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. 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. 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 that will appear on the 7-Segment display. 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 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 display 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. 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. 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 segment BCD inputs and be displayed as normal. Also note that this particular unit is designed for 24VDC use. 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 noisy environments. By making use of the 4-bit latches that are built into the 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 is set using the 4 data switches. If SW1 is then closed, the current BCD input data will enter the latch of the upper IC, and will be passed on to the decoder causing the numeral to displayed. 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 binary 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. By using multiplexed data lines as shown above, we can dramatically binary the number of PLC outputs required 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 display the 16 data lines are internally connected in parallel with these sockets, and this allows us to easily add additional units by segment interconnecting them using a flat ribbon cable assembly. Why this method was chosen, will be investigated now. 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 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 Display one of the four output bits to O 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. 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. There are no hard binary fast rules relating to this issue, but a quick look at the following chart does provide some food for thought. 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. Interfacing to 7-Segment Numeric Displays 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 segment 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. 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 that will appear on the 7-Segment display.
Digital Electronics: Binary Coded Decimal (BCD) to 7 Segment Display
Digital Electronics: Binary Coded Decimal (BCD) to 7 Segment Display
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