The SN74LS147 and the SN74LS148 are Priority Encoders. They
provide priority decoding of the inputs to ensure that only the highest
order data line is encoded. Both devices have data inputs and outputs
which are active at the low logic level.
The LS147 encodes nine data lines to four-line (8-4-2-1) BCD. The
implied decimal zero condition does not require an input condition
because zero is encoded when all nine data lines are at a high logic
level.
The LS148 encodes eight data lines to three-line (4-2-1) binary
(octal). By providing cascading circuitry (Enable Input EI and Enable
Output EO) octal expansion is allowed without needing external
circuitry.