Re: why address pin 14-15 tied for a 27c256 ROM ?

robb wrote:

> my microcontroller board has installed a 23256AC (i.e. 27c256
> style) ROM chip with 8031 MCU...
>
> I traced the ROM chip's pins to be (reasonably) sure that i was
> selecting a correct/compatible eprom chip setup for ***reading***
> this ROM chip on an e(e)prom programmer, as the programmer did
> not show direct sup****t for the chip.
>
> When tracing connections i found that (pin 1 - NC) on the chip
> was tied to (pin 27 - A14) and these  connected to (Addr 14) on
> the MCU
>
> is there a design reason for doing that, maybe prevent using a
> 27c512 chip for example ?
>
> so for different ROM  chips that means that
>
> Chip      Pin 1   ->   Pin 27    =    ?result/purpose?
> --------------------------------------------------------
> 23128     NC      ->  PGM        =    no prob
> 27c128    Vprog   ->  PGM        =    ???
>
> 23256     NC      ->  A14        =    no prob
> 27c256    Vprog   ->  A14        =    ???
>
> 27c512    A15     ->  A14        =    **???
> -------------------------------------------------------
>
> Does this mean i can not use a 27c512 chip ?
> or maybe it is a simple  way to prevent a floating address line ?

I forget the reason, it's been ages since I used an Eprom rather than
parts with internal flash memory but there was some good reason for it
IIRC.

Yes, it may stop a 27512 being used on that board.

Graham