7402: Difference between revisions
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== Signal descriptions == | == Signal descriptions == | ||
Y is low if either A or B is high, and is high if A and B are both low. | 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 conflict]]s (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 [[74161|latch]] writes. | |||
See: [[Cartridge connector]], [[74161]] | |||
[[Category:7400 series|002]] | [[Category:7400 series|002]] | ||
Revision as of 22:51, 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 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
