7402: Difference between revisions
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11 -- /ROMSEL | 11 -- /ROMSEL | ||
12 -- 12 | 12 -- 12 | ||
13 (11+12) -- 08, 09 = ROMSEL | 13 (11+12) -- 08, 09 = ROMSEL AND CPU R/W | ||
08 -- 13 | 08 -- 13 | ||
09 -- 13 | 09 -- 13 | ||
10 (08+09) -- PRG /OE = /(ROMSEL | 10 (08+09) -- PRG /OE = /(ROMSEL AND CPU R/W) | ||
Others -- GND | Others -- GND | ||
Latest revision as of 22:58, 14 December 2022
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 02 -- CPU R/W 03 -- CPU R/W 01 (02+03) -- 12 = /(CPU R/W) 11 -- /ROMSEL 12 -- 12 13 (11+12) -- 08, 09 = ROMSEL AND CPU R/W 08 -- 13 09 -- 13 10 (08+09) -- PRG /OE = /(ROMSEL AND 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
