The SN54/74LS47 are Low Power Schottky BCD to 7-Segment Decoder
/Drivers consisting of NAND gates, input buffers and seven AND-OR-INVERT
gates. They offer active LOW, high sink current outputs for driving
indicators directly. Seven NAND gates and one driver are connected in pairs
to make BCD data and its complement available to the seven decoding
AND-OR-INVERT gates. The remaining NAND gate and three input buffers
provide lamp test, blanking input / ripple-blanking output and ripple-blanking
input.
The circuits accept 4-bit binary-coded-decimal (BCD) and, depending on
the state of the auxiliary inputs, decodes this data to drive a 7-segment display
indicator. The relative positive-logic output levels, as well as conditions
required at the auxiliary inputs, are shown in the truth tables. Output
configurations of the SN54/ 74LS47 are designed to withstand the relatively
high voltages required for 7-segment indicators.
These outputs will withstand 15 V with a maximum reverse current of
250 mA. Indicator segments requiring up to 24 mA of current may be driven
directly from the SN74LS47 high performance output transistors. Display
patterns for BCD input counts above nine are unique symbols to authenticate
input conditions.
The SN54/74LS47 incorporates automatic leading and/or trailing-edge
zero-blanking control (RBI and RBO). Lamp test (LT) may be performed at any
time which the BI /RBO node is a HIGH level. This device also contains an
overriding blanking input (BI) which can be used to control the lamp intensity
by varying the frequency and duty cycle of the BI input signal or to inhibit the
outputs.