Table of Contents
Advertisement
vector ZCB Single Board canputer
It is imr:ortant to note here that in the RS-232C protocol, any given line
has one name, regardless of your point of view.
For example, although a
rrodem,
which is a kind of Data Carnnunications Equipment ("DCE"), receives
its data on line 2, line 2 is still called Transmitted Data.
Notice that
the names of the lines are more meaningful if you look at them from the
point of view of a terminal at the other end of the line, that is fran the
point of view of the Data Terminal Equip:nent ("DI'EI!).
According to RS-232C,
Data Terminal Equipment transmits on the Transmitted Data line and receives
on the Received Data line.
The ZCB board is wired to behave as if it were Data Communications
Equipment.
This is reasonable because the board is IIDst caru:ronly connected
to printers, CRT's, and other kinds of Data Terminal Equipment.
This
explains why RS-232C line 2 - Transmitted Data - is connected to the 8251
RxD (Receive Data) line, and why line 3 - Received Data - is connected to
the 8251 TxD (Transmit Data) line, in the factory configuration.
If
you want to connect the board to a
rrodern,
that is, you want the board
to behave like Data Terminal Equipment, you have to reverse lines 2 and 3
somewhere.
The result would be that RS-232C Transmitted Data is connected
to the 8251 TxD pin
and
RS-232C Received Data is connected to the 8251 RxD
pin, which is exactly the way Data Terminal Equipment sh,?uld behave.
To
send handshaking signals:
Data Carnnunication Equipment connected to
the board may require a +12
VDC
on some other line (most often line 4 or
line 20).
Similarly, a piece of Data Terminal Equipment connected to the
board may require +12
VDC
on some line other than 5 or
6,
though this is
rare.
You can supply +12
VDC
as a constant enabling signal, by connecting
the desired RS-232C line(s) to +12
VDC
through a pull-up resistor.
Alternately, you can allow your software to control the +12
VDC
enabling
signal(s), if desired.
The 8251 has two pins that can be controlled by
software.
They are pin 23 (RTS) and pin 24 (DTR).
Software controls the
status of these pins by outputting a canmand instruction byte to the 8251,
as described in the 8251 references mentioned in the Perspective section of
this manual.
Note that by sending a binary 1 to one of these status lines,
the line is turned ON, which is converted
by
the RS-232C line driver into
+12 VOC.
To receive handshaking signals:
Data Communications Equipment connected
to the board may send +12
VDC
handshaking signals on one-or IIDre lines, IIDst
often RS-232C lines 5 and/or 6.
Similarly, Data Terminal Equipment
connected to the board may send +12 VDC on one or more lines, most often
RS-232C lines 4 and/or 20.
You do not have to monitor these lines,
necessarily.
If you choose to, you can read them through software controL
The hardwired line drivers and receivers are already in place.
The 8251 has
two pins that can be used
to
receive handshaking signals: pin 17 (CTS) and
pin 22 (DSR).
Software can monitor the status of DSR by rronitoring the
appropriate bit in the 8251 status byte.
The RS-232C line receiver causes
+12
vr:c
to make this bit a binary 1 (ON).
Software can also rronitor the
ers
status.
Rather, if the input to CTS is OFF, the 8251 will not transmit
Rev. 1-B
6/11/80
2-17
Advertisement
Table of Contents
