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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 and
RS-232 DTE test modes and can also be controlled with this register. The address of the
Communications Register is Base+4. Table 9 details its bit definitions.
00TCKENRCKEN
RLEN
or
SW_SYNC
LLENEXTSYNCTM ST
Bit 0Bit 1Bit 2Bit 3Bit 4Bit 5Bit 6Bit 7
Table 9 --- Communications Register - Read/Write
Bit 7: TM ST --- Test Mode Status:
This bit can be used to read the status of the Test Mode signal on a DTE, allowing
the user to monitor the signal without generating any interrupts.
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 10 of the DB-25 connector.
Bit 5: LLEN --- Local Loopback Enable: When set
(logic 1), this bit allows the DTE to test the functioning of the DTE/DCE interface
and the transmit and receive sections of the local DCE. The DCE device must
support local loopback for this to work. When cleared (logic 0), no testing occurs.
LLEN can also be used as a software-controlled general-purpose output.
Bit 4: RLEN --- Remote Loopback Enable: SW_SYNC
--- Software Sync On: When
the 'C' option is used with the client driver or enabler, this bit functions as
SW_SYNC, otherwise it functions as RLEN.
RLEN
('C' option not used) If this bit is set
(logic 1), the DTE can test the transmission path through the remote DCE to the
remote DTE interface and the return transmission path. The remote device must
support remote loopback for this to work. When cleared (logic 0), no testing
occurs.
SW_SYNC ('C' option is used)
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