7402

The 7402 (variants 74LS02, 74HC02) is a 74-series logic IC with four independent 2-input NOR gates.

7402 Pinout

        .--\/--.
  1Y <- |01  14| -- +5V
  1A -> |02  13| -> 4Y
  1B -> |03  12| <- 4A
  2Y <- |04  11| <- 4B
  2A -> |05  10| -> 3Y
  2B -> |06  09| <- 3A
 Gnd -- |07  08| <- 3B
        `------'

Signal descriptions

Y is low if either A or B is high, and is high if A and B are both low.

Bus Conflict Suppression Implementation

74HC02     -- Connection
----------------------------
+5V        -- +5V
2          -- CPU R/W
3          -- CPU R/W
1 (2+3)    -- 12 = /(CPU R/W)
11         -- /ROMSEL
12         -- 12
13 (11+12) -- 8, 9 = ROMSEL or CPU R/W
8          -- 13
9          -- 13
10 (8+9)   -- PRG /OE = /(ROMSEL or CPU R/W)
Others     -- GND

This is sometimes used in discrete logic mappers to prevent bus conflicts (e.g. ANROM). By combining /ROMSEL with CPU R/W, the PRG chip can have its output enabled (/OE) only during reads, preventing a conflict with its latch writes.

See: Cartridge connector, 74161