Vector Graphic Bitstreamer II, User Manual, Page: 34 / 54

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Vector
Graphic Bitstrea.mertI~Boara
It is important 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
modern, which is a kind of Data Communications Equipment (llDCE"), 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 tenninal at the other end of t..'1e line, that is from t..~e
point of view of the Data Terminal Equir:ment (
It
DTE
It).
.~ccording to RS-232C,
Data Terminal Equipment transmits on the Transmitted Data line and receives
on the Received Data line.
The Bitstreamer II board is wired to behave as if it were Data
Communications Equipment. This is reasonable because the board is most
commonly connected to printers, CRT's, and other kinds of Data Tenninal
Equipment. This explains why RS-232C line 2 - Transmitted Data - is
connected to tha 8251 RxD (Receive Data) line, and why line 3 - Received
Data - is connected to ~he 8251 TxD (Transmit Data) line, in the factory
configuration.
If you want to connect the board to a rrodem, 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 Equir:ment should behave.
To make the board look like Data Terminal Equipment, you also should
eliminate the +12 VDC signals on lines 5, 6, and 8, unless you are
absolutely sure tl1at the Data Communications Equipnent you are connecting to
does not send handshaking signals on one or mora of these lines. (Many low
speed acoustic couplers do not use these lines.) The reason is that if t..'1e
Data Communications Equipment has an RS-232C line driver connected to one of
these lines, the +12 VDC could blow it out.
To send handshaking signals: Data Communication Equipment connected to
the board may requi ra 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 r12 VDC on some line other t..'1an 5, 6, or 8, though this is
rare. You can 12 VDC as a constant enabling signal, by connecting
the desired RS-232C line
(5)
to +12 VDC through a pull-up resistor.
Alternately, you can connect the required line(s) via an RS-232C line
driver to the 8251 and thus allo\'lyour 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 (RrS) and pin 24 (DTR). Soft\vare
controls the status of these f'ins
by
outputtifB a command instruction byte
to tha 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 t..'1e
RS-232C line driver into +12 VDC.
To receive handshaking signals: Data CorrumunicationsEquipment connected
to ~he board may send +12 vTIChandshaking signals on one or more lines, most
often RS-232C lines 5, 6, 8 and/or 22, Similarly, Data Tenninal Equipment