I've got a dead Apple IIe with suspect EF and and CD ROM's
342-0134
342-0135
I read somewhere that these can be replaced with 27C64 ROM's. Are they 27C64 ROM's which can be read with the programmer to confirm if they are valid?
I've tried reading them as M27C64 ROMs but pin 1 on both fails and if I ignore that it returns 0xFF for all addresses.
So first off what chips are these and can they be read with a moderm programmer, and can they be replaced with a 27C64?
According to this site they are 8k x 8 (27C64) so suprised I can't read them. I can't imagine they are both faulty?
http://www.applelogic.org/APPLEASICs.html
Seems the TL866 has problems reading these chips. Tried my T48 programmer and it reads both chips successfully and they are both fine :)
The motherboard chips use the Synertek part number 2365A, factory-programmed ROMs, not EPROMs. James Sather describes them and discusses compatibility in the opening pages of chapter 6 in Understanding the Apple IIe.
There are a few families of ROMs and EPROMs that can be installed in those sockets. And there are potentially distinct settings for reading the original 2365A's in an EPROM programmer
Socket compatibility: in addition to the original 2365A, the motherboard will accept both 2764 and 27C64 EPROMs with any letter suffix.
EPROM reader compatibility: to read Apple's original ROMs use the 2365 or 2365A setting on your EPROM reader. If your reader doesn't list any variations of "2365" then try "2764" or "2764A" (without a "C" in the number).
Don't use part numbers with a "C" in the middle. Manufacturers inserted the "C" for CMOS EPROMs with extended programming features, activated by putting special voltages on pin 1. Your EPROM programmer probably doesn't recognize Apple's original ROMs as "27C64" because the Synertek 2365A doesn't respond to the extended features of a 27C64.
In post #3, 'S.Elliott' wrote:
" Don't use part numbers with a "C" in the middle. Manufacturers inserted the "C" for CMOS EPROMs with extended programming features, activated by putting special voltages on pin 1. Your EPROM programmer probably doesn't recognize Apple's original ROMs as "27C64" because the Synertek 2365A doesn't respond to the extended features of a 27C64."
Uncle Bernie adds a warning:
Anyone attempting to read out any original Apple ROM on any EPROM programmer is well advised to turn off all the "Device ID" features of the programmer before reading the ROM. Myself, I even go further and disable the "continuity" check, so when I read ROMs, the programmer (I have a BP1200) does only read, but never applies any other tricks.
As 'S.Elliot' correctly stated, some EPROMs have special functions which are enabled by applying so-called "supervoltages" on certain pins (not always pin #1). These supervoltages, when applied to the same pins on ROMs, may damage the ROM and make them unreadable.
DON'T FALL INTO THIS TRAP !
- Uncle Bernie