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The Past, Present and Future issues of G3 CPU Card Upgrades
(Originally Published in 1998)

Updates: In 1999, G3 CPUs were made with higher bus/cpu ratios (10x) which allow up CPU speeds of up to 10 times the bus speed. The original article was written when only 8x (max) ratio G3 CPUs were available. For reviews of 10X G3 CPU uprades (and even G4 CPU upgrades) check the reviews section of the CPU Upgrades page. You can also search the Rate Your CPU upgrade database for owner reports on most every brand of upgrade, searchable by Mac model/series.

(Fall 1998 Update) For a review of the first shipping 400MHz CPU Card upgrade, read my XLR8 G3/400 Review. Info on this card and Phase 5's new cards have been added to the Pros/Cons summary below. This card was plug and play at 50MHz bus speeds/400MHz CPU speed in both a Genesis (9500 based) and 9600/350 Mac. I did not have such good luck with the PowerForce G3/366 however. It failed to boot at 45.67MHz in the 9600, possibly due to a bad card. It has been returned to PowerLogix for investigation.

Many readers have asked for my opinions on a CPU reseller's G3 400MHz upgrade card comments page [the dealer later removed the article from their site, so the link was removed] that talks of possible stability problems that may occur in these new CPU Card upgrades. Apple G3 systems run a faster bus (66MHz), so their ZIF CPU upgrades do not have these limits and as I'll explain later, are excluded from the problems discussed here. Since it is impossible for me to cover this subject in depth in email I decided to post this page with complete coverage of the entire issue.

Rest assured when I can obtain a 400MHz CPU card upgrade I'll test it in all the Macs here to see what issues, if any, arise. My Macs (8600, 9600, PowerTower Pro, PowerCenter Pro, and two Genesis (9500 based) models) are loaded with ram, and expansion cards and should make for a good workout. I think I have the widest array of video, scsi and hard disk configurations of any test site on the net (I should, I've spent $30K since April 97) and I also have licensed versions of most every professional package on the market (from Photoshop to Lightwave 3D).

The information here will be of benefit to any Mac owner looking to purchase a G3 CPU card upgrade, regardless of speed. Forgive the length of this article, I've tried to cover all aspects of the issues involved to educate those readers who are less experienced with the background of hardware developments that lead us to to where we are. I've tried to organize this page into topic areas so that more knowledgeable readers can skip sections.

I decided rather than comment again on the news page that a more permanent page was a better approach and would allow a more complete explanation of where we are, where we have been and where we are going. I'll also include a description of the two contrasting approaches taken by companies in the past and what I have found to be the most and least tolerant systems regarding bus speeds with G3 cards - the core of the real high speed G3 CPU issue. Bus speeds on machines that came before Apple's G3s have long been the source of arguments, hype, myths and often real world disasters in the past and now has come back again to haunt us in our future quests for faster processor speeds.

The (Potential) Problem with 400MHz G3 CPU Slot Cards:


  1. G3 CPU has a max ratio of 8X
  2. A 400MHz G3 therefore requires a 50MHz bus speed
  3. Many Macs have been unable to run greater than 45MHz bus speeds with the current array of G3 cards

Why We're Where We Are Now:

As CPU speeds have continued to increase at a much faster rate than the mass production, affordable RAM technologies, the gap between memory/system bus speeds and CPU speeds have increased manyfold. In the days of the 33MHz processor and 16MHz bus, only a 2X ratio was needed. Now with 400MHz (and higher) processors reaching the market and memory bus speeds (on the Mac) often running at 40-50MHz it's obvious that higher ratios are needed for these CPUs to run at their rated speeds. It seems the G3 designers didn't count on the longevity of our Macs, as they provided only a 8X maximum bus to CPU ratio in the design. I'd hoped that later G3 models might have as high as a 10X ratio setting to compensate but that didn't happen. I suspect 10X will be included in the G4 models if they are to reach their planned speeds in the future.

Basic Explantion of Bus Speeds, Ratios and Why They Matter:

The CPU card determines the motherboard bus speed of your system. The CPU is set to run at a multiple of the bus speed. When the rated speed of the CPU exceeds the clock speeds possible by the highest ratio setting times the maximum possible bus speed - you've hit a brick wall limit on CPU speed that cannot be addressed without a) faster memory bus speeds or b) higher bus to CPU ratios in the CPU design.

Original CPU card designs used a fixed speed oscillator chip running at the bus speed, and the CPU had it's ratio connection fixed at some set value. For instance on an 8500/150 there was a 50MHz oscillator and a fixed 3X CPU/Bus ratio for a 150MHz CPU speed. The original 604 CPU had a 3x ratio, the 604E (180-250Mhz) had up to a 6X ratio setting, the Mach 5 604E (250-350MHz design) has up to 10x settings as they knew the Mac models of the day were generally 50MHz bus speed designs and planned for future models. Production and further development of the Mach 5 design was halted however and replaced by the G3/750 CPU, which is based on the 603e CPU. The G4 CPU (based on the 604e) will soon replace the G3 as the highest performing PowerPC chip. Its performance is said to be much higher than the G3, and address some performance issues with the G3 such as the lack of load/read pipelining and MP support.

As I mentioned here awhile back and I'm sure most speed fanatics know, the G3 CPU chip has an 8X max ratio, so with the low bus speed maximums of most of the current crop of G3 CPU cards (generally 45MHz), that's a top CPU speed of 360MHz (8*45). However, note that some Newer Tech cards currently do have a 50MHz bus setting which some readers have used with success. Therefore it may be possible, with timing changes to the current designs, to make 50MHz bus speeds reliable in many Macs. Only time and real world tests will tell. But the card design is only part of the equation. Your Mac's motherboard design, L2 cache, mix of RAM and even ASIC chips (Application Specific Integrated Circuit) can also be a make or break factor. As I'd commented to several readers last year, I'd hoped the designers of faster CPUs would have provided higher ratios, but so far, with the current line of G3 CPUs, that's not the case.

Since higher ratios than 8X are not available in even the 400MHz G3, these cards will need a 50MHz bus speed just to run their rated CPU speed. That means unless some timing changes are done in most of the cards out there, they are likely to have problems with reliable operation in many Macs, even with matched dimms and with or without a fast motherboard cache. Newer Tech's G3 card may be the exception, as some of them have run 50MHz reliably. I'd hoped my 9600/350 (Kansas motherboard has no cache ) would run higher bus speeds reliably but in the only test so far I had data corruption (as reported at the site) on a PF 300/300 running at 47.5MHz bus speeds during a CDrom to HD large folder copy. The PowerComputing machines (RIP) seem better able to handle faster bus speeds, and my early PowerForce 250 is running fine with a 60MHz bus in my PowerCenter Pro.

As I commented in my first G3 CPU card review last October, the G3 cards seemed far more sensitive to bus speed timing issues than the 604E cards. In the same 8500 that ran 55MHz bus speeds rock solid with a 604E card, my first G3 CPU Card would not run 100% reliably at sub-50MHz speeds. Possibly vendors could add wait states or modify designs to address this but that remains to be seen.

The bottom line is that I fear many Macs will be limited to running 50MHz bus speeds (or lower) given the current G3 card designs. Even assuming that 50MHz was reliable that means the top CPU speed would be 400MHz at best. The smaller ZIF CPU Card of the Apple G3 systems is a much simpler, more reliable design, with fewer parts and shorter circuit paths. It also doesn't require the voltage conversion components of the older CPU card Macs. The newer motherboards used with the ZIF CPU Cards are designed for 66MHz and faster bus speeds and avoid the whole issue of the bus speed/ratio limit as even at the stock 66MHz speed, they could handle CPU speeds as high as 533MHz (66.66MHz x 8 ratio).

Bus Speeds: A little history of the G3 cards:

A bit of history may help explain where we are now. The first G3 Cards released were the Newer Tech MaxPowr Pro (250/125)/Pro+ (275/166) [reviewed here last year] and Powerlogix PowerForce 250/250. The Newer Tech cards had a maximum of 50MHz bus speeds and limited settings but generally ran well even in the Apple Macs. The first PowerLogix PowerForce 250 card production run had a bus speed range of 40 to 70MHz, but as reported in my review most of those settings were not usable in Apple Macs. However, I was able to run it in a PowerCenter and PowerCenter Pro at 60MHz and greater bus speeds (with the stock 1MB cache installed!)

Why Did PowerLogix Lower the Max Bus Speed of the Early PowerForce?

First off, these are my words and logic--not those of Powerlogix. However I base this on conversations with owners, companies and simple common sense. The first PowerForce G3 allowed bus speeds up to 70MHz to be selected, however for most Mac owners most of those settings (sometimes all but two or three) were not usable. Especially in the case of Apple brand Macs, they would usually support bus speeds no faster than 45MHz. The net result was that the card had only two or three working CPU speeds, since the range of bus speed settings below 45MHz was only two or three as I remember. There were 15 settings of bus speeds and spreading them over a 30MHz range meant each step was 2MHz. With a ratio setting of 7X for instance that meant the CPU speeds increased in 14MHz steps. With such a coarse increment and few bus speeds, the resulting range of CPU speeds was severely limited in many Macs.

The next PowerForce production model had 45MHz as the top bus speed for several reasons--

  • it was generally the maximum reliable speed most Apple Macs would run.
  • it offered the optimum memory bus speed according to many experts like Newer Tech.
  • it resulted in a lot less tech support calls, more usable CPU speeds and more satisfied customers.

With 15 settings and the 40-45MHz range you had much finer control of bus speed settings (every 0.33MHz). This also meant you had finer control on CPU speed increments in a search for the maximum reliable speed the card would run. Maybe now you can understand why Newer Tech takes the less complicated road of having only a few settings instead of separate bus and ratio settings--although limiting in some cases, it's less confusing and results in far lower tech support costs.

Which Systems Are Most & Least G3 CPU Tolerant?

Based on my experience and those of the many readers that have written (a small sample is shown here) here is a summary of the bus speed limits in some of the more popular Macs (worst and best bets). Again this is based on current G3 cards, not any new designs or changes that my be present in the new models :

Most tolerant of G3 Bus speeds:
  • PowerCenter Pro
  • PowerCenter

Least Tolerant of G3 bus speeds:

  • 7500
  • 8500
  • 9500/Umax S900/Genesis

Here's a few details on why I listed the above:

PowerCenter/PowerCenter Pro:
The PowerCenter Pro was designed for a 60MHz bus speed (with some headroom - I've seen 64Mhz with a 604E card and the stock cache!). The lack of memory interleaving and basic motherboard design of the original PowerCenter made these same speeds possible in many cases. In fact, that same first model PowerForce 250 has run 300/200/60 fine with the stock L2 cache in my PowerCenter Pro and ran 65MHz bus speeds with no motherboard cache dimm in a PowerCenter 150. These are a few of the reasons I miss Power Computing, but I digress.

Apple 7500

The infamous Val4 motherboard issue may be a myth, but there is no denying that most 7500 owners with a PowerForce G3 card reported a fast cache is needed for even sub-45mhz speeds. There are some amazing reports however from PF 220/110 owners that are very happy with their systems.

Apple 8500

Most owners don't achieve even 45MHz bus speeds but it is possible (I've done it). Mine also ran 50MHz bus with a Newer Tech G3 card (matched ram, aftermarket cache). With today's crop of cards and a mix of RAM don't expect that to be the norm however. Most owners report running at lower bus speeds, either due to bus speed problems or to keep the CPU speed at the maximum reliable speed. (This topic is too long to cover here, but basically with CPU speed is primary to performance and since the current cards in most cases are limited to 40-45mhz bus speeds it's better to sacrifice a few mhz in bus speed when it may allow a higher ratio and therefore higher CPU speed).

Apple 9500/UMax S900/Genesis

Although the stock motherboard cache was designed to run at least a 50MHz bus with the 604E (9500/200), as noted the G3 cards are much more finicky. These owners take more of a risk with G3 upgrades as they can't remove or upgrade the motherboard cache (it's soldered in). Newer Tech does allow disabling the cache via a control panel and in general I've recommended that brand for 9500 owners as the least risky.

Of 4 Powerforce design G3 cards I've tried in my Genesis, two did not work at the same bus speeds as the other two (less than 44MHz bus), which shows there is some variation in identical design cards from different mfrs. Granted I refused to deinterleave RAM as a attempt to solve the problem since I have all slots full (matched dimms, 60NS Macgurus RAM) and I didn't consider that acceptable for this loaded system's intended purpose. As I say in the reviews - there are so many variables involved somtimes only trial and error can determine how well the card will work in your specific system. A XLR8 266/177 currently hums fine in there at 300/200/45. I was impressed with that card so much I bought it after the review. Of course two weeks later the 220/110 cards arrived at half the price , and they often top 300 MHz. I think the world waits until I buy to lower prices 50%.

Summary of G3 CPU Card History:

The Past: Wide bus speed setting model had too few speeds that were usable for most Macs (spread too wide and contained too many settings impossible to use). The only other model had only two or three settings.

The Present: Most cards limited to 45MHz bus speed max (a few have 50MHz, one 55MHz but reliability is unproven). You often have to choose between a model is lower in cost, has more overclocking potential and therefore *possibly* higher speeds, but compatibility issues (see the FAQ) or ones with limited speed settings, less potential CPU speed and higher costs (but less compatibility problems).

Current Choices:

Powerlogix Powerforce designs (also sold by Bottom Line, original Mactell cards [they have a new design now] and original XLR8 [they also have new design cards now] ). [Update: The new XLR8/400Mhz cards are a new design, far more stable at higher bus speeds than any card I've seen to date - see my Review of the XLR8 400 for more details. Initial tests with the PowerLogix G3/366 were disapppointing, but I could have had a bad card. It failed to run at 45.67MHz in the 9600/350]

  • Pros:
    • Generally lower cost
    • Many speed settings
    • 40-45MHz bus speed range
    • Ability to run CPU faster than rated speeds (many settings)
    • Many models are good values based on reported speeds obtained
    • Powerlogix brands maintain warranty regardless of speeds used
    • Large number of mfrs and resellers means more competition and lower prices
  • Cons:
    • Compatibility issues (see FAQ question 31)
    • Adjustability can be confusing for some buyers
    • Potential to run the CPU beyond reliable speeds
    • No capability to disable motherboard cache via software
    • May require either removing or upgrading motherboard cache for stability
    • Manuals on some brands not as well written as competing models

Powerlogix seems to take the approach of pushing the edge and is the favorite card design of risk takers and those that try to squeeze every bit of performance from their Macs. If the compatibility issues don't affect you these cards can be outstanding values for the money.

Newer Technology designs (also sold by Techworks under license).

  • Pros:
    • Software allows disabling motherboard cache
    • Excellent compatability with other hardware and software
    • Good reputation for stability
    • Easier installation due to small number of settings
    • Good manual
    • Readily available from a large number of resellers
  • Cons:
    • Higher cost than similar Powerforce models ($100 or more typically)
    • Limited CPU and bus speed range (but higher bus speeds in some models)
    • Overclocking not warrantied (most cards have one setting to overclock the CPU, most pronounced on the 220/110 model which has a 275MHz setting)

Sonnet Technologies

Since I have not tested these cards and have little feedback on them I can't comment on their bus speed sensitivities or performance. They are a major supplier of Nubus G3 upgrades (and other older Mac upgrades) and owner reports on them are in the G3 Survey Results pagesG3 Survey Results pages.

Vimage (Interware).

  • Pros:
    • Excellent compatability reported (Adaptec SCSI cards, etc.)
    • Owners report easy installation and reliable operation
  • Cons:
    • Slightly higher cost than Powerforce models
    • Overclocking not possible
    • New design with little product history
    • US warranty/repair facility?
    • Less retail channels means less price competition on this model
    • Too new and too few in use for a large sample of owner feedback

XLR8's new design (their 333Mhz-400MHz cards). I and several other readers have had excellent results with the new XLR8 400MHz G3 card at 50MHz bus speeds. It ran plug and play in both a 9600/350 and a Genesis (9500 based) system that was loaded with RAM and PCI Cards. One reader just wrote the card also worked flawlessly in his 8500. Here's a summary of the new 400MHz XLR8 card but see my review for more info.

  • Pros:
    • Able to run 50MHz bus speeds reliably based on current testing.
    • Excellent performer
    • Readily available from a large number of resellers
  • Cons:
    • Early testing shows the Adaptec SCSI card issue (wide drives don't mount) issue is still present. XLR8 hopes to solve this with a revision to the control software.
    • Retrospect backups seem to work fine in OS 8.5 with Virtual Memory off based on limited testing but disabling SCSI manager 4.3 was needed in OS 8.1 to allow backups to complete and verify (see my Retrospect Workarounds page for details). XLR8 said disabling the cache during backups was also another workaround until they released their revised control panel which they hope will resolve the issue.

There are other models like Phase 5's line but there is too little data to draw any conclusions. The above represents 98% of the generally available cards to the mass market. Update: 10/26/98: A reader reports good results with a Phase 5 Maccellerate G3 CPU card and 50MHz (and above) bus speeds:

" Hi,
Regarding your article on xlr8yourmac, I want to add a bit of data. I recently bought a Phase5 Maccelerate! 300MHz card to put it into my PowerMac 7500 (VAL-4 motherboard). It worked right out of the box with the standard 300 MHz CPU/150 MHz backside cache/50 MHz bus setting perfectly. I have no cache in the motherboard L2 cache slot, but tested the card briefly with the original Apple 256k L2 card, with which the Maccelerate! also worked perfectly, albeit a little slower. Changing the card to 300 MHz CPU/200 MHz backside cache/50 MHz bus also works perfectly for me. When I change the setting to 302.5 MHz CPU/ 151.25 MHz backside cache/55 MHz bus, the computer still runs without crashing, but I get black pixels on the video, so I reverted to the safe 300/200/50 MHz setting. I bought the card after getting a strong encouragement from somebody else who also runs this card in a stock 7500 with a VAL-4 motherboard.
Hanno Wirth "

The Future: Design changes are needed for many cards to run 50MHz bus speeds that are required for 400MHz CPU speeds (again we're talking non-Apple G3 systems here). Since I suspect most 333 and 366MHz card owners will be running their CPUs overclocked, this makes the potential market for 400MHz CPU cards is even smaller. Many buyers will ask 'Why pay for a 400MHz when a 333 or 366MHz equivalent might run the same speed?' (assuming the 333/366 models have the same bus speed range as the 400MHz versions and therefore the possibility to run as fast). That's *if* these cards can run anywhere near 50MHz bus speeds. It will be interesting to see how well the designers meet this challenge. With the G4 CPU to be released later this year, there may be a narrow market lifespan of these cards (if the G4 is affordable plentiful). Based on the track record of new high performance CPUs, I expect them to be very high prices and in short supply initially. Remember the first G3 250/125 models that sold for $1500?

My advice? Relax, enjoy your Mac and maybe pick up a bargain as the current models drop lower in price when the new higher speed cards arrive. Higher speed models and competiton have made the $1500 cards of last fall sell for under $400 today. That's sort of rapid change was unheard of even 18 months ago. I still remember paying $589 in Feb. of 1997 for a 180MHz 604e card which was considered an incredible bargain at that time - so low in fact compared to the competition that I had some other dealers tell me there was no way a company could sell a card at that price and make a profit.

Don't Worry - Be Happy

Instead of preaching gloom and doom we all should be thankful in many respects. The G3 CPU Upgrades have many of these older machines running at speeds even Apple never dreamed. I've gotten many reports of older Macs running 300MHz or faster with a G3 card that costs under $400 now (220/110 models). Recently Vimage has given new life to the 54/64xx Macs and Starmax 3000/4000 series by providing G3 CPU upgrades via the L2 cache slot. Powerful upgrades have never been so affordable.

MacCPU does make a valid point on the G4, it may be available early next year in CPU Card upgrades, but I wonder what the cost will be. The market can't support more than the $1900 that the latest 400MHz G3s are listing for in my opinion, as by that time you may be able to buy a G4 equipped new system for a bit more.

What keeps the CPU Upgrade market alive is the fact they are often more affordable than a new system, less headaches to switch over (remember that new machines often bring their own problems) *and* Apple has no 6 slot Mac, new models are often limited in other ways (expandability, PCI slots, RAM slots, power supply rating, expansion bays, compatibility issues, etc.). What faster G3 CPU cards have done is drive down the cost of lower speed models to the point they are affordable for a larger portion of the market. A 220/110 card is a great value for many owners that can't afford a new machine, or have a lot of money invested in FPM RAM that can't be used in the Apple G3 or that need more than the 3 PCI slots the new Macs provide.

I've tried to show in my G3-ZONE and Mach 5 (9600/8600) pages that even the older models, properly equipped can provide practically identical *real world* applications performance as even the high end new models. Apps test scores shown show they are closer than the hype would have you believe in many typical applications if the machines are properly equipped with sufficient RAM.

The Challenge:

The 50MHz+ bus speed design challenge may serve as a true test of the engineering mettle of the current crop of vendors. I'm sure they are testing for compatibilty in most of the popular Macs and it would be folly to do otherwise. If there is a big enough market and demand for a 400MHz G3 card then I'm betting someone will do it right. This may force some innovation in the current designs.

I for one have a loaded Genesis machine that I hope will be a home for one of these new breed of CPU Upgrades (either fast G3 or a G4). I've got too much money invested and it's far too nicely configured to abandon for the current crop of plastic flexy fliers with one drive bay free and a 171w supply. With a wide Cheetah boot disk, internal 4-drive striped RAID array, dual PCI SCSI cards, capture card, dual video cards and a 3Dfx card I've yet to see a machine that could replace it for overall capability.

Stay tuned for results when the new wave of cards arrive. I have a wide array of Macs to test these new cards in - a PowerTower Pro, PowerCenter Pro (60mhz bus), 8600/300, 9600/350, and two different 9500 motherboard based Macs (two Genesis systems built from parts). And I can assure you these systems are loaded - never less than 128MB, and most with 256MB or more (one Genesis has 768MB of ram and all PCI slots full).

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Last Revised: 11/13/98

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