Pages created and updated by Terry Sturtevant Date Posted: October 11, 2019

PC/CP220 Digital Electronics Lab

Decoder Lab



In this lab you will wire a BCD-to-7-segment decoder with a 7 segment display and a pushbutton (or momentary) switch .


Display unit

You will be using a 7-segment display.


This chip has 4 inputs, probably labeled A to D, and 7 outputs, probably labeled a to g, to light up the appropriate segments of the display. There may be additional inputs and outputs for additional functionality. You should choose a decoder chip that corresponds to your display; i.e. if you're using a common-anode display, then use a common-anode decoder. If you're using a common-cathode display, then use a common-cathode decoder. Otherwise you'll need to have inverters on each of the lines between the decoder and the display.

Dealing with enables, gates, strobes, etc.

Many chips which have logic in them also have pins which control overall function of the device, such as whether the outputs should function or not. These pins are usually called enables, gates, or strobes. To determine whether such pins exist, look at the truth table. See if there are any places where the outputs are not what you expect. See which inputs control this.
After having found any of these pins, usually you would tie them to power or ground so that the chip functions as expected.
In this lab, you will use a pushbutton switch to control one of them to show the effect.


Parts list

Wiring a Decoder with I/O

In this lab you will wire a BCD-to-7-segment decoder with a 7 segment display. The schematic diagram will not be given; you will need to figure out your circuit layout and how to connect the circuit components.

  1. Examine the pinout of the NES-5011, (alternate link), 7-segment display used in the lab before you start wiring.
    Since this device doesn't have a notch at one end to indicate pin 1, look at the data sheet to figure out which pin is pin 1. Then figure out which pin corresponds to which input.

    7-segment display pinout

    Write the pin designations on the pinout sheet that looks like this image to help you with your wiring.

    Your display has a common cathode that requires the driver (decoder) to provide a high-level voltage to activate a segment. The common pin is connected internally and only needs to be grounded from one side.

    Does the datasheet actually use the phrase "common-cathode" anywhere? If not, how can you figure out which type it is? (Hint: Look at the diagrams of the circuitry inside the chip, and then look back at the earlier handout about LEDs.)

    Since each segment of the display is an LED, you'll need to use resistors just as you would for discrete LEDs.

    Since all of the LED segments are connected to the common pin, you could use a single resistor between the common pin and power or ground, (depending on whether the display is common anode or common cathode), to limit the current to all of the LEDs. The only drawback with this is that you might notice the display is dimmer when more segments are on.

    (Hint: If you choose to use individual resistors, you can use the resistors to connect the decoder to the display, instead of having wires to connect the decoder to the resistors and wires to connect the resistors to the display then you will save a lot of wiring.)

  2. Put the 7 segment display and resistor(s) at one end of the breadboard.
    This is the output module.

  3. Use a wire to power or ground as needed to test each of the 7 display segments . (As long as the resistor(s) for the 7 segment display are in place, this should be fine.) (You don't have to test the decimal point(s).)

  4. Put the pushbutton switch (with its resistor) at the other end of the breadboard. Wire it for active LOW operation.
    This, along with the debugger board, is the input module.
    Be sure to daisy-chain the breadboard and the debugger board, rather than using several clip leads or having clip leads holding more than one wire.

  5. Using the debugger board, test to see that when the pushbutton is pressed, the output is LOW, and when it is unpressed, the output is HIGH.

  6. Examine the datasheet for the 74LS48 seven segment display decoder .
    Notice that the 74LS48 has 3 "extra" pins; LT, BI, and BI/RBO. Look at the device truth table to figure out which of these pins are inputs, and whether the ones which are inputs need to be tied high or low.

    Is this decoder designed for common cathode or common anode displays? Does the datasheet actually use the phrase "common-cathode" anywhere? If not, how can you figure it out? (Hint: Look at the truth table for a number, such as "zero" and see whether the appropriate segments are "high" or "low".) Is this correct for the display you have, or will you need to invert all of the inputs?

  7. Put the 74LS48 in the middle of the breadboard, and use the debugger board for the BCD inputs to the decoder.
    Use the pushbutton switch for LT.
    Tie the other "extra" inputs high or low, as appropriate.

  8. Connect the input, logic, and output modules and verify that the 7-segment display show the correct output for decimal numbers from 0 to 9.
    See what happens when you press the pushbutton. Do you understand the purpose (and the name) of the LT input?

Demonstrate the circuit to the lab demonstrator before you leave.


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