Interfacing to 7-Segment Displays

Posted on Feb 4, 2014

In industrial PLC applications, one of the old, but simpler methods of displaying numeric information is to use one or more 7-Segment numeric displays connected to an output card of a PLC. Although it is possible to build such a display yourself, it is far more common to employ a pre-manufactured product such as the 4-digit panel mount unit shownat the top of this page.

Interfacing to 7-Segment Displays
Click here to download the full size of the above Circuit.

To correctly interface a PLC to such a display, it helps to first understand what basic electronic components are typically employed in their makeup, and how this effects our task of interfacing to, and programming such a unit. Although both LED and LCD numeric displays are readily available, and interfaced similarly, we`ll concentrate on the more common LED units in the examples to follow. Once 7-Segment LED displays became readily available, a simple IC known as a "BCD to 7-Segment decoder" was quickly developed to simplify their use. Binary formatted data presented to this IC`s inputs results in the IC`s outputs being placed into the correct state to display the equivalent numeral (0 to 9) on a 7-Segment display. Although BCD to 7-Segment decoder ICs are available without built in latches, this particular IC includes a built in 4-bit latch which we will make use of in later examples. For now the latch is set to simply allow input data to freely pass through to the decoder. In the above diagram, the 4 toggle switches, SW0 to SW3 are used to select the desired numeral (0-9) that will appear on the 7-Segment display. By using a decoder, it`s now simply a matter of setting the correct 4-bit BCD pattern feeding the inputs of the decoder, and the decoder takes care of the rest. The decoder section also has two additional inputs. Lamp Test (LT) turns all segments on so you can verify at once that all display segments are...

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