IC7447 Integrated Circuit: BCD to 7-Segment Decoder Explained

The 7447 is a BCD-to-seven-segment decoder and driver for a common-anode display. Its seven segment outputs are active low and open collector. The 7447 uses a 16-pin package, not a 14-pin package. Viewed from the top, find pin 1 beside the notch, then count counterclockwise.

7447 BCD-to-Seven-Segment Truth Table

Use this table with LT, BI/RBO, and RBI inactive at a high level. A is the least significant BCD bit, and D is the most significant bit. ON means the active-low output sinks current and lights that segment.

What Is the 7447 IC? 

The 7447 accepts four BCD inputs and controls seven display segments. The input weights are A = 1, B = 2, C = 4, and D = 8. The decimal-point segment is not driven by this IC.

The 7447 and 7448 are not interchangeable display drivers. Their output polarity and intended LED connection are opposite.

Source: Texas Instruments BCD-to-Seven-Segment Decoders/Drivers datasheet.

How to Connect a 7447 to a Common-Anode Display

Connect VCC at pin 16 to the 7447 logic supply and pin 8 to ground. Connect the display's common-anode terminal or terminals to the positive display supply, and connect that supply's return and pin 8 to the same ground. Feed the four-bit code to A, B, C, and D. Connect each output a through g to the matching segment cathode through its own external current-limiting resistor. The 7447 does not provide these resistors internally.

TI document SDLS111 specifies common-anode displays, active-low outputs, and the open-collector sink stage. An active-low output turns its segment on by sinking current. An output in the off state does not source current like a normal active-high logic output. This is why copying a 7448 common-cathode circuit produces the wrong result. Source: Texas Instruments SN5447A and SN7447A datasheet.

Choose each resistor for the intended segment current using the display-supply voltage, the segment's forward voltage at that current, and the 7447's low-output voltage. Then verify that the current stays within both the display rating and the driver's per-output sink-current limit. The current-limiting resistor guide explains the resistor calculation.

Inputs 0 through 9 display decimal digits. Inputs 10 through 14 are not blank or undefined on the 7447. They produce the exact nonnumeric patterns listed in the truth table above.

7447 Lamp Test and Blanking Controls

LT, BI/RBO, and RBI are active-low controls. BI/RBO is the main gotcha: pin 4 is a bidirectional wire-AND node. It acts as a blanking input when driven low and as the ripple-blanking output during zero suppression.

Cascaded digits can use RBI and RBO for leading-zero or trailing-zero suppression. Treating pin 4 as an output-only RBO pin can make blanking behave unexpectedly.

7447 Electrical Ratings

Use the full part marking when applying electrical limits. The original SN7447A and the later SN74LS47 share the logic function and pinout, but their permitted segment sink currents differ.

These are operating limits, not targets. Segment current must also stay within the selected display's ratings.

7447 Wiring Checklist

The 7447 is a DIP-16 BCD-to-seven-segment driver for common-anode displays. Its a through g outputs are active-low open collectors. Match each numbered output pin to the correct segment instead of assuming alphabetical pin order.

Keep LT, BI/RBO, and RBI high for ordinary decoding. Remember that BI/RBO is bidirectional and that inputs 10 through 14 produce defined nonnumeric patterns.

For a complete counter application using this driver, see the frequency-counter circuit guide.