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
as the 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'll concentrate 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 the equivalent numeral (0 to 9) on a
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...
|BCD Input Data
| Numeral Displayed
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 forces the 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 were
available, 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 data sheet
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 24VDC use... This is by far the most common DC voltage
level used in industrial installations, and PLC I/O cards designed for such use are therefore extremely
common... In comparison to 5VDC circuits, 24VDC circuits can typically tolerate far greater supply voltage
excursions, are less sensitive to the effects of contact resistance, and more tolerant of electrically
By making use of the 4-bit latches that are built into the 4511 IC, we can easily multiplex the digits
if so desired... By properly controlling the state of each latch enable pin (LE) we can use the same input
data lines (4 switches) to selectively write to each 7-Segment display independently... With just a minor
modification to our circuit, we will be able to essentially treat each digit as a unique 4-bit memory
location where BCD data of our choosing can be stored and retained...
In the above schematic diagram, each display may be written to separately... First the BCD equivalent of
the desired numeral (0-9) is set using the 4 data switches... If SW1 is then closed, the current BCD input
data will enter the latch of the upper 4511 IC, and will be passed on to the decoder causing the numeral
to displayed... if SW1 is then opened, the latch will retain the current data, but will now ignore any
changes on it's inputs... The desired numeral will continue to be displayed by the upper LED display until
power is lost, or SW1 is again closed and new data is allowed to enter it's latch...
The Lower 7-Segment display may be written to in a similar fashion... Set the BCD equivalent of the
desired digit using the 4 data switches, then close SW2 momentarily to store and retain the current
BCD data... We might say that we are strobing the data into the display...
We could readily replace the 4 data switches and 2 latch switches if we had 6 5VDC outputs available
on our PLC... If we wished to add additional digits, we would require 1 more PLC output for each digit added...
By multiplexing the data in this fashion we would only require 8 PLC outputs to control a 4 digit display...
4 Digit Display configured for Multiplexed Digits
By using multiplexed data lines as shown above, we can dramatically reduce the number of
PLC outputs required (8) to control this 4 digit display... The down side is that writing a ladder logic
program to update a display wired in this fashion will be somewhat more complicated then one written to
control 4 digits wired in parallel...
If you have been looking closely, you may have questioned the purpose of the dual inline sockets
located on the wiring PCB of this manufactured display unit.. Well essentially the 16 data lines are
internally connected in parallel with these sockets, and this allows us to easily add additional
units by simply interconnecting them using a flat ribbon cable assembly...
Multiple 4 Digit Displays configured for Multiplexed Digits
For those with a fundamental knowledge of computers, you'll likely note that we have essentially
created a 4-bit data bus, and the strobe lines are in effect our address bus... We can continue to
add additional digits, and for each digit added we will require 1 additional PLC output, or 4 outputs
per quad display...
1 Quad Display requires 4 data + 4 strobes = 8 outputs
2 Quad Displays requires 4 data + 8 strobes = 12 outputs
3 Quad Displays requires 4 data + 12 strobes = 16 outputs
4 Quad Displays requires 4 data + 16 strobes = 20 outputs
If you are reading through this tutorial because you are having difficulty determining how to
control the 4 quad displays in the Bottle Line Simulation, you may already have noted that the displays
in that simulation do not employ muxed (multiplexed) digits, but rather the quad displays themselves are
muxed... Why this method was chosen, will be investigated now...
Multiplexed 4 Digit Displays ..... (Bottle Line Simulation)
The above diagram functionally depicts how the 4 panel mounted display units in the Bottle Line
simulation are deemed to be configured... In this case, the 4 manufactured displays are
wired to a shared 16-bit data bus (wires) which is connected to a 16-bit 24VDC output card (O:4) that
is dedicated to this purpose... 4 unused outputs from card O:2 were selected, and appropriately wired
to allow control of the strobe lines associated with each display unit...
Each unit may be written to individually by first sending the 16 bit BCD representation
of the 4 decimal numerals to output card O:2... If one of the four output bits 0:2/11 to O:2/14 is
then taken from an initial low, to high state, the BCD data will enter the selected latches, and the
numerals will be displayed... If this strobe line is then returned to the initial low state,
the BCD data will (be latched) remain in the latches, and continue to be displayed... Any changes in
the state of the data lines will now be ignored, and this will continue until the next low-to-high
transition of this unit's strobe line occurs...
You may have noted that this configuration requires a high level on the strobe line to allow data
to enter the latch, while some of our earlier examples required a low state... The polarity of the
strobe is non critical, and with this particular make of display, the choice of polarity is set by
simply positioning a configuration DIP switch appropriately...
Multiplexed Digits vs Displays
There are no hard and fast rules relating to this issue, but a quick look at the following
chart does provide some food for thought...
................................Muxed Digits....Muxed Displays
1 Quad Display requires.......8 outputs......16 outputs (no muxing)
2 Quad Displays requires.....12 outputs......18 outputs
3 Quad Displays requires.....16 outputs......19 outputs
4 Quad Displays requires.....20 outputs......20 outputs
5 Quad Displays requires.....24 outputs......21 outputs
6 Quad Displays requires.....28 outputs......22 outputs
10 Quad Displays requires....44 outputs......26 outputs
Another consideration relates to ease of programming... The task of writing digits one at a time
is obviously going to be a greater challenge then writing 4 digits at a time... Also, fewer
instructions typically equates to an easier to read program, plus a quicker scan... If you need to
control 4 quad displays, muxed displays would be my choice... Need 10 displays, and it might be time to
consider some other means of presenting information to the operator... HMI?...