User`s manual

8.4 Support for SCC Channel B
The MPAC-100 is a single-channel device. Portions of SCC channel B are used to
augment channel A. Channel B cannot be used for transmit, but may be used for
receive, subject to certain limitations.
8.4.1 Receive data and clock signals
The receive data signals RXDA and RXDB are tied together. The receive clock
input signals RTxCA and RTxCB are also tied together. This can be useful in unusual
applications. It would be possible to run the receiver and transmitter at different baud
rates, using channel B's baud rate generator and receiver for the received data. Of
course, the channel A transmitter and receiver can be run at different speeds simply by
having external data clocks supplied to TRxCA and RTxCA from the cable.
The W/REQB signal is used to generate DMA requests between the SCC and the
internal FIFOs if channel B is used for receive.
8.4.2 Extra clock support for channel A
The TRxCB clock output can be routed back to RTxCA as another way to use the
channel B baud rate generator to derive an independent clock for the channel A
receiver. This is controlled by the RCKEN bit in the Communications Register (see
page 39).
8.4.3 Extra handshaking for channel A
The SCC does not provide a DSR input for either channel. The MPAC-100 routes
the DSR signal from the connector to the DCDB input of the SCC. Software can
therefore use DCDB as a surrogate for DSR on channel A.
8.4.4 Other signals are not used
All channel B signals not listed above are not available at the connector. The
CTSB and SYNCB inputs are tied to their inactive states. The TXDB, DTR/REQB, and
RTSB outputs are left open.
Quatech MPAC-100 User's Manual
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