Zilog Z16C35 - Manual (Page 80)

ISCC
User Manual
UM011002-0808
74
4.4.2 SDLC Receive
The receiver in the ISCC™ always searches the receive data stream for flag characters in
SDLC mode. Ordinarily, the receiver transfers all received data between flags to the
receive data FIFO. However, if the receiver is in Hunt mode no flag is received. The
receiver is in Hunt mode when first enabled, or the receiver may be placed in Hunt mode
by the processor issuing the Enter Hunt mode command in WR3. this bit (D4) is a com-
mand, and writing a “0” to it has no effect. The Hunt status of the receiver is reported by
the Sync/Hunt bit in RR0.
Sync/Hunt is one of the possible sources of external/status interrupts, with both transitions
causing an interrupt. This is true even if the Sync/Hunt bit is set as a result of the processor
issuing the Enter Hunt mode command.
The receiver will automatically enter Hunt mode if an abort is received. Because the
receiver always searches the receive data stream for flags and automatically enters Hunt
Mode when an abort is received, the receiver will always handle frames correctly, and the
Enter Hunt Mode command should never be needed. The ISCC will drive the SYNC pin
Low to signal that a flag has been recognized. The timing for the SYNC signal is shown in
Figure 4-12.
The first byte in an SDLC frame is assumed by the ISCC to be the address of the second-
ary station for which the frame is intended. The ISCC provides several options for han-
dling this address.
WR10 7 Select CRC preset value
WR0 6-7 Reset Tx CRC
WR10 1 Abort/flag on underrun
WR0 6-7 Tx underrun
WR7 6-7 Flag
Table 4–19. Initializing the Transmitter in SDLC Mode
Register Bit No Description
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