74139

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.

       ,---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
       `-------'

Each side computes the following function:

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

• Philips 74HC139 datasheet