Date   : Sun, 29 Mar 2009 10:30:33 +0100
From   : jumbos.bazzar@... (Mark Haysman)
Subject: A quick question about EPROM programming

>> I suppose I have a related question, can I drop in a 27512 for example, 
>> tell
>> the programmer it's a 27128 and expect it to work like one? Obviously I
>
> To be safe you'd have to program both 16Ks of the EPROM with the
> same data as you wouldn't know which half would get selected once
> it was in the computer. Most programmers that can program 16K
> EPROMs can also program 32K ones, often you program each 16K half
> at a time. If the programmer doesn't support programming 32K
> EPROMs, you can do it manually with a wire link.

That's fine for 27256s, but 27512s have 4 banks of 16K, and the Vpp is moved 
to Pin 22, so it's unlikely to work (I never say never, as there's always 
someone that's done it!). 27256s only work because of a fluke in the way the 
/CE signal is also the /PGM signal on that size of EPROM . The problem with 
using a 27512 in the Model B is that Pin 1 in the socket is not connected to 
anything, as it's the Vpp pin in the smaller ROMs. You would have to tie 
that either up or down manually to prevent spurious accesses to different 
locations on the ROM. Even if you program all 4 banks the same, some ROMs 
you may find will be okay with that, whilst it will upset others. Even 
better, tie it to Pin 12 of the LS163 paging register, then you can have 2 
different ROM images in the 27512. To use all 4 banks, cut the link between 
pin 27 and 28 under the motherboard, and connect pin 27 to Pin 11 of the 
LS163.

> Drop the EPROM into the ZIF socket, but ensure pin 27 does not go
> into the socket. Connect a wire link between the free pin 27 and 0v
> to program the bottom 16K, and to +5v to program the top 16K. *DO*
> *NOT* put a wire link into pin 27 of the ZIF socket as when
> programming a 16K EPROM that pin has 12v/21v at it.

Pin 27 is the /PGM pulse, and is only a TTL level signal, the Vpp is Pin 1 
on a 27128/27256, and pin 22 on a 27512. But yes, still leave it not 
connected.

Mark.