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?

  • 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. – Chromableed Studios Feb 22 '13 at 3:57
up vote 2 down vote accepted

enter image description hereenter image description here

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)

  • "bridge the lifted pin 27 to Pin #28 of the '28" exactly what I was looking for, thanks! – Chromableed Studios Feb 13 '13 at 3:35
  • May I ask is there significance to the blue color of the wire? Pardon my noobness if this is basic. – Chromableed Studios Feb 13 '13 at 3:57
  • 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. – placeholder Feb 13 '13 at 4:13
  • Loved the blue wire reference from ancient times! – Anindo Ghosh Feb 13 '13 at 5:04
  • 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 – Chromableed Studios Feb 13 '13 at 14:53

Your Answer

 
discard

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.