receive overrun error is made, a special condition
interrupt occurs. The SCC presents vector %2E to the
CPU, and the service routine located at address %447A is
executed. Register RR1 is read to determine which error
occurred. Appropriate action to correct the error should be
taken by the user at this point. Error Reset and Reset
Highest IUS commands are given to the SCC before
returning to the main program so that the other low-priority
interrupts can occur.
In addition to searching the data stream for flags, the
receiver also scans for seven consecutive 1s, which
indicates an abort condition. This condition is reported in
the Break/Abort bit (D7) in RR0. This is one of many
possible external status conditions. As a result transitions
of this bit can be programmed to cause an external status
interrupt. The abort condition is terminated when a zero is
received, either by itself or as the leading zero of a flag.
The receiver leaves Hunt mode only when a flag is found.
SOFTWARE
Software routines are presented in the following pages.
These routines can be modified to include various other
options (e.g., SDLC Loop, Digital Phase Locked Loop
etc.). By modifying the WR10 register, different encoding
methods (e.g., NRZI, FM0, FM1) other than NRZ can be
used.
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