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run the current application software.
Serial Ports
There are two basic kinds of general purpose I/O (Input/Output)
port. One kind is a “serial” port, where data bits are sent one
at a time along a single wire, in series. Additional wires are
used to send control signals between the transmitting and
receiving devices. These control signals allow the two
communicating devices to determine which of them will send
or receive the data, and when the first bit of a particular byte is
being sent, among other things.
Standardized “protocols” for these signals have been agreed
upon by the microcomputer industry so that the different
manufacturer’s machines can communicate with each other.
Your computer system uses the “RS-232C” asynchronous serial
communications interface. The serial port connector is the left-
hand 9-pin “D-type” connector mounted on the rear pannel of
the system motherboard. Electrical information for the serial
port is in Appendix A “Specifications.” For details on the
serial port pin connections, see Appendix D, “Connector
Pinouts.”
MS-DOS uses labels to refer to the various I/O ports on your
computer. The RS-232C
serial ports on the system
motherboard are assigned the “COM1” (Communications port
#l) label and the second port is "COM2" (Communications
port #2) label.
Expanding Your System
7-3