7402: Difference between revisions

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m (→‎Bus Conflict Suppression Implementation: does capital AND make its logical function clearer?)
 
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Line 24: Line 24:
  11        -- /ROMSEL
  11        -- /ROMSEL
  12        -- 12
  12        -- 12
  13 (11+12) -- 08, 09 = ROMSEL or CPU R/W
  13 (11+12) -- 08, 09 = ROMSEL AND CPU R/W
  08        -- 13
  08        -- 13
  09        -- 13
  09        -- 13
  10 (08+09) -- PRG /OE = /(ROMSEL or CPU R/W)
  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