Re: Burning EPROMs

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

On Tue, 17 Dec 2002 20:56:14 +0000, Michael J. Mahon wrote:

> Stephen Shaw replied:
> 
>>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
>>> 
>>> 
>>Thanks. I'll save this one for when I get a chance to tackle my APL
>>cards.
> 
> Stephen, I overlooked one pin:  pin 21 of the 2708 is -5v, and this pin is
> Vpp on the 2716, so the trace to this pin should also be cut and it should
> be grounded.
> 
> -michael
> 
> Check out amazing quality 8-bit Apple sound on my Home page: 
> http://members.aol.com/MJMahon/

OK added to the list :)