11 Communications Register
The Communications Register is used to set options pertaining to the clocks. The source
and type of clock to be transmitted or received can be specified. External synchronization can
also be controlled with this register. The address of the Communications Register is Base+4.
Table 9 details its bit definitions.
TXDEN
RXDEN
TCKEN
RCKEN
SW_SYNC
0
EXTSYNC
0
Bit 0
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
Table 9 --- Communications Register - Read/Write
Bit 7:
Reserved, always 0.
Bit 6:
EXTSYNC --- External Sync Enable:
If this bit is set (logic 1), software-controlled sync is disabled and the SCC's
SYNCA input is driven by the signal coming on pin 21 of the DB-25 connector.
If this bit is clear (logic 0), the SW_SYNC bit can be used to drive SYNCA.
Bit 5:
Reserved, always 0.
Bit 4:
SW_SYNC --- Software Sync On:
This bit is
used to drive the active-low SYNC input of the channel A receiver. The SYNC
signal is asserted when this bit is set (logic 1), and is deasserted when this bit is
clear (logic 0). This is useful in situations where it is necessary to receive
unformatted serial data, as it allows the SCC receiver to be manually placed into
sync under program control. This bit is ignored if bit 6 is set (logic 1).
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