Lab 6: Shift Register

Introduction

The objective of this lab is to demonstrate the operation and characteristics of bipolar integrated circuit shift registers. A shift register is a device that inputs data loaded serially into a basic memory unit (flip-flop of latch) and then shifts this information to an adjacent storage element, leaving the original memory unit open for additional input data.

There are a large number of shift register types and configurations. Shift registers can be made from TTL standard flip-flops or can be packaged for specific operation. In today's lab we will use a standard J-K flip-flop to creat a general purpose shif t register. We will also investigate a specific shift register device wired in various configurations (shift right register, shift left register, write/recirculation shift register and a Johnson counter).

Equipment needed

Figure 1

Procedure

Part 1--J-K flip-flop shift right shift register

  1. Construct the four bit shift register shown in figure 1. Shift pulses for this circuit will be derived from logic switch A. Logic switch B is connected to reset the shift register via the asynchronous clear inputs on the flip-flops. The serial input to the shift register will come from data switch SW1.

  2. Apply power to the circuit. Reset the shift register by actuating the B logic switch. Set the data switch SW1 to binary 1 position.

  3. Using the A logic switch, apply 4 shift pulses. Observe the LED outputs as you do. Record the binary value of the register contents after four pulses.

  4. Repeat step 3 with SW1 set to binary 0.

  5. Using the SW1 data switch and the A logic switch, load the shift register by using the procedure below.

      SW1 = 0, depress switch A
      SW1 = 1, depress switch A
      SW1 = 0, depress switch A
      SW1 = 1, depress switch A

    Observe the LED indicators and record the decimal equivalent of the binary number in the shift register.

    There are actually two decimal numbers represented by the binary number in the shift register. Which bit is the LSB?

Part 2--74LS95 MSI shift register--shift right and shift left

  1. Remove the J-K flip-flop shift register from the breadboard and construct the circuit shown in fiqure 2. Use the TTL handbook to verify the correct pin connections for the 74LS95.
    (This shift register was discussed in class; refer to your lecture notes on it's capabilities and drawbacks) Recall this device can provide both parallel and serial inputs dependent on the mode switch.

  2. First we test the parallel operation. Apply power to the circuit. Disregard the LED indicators states at this time. Set all the data switches to binary 0. Then set the B logic switch and hold it in the set position. Then momentarily set the A logi c switch high. Record the output of the LED indicators.

  3. Set all of the data switches to binary 1 and repeat step 2. Record the output of the LED indicators.

  4. Depress the A logic switch four times and note the LED indicators states. After you have applied four shift pulses, record the LED indicator states.

  5. Modify the 74LS95 shift register as shown in figure 3. Notice that the outputs L2-L4 are feedback to the parallel inputs. In this configuation the mode switch will select either shift right of shift left operations. What is the binary state of the mode control input to perform the shift left operation?

  6. Set data switches SW1, SW2 and SW4 to the binary 0 state. Apply power to the circuit and set the A logic switch four times. Record the state of the LED indicators.

  7. Set switch SW1 to the binary 1 position. Set the A logic switch four times. Note the direction of the shifting as the shift pulses are applied. Record the output after four pulses.

  8. Set the SW1 to binary 0. Again set the A logic switch four times. Note the direction in which the data shifts. Next, set SW2 and SW4 to binary 1. Apply four shift pulses with A logic switch. Again note both the direction of shifting and the conte nts of the shift register.

  9. Set the SW4 switch to binary 0 and apply two shift pulses. Note the direction of shifting and record the LED indicator states.

Part 3--Recirculation shift register

  1. Modify The 74LS95 shift register as shown in figure 4. This configuration the shift register contents to be recirculated.

  2. Apply power to the circuit and set SW1 to binary 1 and SW4 binary 0. Set the A logic switch four times. This should clear the register. Next, set SW4 to binary 1. Set A logic switch two times and record the binary number stored in the register.

  3. Set SW1 to binary 0. Set A logic switch 4 times, recording the register contents after each pulse. Make a table of the output.

    Is the data in the shift register lost or retained as a result of shifting the data? Explain how you would clear the data from the shift register?

    Notice that the mode control input lines lets you put in data from an external source. In this circuit that data can be shifted out of the register and used externally as well as recirculated so that it is retained for another operation.

Part 4--Johnson counter
  1. Modify the 74LS95 shift register as shown in figure 5. This configuation is called a shift right Johnson counter.

  2. Apply power to the circuit. Set the A shift input switch until the shift register contains all 0's. Starting with all zero's (0000), set the A logic switch and record the new contents of the shift register. Repeat this procedure recording the shift register contents after each A logic switch actuation. Tabulate all this data.

  3. 3. Connect the A logic switch to a 1 Hz clock and observe the output states.

    Make a timing diagram of a Johnson counter.

Figure 2 


     
    
Figure 3
    
 
    

     
    
Figure 4
    
 
    

     
    
Figure 5