Parallel
Input/Output
Interface
The
general-purpose parallel interface
is
a 25-pin male connector on the back panel of
the computer. This connector is generally used for a parallel printer.
For
pin connections, see appendix E.
Serial
Interface
A 25-pin D-type female connector on the back panel of the computer serves as the gen-
eral purpose serial interface. This connector can drive a wide range of different peri-
pherals, including an external modem or a serial printer.
For pin connections, see appendix E.
INTRODUCTION
TO
SERIAL CIRCUITRY
The
circuit
that
controls the serial link
to
the outside world is called a UART, which
is
short for Universal Asynchronous Receiver/Transmitter. The UART
is
able
to
commun-
icate
at
baud rates (bit-rate of transmission of data)
that
you preset.
It
can receive or
send
data
with a programmable length of eight or nine bits.
The
UART
is
also capable of detecting overrun errors, which occur when some other sys-
tem sends
in
data
faster than you remove
it
from the data-receive register. There are
also
status
bits
that
you can read
to
find out when the receive buffer
is
full or when the
transmit buffer is empty. An additional
status
bit is provided
that
indicates "all bits
sent." All of these topics are discussed below.
SETTING
THE BAUD
RATE
Baud rate (rate of transmission) is controlled by the contents of the register named
SERPER. Bits 14-0 of SERPER are the baud-rate divider bits. If you consider the con-
tents of these bits
to
be the number
N,
then N+1 color clocks (each 279.4 ns) occur
between samples of the
state
of the input pin (for receive) or between transmissions of
output
bits (in the transmit mode).
240 Interface Hardware
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