BCD - to Decimal Decoder Driver

Student Information

Part 1. Objectives

• To learn 8 - 4 - 2 - 1 BCD Code
• To verify the operation of a BCD - to - decimal Decoder

Part 2. Discussion

In 8 - 4 - 2 - 1 BCD code, the decimal numbers 0, 1, 2, 3,..., 9 are represented in weighted binary form. There are six representations which are not used in this code, namely, 1010, 1011, 1100, 1101, 1110, and 1111. Thus, there is an error if any of these six numbers appears in a system using 8 - 4 - 2 - 1 code.

Changing a BCD number to its equivalent decimal number requires the use of a decoder with four inputs and ten outputs. One decoder for each digit of the decimal number to be decoded is needed. An example IC is the 7445 which is a BCD - to - decimal decoder/driver.

Part 3. Equipment and parts required:

• 1 BCD - to - decimal decoder/driver, 7445
• 1 indicator lamp, no. 49(a VOM may be used if no lamp is available)
• 1 power supply, 5 V dc
• 1 VOM
• 1 resistor, 47 ohms, ¼ W

Part 4. Procedure

1. Read the description on the 7445 data sheet. Note the required power,
Vcc: Icc:
2. Determine the proper pin connections and power to a 7445.. note the output levels with all inputs unconnected.
Vout0: Vout1: Vout2: Vout3: Vout4: Vout5: Vout6: Vout7: Vout8: Vout9:
3. Connect a lamp to pin 1, apply the BCD code 0000 to pins 15, 14, 13, and 12 (0 = 0 Voc), and note the lamp lights. Move the lamp to the other 9 outputs to determine that the lamp will indeed light only on pin 1 for this given input. Use a VOM if no lamp is available.
4. Verify the proper operation of the 7445 by applying the inputs given in the data sheet truth table and observing the corresponding truth - table outputs.
5. Measure the actual value of Von at 1 of the outputs.
Von:
6. Apply each of the 6 illegal input combinations to the 7445, and observe the resulting output signals.

Part 5. Questions

1. List all illegal condition for the 7445

2. What occurs at the output of the 7445 if 1 of the illegal is applied?

3. By examining the 7445 data sheet, can you determine the approximate input threshiod?

3. The 4 inputs to the 7445 will usually be driven by another electronic circuit. This circuit must be capable of sinking current from the 7445 to ground when the input level is a logical 0 (+0.8 V dc). Refer to the schematic diagram on the 7445 data sheet. What maximum allowable resistance can be connected between the input and ground if the voltage at the input is not to exceed +0.8 V dc?
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