74147 - 10-Line to 4-Line Priority Encoder

Overview

The 74147 is a high-speed priority encoder designed to convert nine active-low input lines into a four-line binary-coded decimal (BCD) output. This integrated circuit ensures that only the highest-order active input is encoded, providing reliable data prioritization in digital systems. The device features implied decimal zero encoding when all inputs are high (inactive), making it suitable for applications requiring BCD output generation from multiple input sources.

Key Features

• High-speed priority encoding of 9 input lines to 4-bit BCD output
• Active-low inputs and outputs for improved noise immunity
• Implied decimal zero encoding when all inputs are inactive
• Standard TTL compatibility for easy system integration
• Designed for reliable operation in industrial temperature ranges
• Low power consumption compared to discrete implementations
• Input priority ensures only highest-order active input is encoded

Applications

The 74147 priority encoder is widely used in digital systems requiring input prioritization and encoding:

• Keyboard encoding systems
• Digital control panels
• Interrupt priority controllers
• Data multiplexing systems
• Industrial control systems
• BCD conversion circuits
• Address decoding in memory systems
• Priority-based event detection systems

Electrical Characteristics

Absolute Maximum Ratings

• Supply Voltage (Vcc): 7V
• Input Voltage (Vin): 5.5V
• Operating Temperature Range: -55°C to +125°C
• Storage Temperature Range: -65°C to +150°C

Recommended Operating Conditions

• Supply Voltage (Vcc): 4.75V to 5.25V
• High-Level Input Voltage (VIH): 2V min
• Low-Level Input Voltage (VIL): 0.8V max
• Operating Temperature: 0°C to 70°C (commercial grade)

DC Characteristics

• High-Level Output Voltage (VOH): 2.4V min @ IOH = -0.4mA
• Low-Level Output Voltage (VOL): 0.4V max @ IOL = 8mA
• Input Clamp Voltage (VIK): -1.5V max @ IIK = -18mA
• Supply Current (ICC): 34mA max @ Vcc = 5.25V

AC Characteristics

• Propagation Delay (Input to Output): 33ns max @ CL = 15pF
• Input Capacitance: 5pF typical
• Power Dissipation Capacitance: 25pF typical

Functional Description

The 74147 implements a priority encoding function where the highest-numbered active input determines the output code. The device features nine active-low inputs (1-9) and produces a four-bit active-low BCD output (A-D). When all inputs are high (inactive), the output represents decimal zero (all outputs high). The truth table clearly shows the priority encoding scheme, with input 9 having the highest priority and input 1 the lowest.

Pin Configuration

16-Pin DIP Package

• Pin 1: Input 5 (active low)
• Pin 2: Input 6 (active low)
• Pin 3: Input 7 (active low)
• Pin 4: Input 8 (active low)
• Pin 5: Input 9 (active low)
• Pin 6: Output D (LSB, active low)
• Pin 7: Output C (active low)
• Pin 8: GND
• Pin 9: Output B (active low)
• Pin 10: Output A (MSB, active low)
• Pin 11: Input 4 (active low)
• Pin 12: Input 3 (active low)
• Pin 13: Input 2 (active low)
• Pin 14: Input 1 (active low)
• Pin 15: NC (No Connection)
• Pin 16: Vcc (+5V)

Package Information

Available Packages

• 16-pin Plastic Dual In-line Package (PDIP)
• 16-pin Ceramic Dual In-line Package (CDIP)
• 16-pin Small Outline Package (SOIC)

Physical Dimensions (PDIP)

• Package Length: 19.3mm max
• Package Width: 6.35mm max
• Lead Pitch: 2.54mm
• Lead Length: 3.3mm min
• Package Height: 4.57mm max

Design Considerations

When implementing the 74147 in a system design, several factors should be considered:

• Input pull-up resistors may be required if inputs are driven by open-collector devices
• Output loading should be limited to maintain proper voltage levels
• Bypass capacitors (0.1μF) should be placed close to Vcc and GND pins
• Signal integrity should be maintained for high-speed operation
• Unused inputs should be tied high to prevent floating input conditions

Alternative Components

For applications requiring similar functionality, consider these alternatives:

• 74LS147: Lower power Schottky version
• 74HC147: CMOS version with higher noise immunity
• CD40147: CMOS priority encoder with similar functionality
• 74F147: High-speed version with faster propagation times

Conclusion

The 74147 priority encoder provides a reliable solution for converting multiple input signals into a compact BCD output code. Its priority encoding feature ensures proper handling of simultaneous inputs, while its TTL compatibility makes it suitable for integration into a wide range of digital systems. With robust electrical characteristics and multiple package options, the 74147 remains a versatile component for digital encoding applications.