!pr1
A Different Patch for 65C02 & Old Apples...William O'Ryan Jr.

Since my earlier letter (Jun 84) on the 65C02 and the Apple II+ I was interested and gratified to read Andrew Jackson's (Dec 84) and Jim Sather's (Mar 85) letters on the same subject.  However, two things began to worry me.  First, the smallness of the time gain in the F257 chips (around 7 nanoseconds, I understand).  That did not seem enough to be very reliable.  Second, a friend in town has an Apple whose speed was not sufficiently improved to allow the 65C02 to work (although there was some noticeable improvement).

After reading the first few chapters of Jim Sather's book, "Understanding the Apple II", I was able to come up with a new solution.  As I figure it, this new solution yields an improvement of around 70 nanoseconds, more than enough.  Simply put,  just replace the -RAS line inputs to the 74LS174 chips at B5 and B8 with AX.  AX rises 70 nsec earlier than -RAS, enabling those chips to latch RAM output 70 nsec earlier.  It is a simple patch and may be done either with or without altering the motherboard.

I tried it first without altering my motherboard, on a Rev 44-1 Apple using 200 nsec 16K RAM chips. I was surprised to see it work, as I had expected that 200 nsec RAM chips would be too slow for the patch.  (I haven't tried it yet with 250 nsec RAM chips.)  Actually, this particular Apple did not need any speed-up -- the 65C02 was already working in it.

To do this patch:  remove the chips at B5 and B8; seat an extra socket under each of them; pin 9 on these sockets should be bent out so they do not go into the motherboard sockets; remove the chip at C2 and put an extra socket under it; connect a wire from pin 14 of the C2 socket to the bent out pins 9 of B5 and B8.  Pin 14 of the 74S195 at C2 is a source of the AX signal; pin 9 of B5 and B8 was previously connected to -RAS.


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I have another Apple (Rev 4) which has 24 150 nsec 64K RAM chips (using the Cramapple mod).  This Apple already had F257's in it with a 65C02.  I put the old LS257's back in, and sure enough the 65C02 began to stumble.  Then I removed the motherboard and on the underside cut the trace to -RAS and soldered in a jumper wire to pin 14 of C2.  It worked perfectly!


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Naturally those who try any of these patches do so at their own risk.

I must thank Jim Sather for his book; it was only by studying the timing diagrams in that book and staring at the circuit diagram published by Apple that I was able to do this.  I hope some of the hardware types will be able to tell me if I have built a time bomb.  I am also very interested to hear whether the problem with the 65802 is the same.





Jumper wires

Pins 9 not
plugged into RAS

View from top front

74LS195

74LS174

74LS174

AX


Underside of
motherboard
viewed from rear

Jumper wire

    Cut
trace here

AX

RAS

RAM
