Power 100 Speedup


Well, here's the scoop on clock chipping the Power Computing Power 100's.

The Power 100 is based on a PowerPC 601+ processor rated at 100MHz. The Bus runs at 33.3MHz giving a Bus to Processor ratio of 1:3. This is the same as the Apple PM8100/100 and 110 machines. The signal is generated by a Motorola clock generator/PLL part # PC955 located at U12 on the motherboard. This part is "set" by the presence/absence of resistors on 8 sets of resistor pads on the board. These pads are located on the top side of the mother board on the left, front side. I will refer to the pads as 1 2 3 4 and A B C D since they are not all labeled.

My Notation Board Notation
1 2 3 4 R171 R191 R280 R4
A B C D       R6

These resistors can be changed to give 75, 90, 100, and 112.5MHz machines, and the Bus to Processor ratio can be set to 1:2, 1:3, or 1:4 in many cases. The potential for 50 and 60MHz machines exists, but I've been unable to tweak the System 7.1.2 or 7.5 software to allow me to get the machine up at these speeds.

The table below tabulates all of the possible resistor configurations. The resistors used are surface mount 0805 5% resistors. Pads 1, 2, 3, and 4 take 10K Ohm resistors; they are marked (103). Pads A, B, C, and D take 1K Ohm resistors; they are marked (102). 1 and A are matched sets, so a resistor need only be on pads 1 or A. The same follows for 2, 3, 4 and B, C, D respectively. The gestalt is a number reported by the computer that signifies the type of computer it thinks it is. (see the table just below this one)

1 2 3 4 A B C D Gestalt Bus (MHz) Processor (MHz) Bus:Processor Note
        102 102 102 102   25 50 1:2 I
103         102 102 102 55 50 100 1:2  
  103     102   102 102 65 25 75 1:3  
    103   102 102   102   20 60 1:3 I
      103 102 102 102     30 60 1:2 I
103 103         102 102 12 37.5 112.5 1:3  
103   103     102   102 55 25 100 1:4  
103     103   102 102   55 33.3 100 1:3  
  103 103   102     102 55 30 90 1:3  
  103   103 102   102   55 45 90 1:2  
    103 103 102 102     65 37.5 75 1:2  
103 103 103         102   1.5 6 1:4 II
103 103   103     102   12 28 112 1:4  
103   103 103   102       56 112 1:2 III
  103 103 103 102       55 22.5 90 1:4  
103 103 103 103                 IV

    Notes:
  1. Illegal instruction, need new system
  2. No Sync
  3. Bus Error
  4. No signal

Gestalt Apple Computer Notation PowerComputing Notation
12 Power Macintosh 8100/120 Power 120
55 Power Macintosh 8100/100 Power 100
65 Power Macintosh 8100/80 Power Macintosh 8100/80


The Modification:

(for 112.5MHz operation) That's it!
If you want to experiment further, and you cannot find these surface mount resistors, send a Self Addressed Stamped Envelope (SASE) to:
Output Enablers
871 Industrial Rd., Suite K
San Carlos, CA 94070
with a note, and we will send you a set.

A few notes: