Re: Burning EPROMs

["Stephen Shaw" <bottrawler@junkbuster.zzn.com>]

On Mon, 16 Dec 2002 21:11:22 +0000, Michael J. Mahon wrote:

> Stephen Shaw wrote:
> 
>>On Sun, 15 Dec 2002 16:13:45 +1300, David Empson wrote:
>>
>>> Herbert Fung <lemonherb1@netscape.net> wrote:
>>> 
>>>> Hi, I was wondering if there is any specific type of EPROM one would
>>>> have to get to burn repacements for various Apple II related cards.
>>> 
>>> It depends on the card.  Older cards (or simpler ones) might use EPROMs
>>> as small as the 24-pin 2708 (1KB).  The 24-pin 2716 (2KB) and 2732/2532
>>> (4KB) were also commonly used on older cards.
>>
>>2708s are obsolete and most EPROM programmers that can handle the 2716s
>>upwards do not seem to handle the 2708 (different programming voltage?).
>>I have a lot of Epson APL printer cards minus their 2708s :(
> 
> You can substitute the much more popular 2716, perhaps with one circuit
> mod.
> 
> The only difference in pinout is that:  1) pin 18 is the "program" control
> line on the 2708, but is /CE on the 2716.  If this is kept at logic low by
> the card, it's fine.  2) Pin 19 is Vdd (the negative supply) on the 2708,
> but is A10 for the 2716.  You will probably need to cut the trace to this
> pin and ground it.  This will result in locking the EPROM to the lower 1KB
> of its address range, which is where the firmware should go. 
> (Alternatively, you could tie pin 19 to A10 on the card bus connector, and
> have a 2KB EPROM space to play with. ;-)
> 
>>> More recent cards tended to use larger EPROMs even if they didn't need
>>> the extra space, because the EPROMs were easier to burn and cheaper.
>>> The 28-pin 2764 (8KB) and 27128 (16KB) were used on several cards,
>>> including the original Apple SCSI card.  The 27256 (32KB) may have been
>>> used on cards that needed a lot of firmware (with a significant amount
>>> of bank switching or an onboard CPU).
>>> 
>>> It is usually possible to replace a 28-pin EPROM with one of larger
>>> capacity, but it depends on the signals that are connected to the extra
>>> address pins.  To be on the safe side, you should try to use a
>>> replacement ROM of the same type as the original.  If you only have a
>>> larger capacity ROM available, then you have the best chance of it
>>> working by duplicating the contents of the smaller ROM into both halves
>>> (or all quarters or eighths) of the larger ROM.
> 
> -michael
> 
> Check out amazing quality 8-bit Apple sound on my Home page: 
> http://members.aol.com/MJMahon/


Thanks. I'll save this one for when I get a chance to tackle my APL cards.