Typically asynchronous communications adapter system control signals and
voltage requirements are provided through a 2 by 31 position card-edge
tab.  Two jumper modules may be provided on the adapter. One jumper module
selects either RS-232C or current-loop operation.  The other jumper module
selects one of two addresses for the adapter, so two adapters may be used
in one system.

The adapter is fully programmable and supports asynchronous communications
only.  It will add and remove start bits,stop bits, and parity bits.    A programmable
baud rate generator allows operation at various selected baud rates. Five, six, seven
or eight bit characters with 1, 1-1/2, or 2 stop bits are supported. A fully prioritized
interrupt system controls transmit, receive,error, line status and data set
interrupts.  Diagnostic capabilities provide loopback functions of transmit/receive
and input/output signals.

The heart of the adapter is a INS8250 LSI chip or functional equivalent.
Features in addition to those listed above are:
        Full double buffering eliminates need for precise synchronization.
        Independent receiver clock input.
        Modem control functions: clear to send (CTS), request to send (RTS),
        Data set ready (DSR), data terminal ready (DTR), ring indicator (RI),
          and carrier detect.
        False-start bit detection.
        Line-break generation and detection.

All communications protocols are a function of the system microcode and
must be loaded before the adapter is operational. All pacing of the
interface and control signal status must be handled by the system
software.

Perhaps an easy way to sort this information is to categorize it with respect to
hardware, firmware and software protocols.  Please allow that for this presentation,a
liberal application of the idea of a protocol has been executed.
 
    Applications Issues