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7 segment rolling display using
PC Click here for the circuit diagram
is very interesting and convenient to be able to control
everything while sitting at your PC terminal. Here,
a simple hardware circuit and software is used to interface
a 7-segment based rolling display. The printer port
of a PC provides a set of points with some acting as
input lines and some others as output lines. Some lines
are open collector type which can be used as input lines.
The circuit given here can be used for interfacing with
any type of PC?s printer port. The 25-pin parallel port
connector at the back of a PC is a combination of three
ports. The address varies from 378H-37AH. The 7 lines
of port 378H (pins 2 through 8) are used in this circuit
to output the code for segment display through IC1.
The remaining one line of port 378H (pin 9) and four
lines of port 37AH (pins 1, 14, 16, 17) are used to
enable the display digits (one a time) through IC2.
The bits D0, D1 and D3 of port 37AH connected to pins
1, 14 and 17 of ?D? connector are inverted by the computer
before application to the pins while data bit D2 is
not inverted. Therefore to get a logic high at any of
former three pins, we must send logic 0 output to the
corresponding pin of port 37AH. Another important concept
illustrated by the project is the time division multiplexing.
Note that all the five 7-segment displays share a common
data bus. The PC places the 7-segment code for the first
digit/character on the data bus and enables only the
first 7-segment display. After delay of a few milliseconds,
the 7-segment code for the digit/character is replaced
by that of the next charter/digit, but this time only
second display digit is enabled. After the display of
all characters/digits in this way, the cycle repeats
itself over and over again. Because of this repetition
at a fairly high rate, there is an illusion that all
the digits/characters are continuously being displayed.
DISP1 is to be physically placed as the least significant
digit. IC1 (74LS244) is an octal buffer which is primarily
used to increase the driving capability. It has two
groups of four buffers with non-inverted tri-state outputs.
The buffer is controlled by two active low enable lines.
IC2 (75492) can drive a maximum of six 7-segment displays.
(For driving up to seven common-cathode displays one
may use ULN2003 described elsewhere in this section.)
The program for rolling display is given in the listing
DISP.C above. Whatever the message/characters to be
displayed (here five characters have been displayed),
these are separated and stored in an array. Then these
are decoded. Decoding software is very simple. Just
replace the desired character with the binary equivalent
of the display code. The display code is a byte that
has the appropriate bits turned on. For example, to
display character ?L?, the segments to be turned on
are f, e and d. This is equivalent to 111000 binary
or 38 hex. Please note that only limited characters
can be formed using 7-segment display. Characters such
as M, N and K cannot be formed properly
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