It is very interesting and convenient to be able to controleverything while sitting atyour PC terminal. Here, a simplehardware circuit and software isused to interface a 7-segment basedrolling display.The printer port of a PC providesa set of points with some actingas input lines and some othersas output lines. Some lines are opencollector type which can be used asinput lines. The circuit given herecan be used for interfacing with anytype of PC’s printer port.The 25-pin parallel port connectorat the back of a PC is a combinationof three ports. The addressvaries from 378H-37AH. The 7 linesof port 378H (pins 2 through 8) areused in this circuit to output thecode for segment display throughIC1. The remaining one line of port378H (pin 9) and four lines of port37AH (pins 1, 14, 16, 17) are usedto enable the display digits (one atime) through IC2.The bits D0, D1 and D3 of port37AH connected to pins 1, 14 and 17of ‘D’ connector are inverted by thecomputer before application to the pinswhile data bit D2 is not inverted. Thereforeto get a logic high at any of formerthree pins, we must send logic 0 output tothe corresponding pin of port 37AH.Another important concept illustratedby the project is the time division multiplexing.Note that all the five 7-segmentdisplays share a common data bus. The PC places the 7-segment code for the firstdigit/character on the data bus and enablesonly the first 7-segment display. Afterdelay of a few milliseconds, the 7-segmentcode for the digit/character is replaced bythat of the next charter/digit, but this timeonly second display digit is enabled.After the display of all characters/digits in this way, the cycle repeats itselfover and over again. Because of this repetitionat a fairly high rate, there is anillusion that all the digits/characters arecontinuously being displayed. DISP1 is tobe physically placed as the least significantdigit.

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