!pr2
Comments on O'Ryan's 65C02 Mod for Apple II.........Jim Sather

William O'Ryan's method (October 1985 AAL) of modifying old Apples to accept 65C02s looks like a very reliable fix.  I notice no negative consequences in RAM or video timing.  I do however recommend switching to 150 nanosecond motherboard RAM.

Apple motherboard RAM read access is CAS' limited, meaning TCAC (delay from CAS' falling to read data valid) is the critical RAM chip specification.  In an Apple with O'Ryan's fix, RAM chips have 140 nsec minus 74LS139 pin 1 to pins 4,5,6 high/low propagation delay to get RAM read data valid after CAS' falls at the RAM chips.  This means TCAC needs to be 119 nsec or less with a typical LS139.  TCAC specifications are 100 nsec for 150 nsec RAM and 135 nsec for 200 nsec RAM, so 150 nsec or faster chips should be installed to be within RAM chip specifications with O'Ryan's fix.

A given Apple II may work with O'Ryan's fix and 200 nsec RAM chips, but operation may not be reliable over a wide range of room temperatures.  Again I say, O'Ryan's fix calls for 150 nsec RAM chips.  To operate with slower chips is asking for trouble.

Incidentally, 16K RAM chips don't cost as much as they used to.  The cheapest 150 nsec 16K RAM chips I can find in my current mail order catalogs are 45 cents apiece at Jameco Electronics, 1355 Shoreway Rd., Belmont, CA 94002.  [Slower ones were $65.00 apiece in 1978!]

As an alternative to replacing slow motherboard RAM chips, one can replace the 74LS139 at F2 with a 74S139.  This changes the TCAC requirement with O'Ryan's fix to 133 nsec for a typical S139, and to 130 nsec for a worst case S139.  These are barely less than the 135 nsec specification of 200 nsec RAM, so operation with 200 nsec RAM is probably reliable.
