theory.
More readily available
references on the 8251, but
ones
that
have
less
to say
about
communication
theory, are the "INTEL 8080 Microcomputer
Systems User's Manual," available
either from Intel or most canputer
retail
stores,
and Adam
Osborne's
IIAn Introduction
to Microcomputers,
Volume II -
Some Real Products,1I also available
in many computer stores.
1.3
serial
asynchronous
coommication
Separately
for
each
serial
channel,
you
can
select
the
rate of
transmission
and reception from a choice of 110, 150, 300, 600, 1200,
2400,
4800,
or 9600 bits/second.
You choose the rate using small dip-switches
on
the board.
If you are writing custom software, there is in addition a simple way in
software to divide the chosen rate of a desired channel by 4, allowing
some
control
of
the
rate
without
physically
opening
up the computer.
For
example,
if you
have
a modem
that
is swi tch selectable
for
either
1200
bits/second
or 300 bits/second,
you can write a program that enables the
operator usi1'l3the keyboard
to change the computer's
rate
of commun ication
to match that of the roodem at any given time.
Separately
for each
serial
channel,
you
can
select
via
software
the
number of data bits in each ASCII character, selecting either
5, 6, 7, or 8.
You can also select the number of stop
bits
in each
character,
selecting
either
1, 1 1/2, or 2.
Finally, you can select whether or not a parity bit
is included for each character, and if chosen, whether or not it is even
or
odd
parity.
For how
to do this
in software,
see
the
references
given
earlier for the 8251 USART.
You
can enable one or more serial channels to communicate
in synchronous
mode.
Modifications
to the board will be required to accompl ish this.
In
brief,
the
8251
SYNDET pin and the TxC and RxC clock pins must be connected
to the outside world, which is not the case in the standard configuration
of
the board.
Once set up for synchronous
comnunication,
you can select separately
for
each
such
channel
the
rate
of communication,
by using an external clock
between"
and 56K bits/second.
As with asynchronous
comrrunication, you
can
select via software the character
length,
selecting
either
5, 6, 7, or 8
bits.
You can
also
select
via
software
whether
or not
a parity
bit is
included for each character, and if chosen, whether or not it is even or odd
parity.
Also
via
software,
you can
select
separately
for each channel
wheG~er you are using internal or external synchronization,
and whether
one
or
t~
synch characters are used.
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