Serial Synchronous Cammunications - Vector ZCB User Manual

vector ZCB Single Board Computer
It is not within the scope of this manual to detail the functioning of
the 8251 USART chip, nor to teach the theory of serial camnunication. In
order to write your own camnunications software or to nodify the ZCB serial
port, you will need to be thoroughly familiar with this chip. You can refer
to Intel's Application Note #16, entitled "Using The 8251 Universal
Synchronous/Asynchronous Receiver/I'ransmitter," which Intel will provide.
This Note is also an excellent reference on basic communications theory.
More readily available references on the 8251, but ones that have less to
say about camnunication theory, are the "INTEL 8080 Microcomputer Systems
User's Manual," available either from Intel or most canputer retail stores,
and Adam Osborne's
"An
Introduction to Microcanputers, VolUIre II - Sane Real
Products," also available in many canputer stores.
1.5.2 serial asynchronous communication
You can select the rate of transmission and reception fran a choice of
110, 150, 300, 600, 1200, 2400, 4800, or 9600 bits/second. You choose the
rate using a small DIP-switch on the board.
If you are writing custan software, there is also a simple way to divide
the chosen rate of a port by 4, allowing some software control of the rate
without physically opening up the computer. For example, if you have a
modern that is switch selectable for either 1200 bits/second or 300
bits/second, you can write a program that enables the operator using the
keyboard to change the canputer's rate of camnunication to match that of the
Il'Ddem at any given time.
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 USARI'.
1.5.3 Serial synchronous communication
You can enable the serial port to camnunicate in the synchronous mode.
Modifications to the board will be required to accanplish this. In brief,
the 8251 SYNDEr pin and the TxC and P..xC clock. pins must be connected to the
outside
~rld,
which is not the case in the standard configuration of the
board.
Once set up for synchronous communication, you can select the rate of
camnunication, by using an external clock between
0
and 56K bits/second.
As
with asynchronous canrnunication, 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 port whether you are using internal or external
synchronization, and whether one or two synch characters are used.
Rev. l-B 6/11/80
1-3