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Interfacing to
7-Segment Numeric Displays

7-Segment Numeric LED Displays
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 asthe 4-digit panel mount unit shown at the top of this page...
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'llconcentrate on the more common LED units in the examples to follow...

 

 
BCD to 7-Segment Decoder c/w 4-bit Latch
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 theequivalent 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 selectthe 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...
 

BCD Input Data
 

 SW3 
 SW2 
 SW1 
 SW0 
 Numeral Displayed 

0
0
0
0
0

0
0
0
1
1

0
0
1
0
2

0
0
1
1
3

0
1
0
0
4

0
10
1
5

0
1
1
0
6

0
1
1
1
7

1
0
0
0
8

1
0
0
1
9

 
 

  
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 working, or identify display units that need to be replaced... This input is normally left at logic 1... The Blanking (BL) input is just the reverse; it forcesthe entire display off... This is used in many cases to blank out leading or trailing zeros from a long display... LT will override BL so you can test even blanked-out display digits...
One should also note that the same circuit could conceivably be controlled by a PLC, if 4 output bits from a 5VDC PLC output card were used in place of the 4 switches shown... If an 8-bit output card wereavailable, then two such circuits (2 digits) could be controlled... A 16-bit card would in turn allow us to control four such circuits (4 digits)..etc...
 
Parallel Non-Multiplexed Multiple Digit Display

The figure (above) on the left is taken from the LogixPro I/O simulator screen, and depicts a common method of interfacing to a 4 digit display... The figures on the right are taken from the datasheet of a pre-manufactured 4 digit display unit which could be readily employed in this particular application...
The manufactured unit does contain four separate circuits, and each circuit (digit) has it's own decoder, but compared to our earlier circuit example, this unit employs additional components and circuitry making it far more versatile and easy to use...
Note that there are 4 "Strobe"lines shown; one for each digit... These strobe lines control built in IC latches which provide us with the option of multiplexing the digits, or displays, if we wished to do so... In the above non-multiplexed application, the strobes are permanently enabled allowing data to simply pass through from the BCD inputs and be displayed as normal...
Also note that this particular unit is designed for...
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