Re: Burning EPROMs

[mjmahon@aol.com (Michael J. Mahon)]

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/