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

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

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:

Illegal instruction, need new system
No Sync
Bus Error
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

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)

Remove the SMT resistor from pads 4
Put the SMT resistor on pads 2

Remove the SMT resistor from pads B
Put the SMT resistor on pads D

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
1678 Shattuck Ave. Suite #247
Berkeley, CA 94709
with a note, and we will send you a set.

A few notes:

The PPC 601 chip has a normal heatsink on it, and a fan blows over it. This appears to be quite adequate for this modification.
The gestalt for this new configuration, 12, is listed as a "Power 120" by the modified System 7.5 Software that comes with the Power 100. If you try to install the Apple System 7.5 Update, your machine will no longer be recognized, and it will not boot. You will need to apply the Output Enablers' System 7.5 Updater Patch before you do the modification.
I have observed no serial, video, floppy, adb, or SCSI problems, and MacEKG reports no hardware anomalies.
Take a look at the Speedometer 4.0 Files for the Power 100. You will notice that the video performance for the 100MHz machine with the 1:2 bus to processor ratio is substantially better than the 112.5MHz machine with the 1:3 ratio. But other than that, the 112.5MHz setup seems to be the way to go. I guess if video is super important, then you might want to go the 100MHz 1:2 route.

schrier@mac.com