SN54/74LS156

The LS155 and LS156 are Dual 1-of-4 Decoder/Demultiplexers with common Address inputs and separate gated Enable inputs. When enabled, each  LS155 and LS156 decoder section accepts the binary weighted Address inputs (A0, A1) and provides four mutually exclusive active LOW outputs (O0O3). If the Enable requirements of each decoder are not met, all outputs of that decoder are HIGH.

Each LS155 and LS156 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 LS155 and 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 toAND the minterm functions by tying outputs together. Any number of terms can be wired-AND as shown below.