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vector ZCB Single Board Computer
from the original purpose for RS-232C - to connect a terminal with a
communication device such as a modem.
A
computer does not have to be
involved at all.
Since a canputer can either play the part of a terminal,
when connected to a modem, or it can play the part of communication
equipment, when connected to a terminal, a canputer serial port can be used
either as OCE or DrE.
However, a given serial :port can only
be
wired up as
one or the other at anyone time.
If the port happens to be wired up to
look like DeE, and you want to connect it to another OCE such as a modem,
then the RS-232C connection will not VoOrk.
Both ends VoOuld
be
transmi tting
on the same lines and receiving on the same lines.
Before the RS-232C
connection can
be
made, in this case, you must rewire the canputer's serial
:port so that it receives and transmits on the lines specified for DrE.
1.5.5 RS-232C on the ZCB
To enable
the serial :port to canmunicate over an RS-232C line, you will connect one
end of this line to the serial port socket on the ZCB board, and the other
end, having an RS-232C standard DB-25 female connector, to the back panel of
the computer.
The cable is designed so that appropriate signals fran the
board are directed to the RS-232C lines at the DB-25 connector as if it were
Data Canmunications Equipment.
Thus, the resulting DB-25 socket at the rear
of the canputer is a DCE RS-232C port.
Drivers are provided for the serial :port to enable the 8251 Transmit Data
and Receive Data lines to input or output at RS-232C voltage levels.
These
receivers and drivers are already connected on the board, requiring no
jl.llTIpering.
When a serial I/O cable is installed these signals are connected
to RS-232C lines 3 and 2 respectively.
In addition, four of the RS-232C control lines are connected to the
appropriate pins on the 8251.
These are DrR, DSR, RTS and
ers.
See the
table "Additional RS-232C handshaking lines available" in Section 2.4.2 for
the pin assignments.
When a serial I/O cable is installed, these signals
are available on the cable's DB-25.
There are four other signals available
fran the 8251 which can
be
accessed:
TxRDY, TxMT, SYNDEr
and
RxRDY.
You can
install jl.llTIpers
and
RS-232C line drivers
to
enable the 8251 to dynamically
control any two of them via software.
Further, a number of other RS-232C lines are available on the board using
the standard serial I/O cable mentioned above.
These are both input and
output lines, but they are not connected to anything other than pads on the
board, nor are drivers and receivers connected to them.
The table "RS-232C
and Connections on
ZCB
Board" in Section 2.4.6 lists the functions of each
of these lines.
Any RS-232C line can
be
connected to +12
VOC
on the board.
In addition,
for each port, the 8251 can
be
used to rronitor in software anyone RS-232C
handshaking line, and the 8251 can be used to control fram software the
output of any two RS-232C handshaking lines, and lastly, the 8251's
transmitter can be disabled or enabled by anyone RS-232C incoming
Rev. I-B
6/11/80
1-5
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