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I am looking to replace a 27C256 compatible maskRom (nintendo Nrom PRG and CHR if it matters) with a socketed 28C256 in order to make a development cart.

It looks like the pin for A14 is in the wrong place and will need to be wired to hole 27

/WE(pin 27) is active low on the 28C256

Can I just float pin 27(leave unconnected)? Or would I connect /WE to +5v?

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  • \$\begingroup\$ Just as a follow up after some experimenting and circuit tracing, if you limit the scope to nrom128 rev4 prg for example (which only uses 16K of the 28c256) the A14(pin27) is already connected to pin 1 and 28 on the board and the 28c256 can be socketed without any extra wires, I guess since the highest address line wouldn't be needed. \$\endgroup\$
    – Chromableed Studios
    Feb 22, 2013 at 3:57

1 Answer 1

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Comparing them side by side you can see the differences:

  1. The '27 has: a. pin #1, Vpp which in circuit is not used (either Gnd or Vcc) b. pin #27, A14 which is a driven address line.
  2. The '28 has: a. pin #1, A14 is a driven address line. b. pin #27, /WE Which must be tied high in circuit.

When you place the '28 in circuit you must:

  • Lift pin #1 of the '28 so it does not contact the Vpp pad

  • Lift Pin # 27 of the '28 so that it does not contact the A14 Pad

  • solder or join or bridge the lifted pin 27 to Pin #28 of the '28 (tying /WE high)

  • connect the PAD at Pin location #27 on the board with a blue wire to the lifted pin #1 of the '28. (connecting the address line)

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  • \$\begingroup\$ "bridge the lifted pin 27 to Pin #28 of the '28" exactly what I was looking for, thanks! \$\endgroup\$
    – Chromableed Studios
    Feb 13, 2013 at 3:35
  • \$\begingroup\$ May I ask is there significance to the blue color of the wire? Pardon my noobness if this is basic. \$\endgroup\$
    – Chromableed Studios
    Feb 13, 2013 at 3:57
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    \$\begingroup\$ You know in those situations where the terrorists always follow the standard wiring code? "careful McGyver, ONLY cut the red wire"? Same thing here ... Nope, it's just called blue wiring because in the wirewrap days SOME companies reserved certain colors for repairs and fixes. \$\endgroup\$
    – placeholder
    Feb 13, 2013 at 4:13
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    \$\begingroup\$ Loved the blue wire reference from ancient times! \$\endgroup\$
    – Anindo Ghosh
    Feb 13, 2013 at 5:04
  • \$\begingroup\$ Oh! I get it! encyclopedia2.thefreedictionary.com/blue+wire I like their definition for red wire--probably closer to what I'm doing x.x Thanks again great answer \$\endgroup\$
    – Chromableed Studios
    Feb 13, 2013 at 14:53

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