74139

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The 74139 (common variants 74LS139, 74HC139) is a dual 2-to-4-line decoder/demultiplexer in the 7400 series. Each side of the chip is a decoder with an active low enable from a 2-bit address to four active low signals. The two decoders can be wired independently, or a /Y from one decoders can be wired to the /E on the other to calculate more complex combinational logic functions.

74139 Pinout

       .---v---.
 1/E -o|1    16|-- Vcc
 1A0 --|2    15|o- 2/E
 1A1 --|3    14|-- 2A0
1/Y0 -o|4    13|-- 2A1
1/Y1 -o|5    12|o- 2/Y0
1/Y2 -o|6    11|o- 2/Y1
1/Y3 -o|7    10|o- 2/Y2
 GND --|8     9|o- 2/Y3
       `-------'

Signal descriptions

  • /E, A0, A1 are inputs
  • /Y0, /Y1, /Y2, /Y3 are outputs

Each side computes the following function:

/E A1 A0 /Y0 /Y1 /Y2 /Y3
0 0 0 0 1 1 1
0 0 1 1 0 1 1
0 1 0 1 1 0 1
0 1 1 1 1 1 0
1 x x 1 1 1 1

The NES contains a 74LS139 to produce the PRG /CE signal from M2 and A15.

References