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LAB NOTES: Apple Beige G3 MB Jumper Settings



Note: This information is provided for reference only. Overclocking of course voids any warranty, and can cause problems if the cpu or cache can't handle the higher speed. If the jumpers are set incorrectly, your system will not boot. If you boost the bus speed, you'll need faster memory than the original PC66 rated dimms.

(This page was original posted in 1998, shortly after the Beige G3s were introduced and updated in December 1999 for 7.5x and 8x ratio settings)

The first person to come up with information on PowerMac G3 overclocking, to my knowledge, is Takashi Imai. He's also come up with information on several other machines, including the PowerBook G3. His web site links are as follows:

(NOTE: Twice I have changed URLs to match changes to the sites below - and now the site has moved again (or no longer on the web). If you find the URL to the current site, let me know.-Mike)

http://www.bekkoame.ne.jp/~t-imai/g3ae1.html (PowerMac G3)

http://www.bekkoame.ne.jp/~t-imai/g3be1.html (PowerBook G3)

http://www.bekkoame.ne.jp/~t-imai (Homepage)

If you go through his homepage, you'll find some links to photographs of some beautiful islands, as well as information on overclocking other Macintosh models.

Now, on with the show!

The G3 motherboard has nine jumper positions. #1 is at the front of the board, and is marked by a nearby silkscreened "1". The factory puts in a one-piece jumper block covering all the positions. In order to do any modifications, you must remove a piece of tamper-resistant tape from the block, remove the block itself, and put in your own set of jumpers (you'll need the small kind with 2mm pin spacing, not the normal 0.1" spacing). take a note of the original jumper positions in case you make a mistake and have to revert back to them.

The lone exception to having to remove the jumper block is that if you have a G3/233, and only want to go to 266, it works out that all you need do is short jumper position 2 -- the rest of the jumpers are the same as before. Since the tamper proof sticker doesn't cover position 2, you can take a piece of wire, bend it in a U, and insert it into the upper surface of the jumper block at position 2. Officially, you don't have a warranty any more, but unofficially, they probably aren't going to notice if you take the wire out before returning it for service.

Jumpers 1-4 set the PowerPC 750 clock multiple. The 750 multiplies the system bus clock to generate its internal operating frequency. From the factory, the 233 MHz PowerMac G3 uses a system bus running at 66 MHz, and multiplies it by 3.5 to get 233. The 266 uses the same system bus speed with a 4x multiple. (If you look at the table, you'll see why it's only necessary to short position 2 to go from 233 to 266.)

(S = Jumper present or "set", " " = no jumper)



Multiple | 1 | 2 | 3 | 4 |
---------+---+---+---+---+
3x       |   | S | S | S |
---------+---+---+---+---+
3.5x     |   |   |   | S |
---------+---+---+---+---+
4x       |   | S |   | S |
---------+---+---+---+---+
4.5x     | S |   |   |   |
---------+---+---+---+---+
5x       |   | S |   |   |
---------+---+---+---+---+
5.5x     |   | S | S |   |
---------+---+---+---+---+
6x       |   |   | S |   |
---------+---+---+---+---+
6.5x     | S |   | S |   |
---------+---+---+---+---+
7x       | S | S |   | S |
---------+---+---+---+---+
7.5x     | S | S | S |  |
---------+---+---+---+---+
8x       |   |   | S | S |
---------+---+---+---+---+

Jumpers 5 and 6 control the MPC106 PCI to system bus multiple. The MPC106 is the center of the PowerMac G3; it implements the PCI interface, controls the SDRAM, and talks to the CPU. It also generates the system bus clock by multiplying the PCI clock. Although the MPC106 supports 1x, 2x, 2.5x, and 3x, the two jumpers will only allow you to select 2x and 2.5x. Here are the settings:



Multiple | 5 | 6 |
---------+---+---+
2x       | S | S |
---------+---+---+
2x       | S |   |   (do not use)
---------+---+---+
2.5x     |   | S |
---------+---+---+
2.5x     |   |   |   (do not use)
---------+---+---+

These jumpers also select the MPC106 VCO mode. The VCO is a part of the MPC106 PLL circuit which multiplies the PCI clock to generate the system clock. The entries marked "do not use" configure the VCO for operation with PCI frequencies of 25 MHz or less. Since the PowerMac G3 always operates the PCI bus between 30 and 35 MHz, we want to use the other VCO mode, selected by the other two entries.

Finally, jumpers 7, 8, and 9 select the PCI clock speed. It turns out that only certain combinations of PCI speed and PCI to system bus multiple are possible, so I show the multiplier setting in the following table as well. (I'm reasonably certain that the following is a complete table for working settings of jumpers 5-9.)
NOTE: If you're increasing the system/memory bus speed - PC66 RAM may not run reliably! If you're determinted to do this - PC100 RAM should be used. If the PC66 dimm has 10ns chips or faster, it may be OK at higher speeds. (Note: I bought a PC66 DIMM in fall 1998 and it appears to have 9ns chips on it by the chip marking. It ran fine at 83mhz bus speeds, but your mileage may vary-Mike)


PCI speed | Multiple | System Bus Speed | 5 | 6 || 7 | 8 | 9 |
----------+----------+------------------+---+---++---+---+---+
30 MHz    | 2.5x     | 75 MHz           |   | S || S | S |   |
----------+----------+------------------+---+---++---+---+---+
31.5 MHz  | 2.5x     | 78.75 MHz        |   | S || S |   | S |
----------+----------+------------------+---+---++---+---+---+
33.33 MHz | 2x       | 66.67 MHz        | S | S || S |   |   |
----------+----------+------------------+---+---++---+---+---+
35 MHz    | 2x       | 70 MHz           | S | S ||   | S | S |
----------+----------+------------------+---+---++---+---+---+
30 MHz    | 2x       | 60 MHz           | S | S ||   | S |   |
----------+----------+------------------+---+---++---+---+---+
33.33 MHz | 2.5x     | 83.33 MHz        |   | S ||   |   |   |
----------+----------+------------------+---+---++---+---+---+

Note that one of the settings slightly overclocks the PCI bus, to 35 MHz (the PCI specification limits legal bus speeds to 33.33 MHz). This probably won't result in anything bad, it's just something to keep in mind. (Not all PCI cards may tolerate a faster PCI bus speed-Mike.)

My current setting is 33 MHz PCI, 83 MHz system bus, 292 MHz CPU. To get this, I use the 3.5x PowerPC 750 bus multiplier (jumper 4 set) and the 33/83 PCI/system bus setting (jumper 6 set). The best absolute CPU speed I've gotten is 300 MHz, but that particular clock rate is only possible with a bus clock of 66 MHz.

For those of you who started out with a 266 MHz CPU, it might be interesting to try 35/70 multiplied by 4.5 for a CPU speed of 315 MHz. From the first table, you'd set jumper 1 for the 4.5x multiple. From the last table, you'd set jumpers 5, 6, 8, and 9 to select 35 MHz PCI / 70 MHz system bus.

Note that frequencies may vary slightly from machine to machine.
- Tim


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