DESCRIPTION
If the Enable requirements of each decoder are not met, all outputs of that decoder are HIGH. Each decoder section has a 2-input enable gate. The enable gate for Decoder “a” requires one active HIGH input and one active LOW input (Ea●Ea). In demultiplexing applications, Decoder “a” can accept either true or complemented data by using the Ea or Ea inputs respectively. The enable gate for Decoder “b” requires two active LOW inputs (Eb●Eb). The LS155 or LS156 can be used as a 1-of-8 Decoder/Demultiplexer by tying Ea to Eb and relabeling the common connection as (A2). The other Eb and Ea are connected together to form the common enable. The LS156 can be used to generate all four minterms of two variables. These four minterms are useful in some applications replacing multiple gate functions as shown in Fig. a. The LS156 has the further advantage of being able to TRUTH TABLE ADDRESS ENABLE “a” OUTPUT “a” A0 A1 Ea Ea O0 O1 O2 XXL XHHH XXXHHHH L LH L LHH HLHLHLH L HH L HH L HHH L HHH H = HIGH Voltage Level L = LOW Voltage Level X = Don’t Care AND the minterm functions by tying outputs together.
onsemi
