Low End Mac’s Compleat* Guide to Cards for the iMac’s Mezzanine Slot

When Apple designed the original iMac‘s system board, it left a connector marked “mezzanine” without explaining its purpose. The official explanation is that it was used for testing logic boards before installing them in the iMac’s swoopy case. But once people saw prototype iMacs with video output, the cat was out of the bag and people knew you could do more with it than Apple had thus far let on. The mezzanine connector could be used to expand the iMac!

Truth be told, Apple never documented the mezzanine slot (a.k.a. PERCH slot) and told developers that it was completely unsupported. However, it survived the first two generations of iMac before disappearing when the iMac Rev. C shipped in January 1999, along with the infrared (IrDA) port that only existed on the Rev. A and B iMacs.

iMac mezzanine slot

Mezzanine slot on original iMac G3 logic board.

Despite Apple’s warnings and prohibitions, a few cards were developed for the “secret” expansion slot. We have done our best to compile a complete list, although in the 20 years since the iMac was introduced, some of them may have been forgotten.

Over time, the mezzanine port was documented by users, and the pinout is available on Applefritter. It apparently provides a 32-bit PCI bus running at 33 MHz or 66 MHz and uses a Molex 52760-1609 connector. (The PCI Mezzanine Card is an industry standard defined in IEEE P1386.1. The iMac appears to implement that standard.)

Game Wizard Video Card

Micro Conversions created an iMac version of its popular PCI Game Wizard accelerated video card, which sold for $199. The card had Voodoo2 graphics and 8 MB of video RAM. There wasn’t room in the cramped iMac for the same 12 MB in the PCI version of the video card.

Based on feedback in the Low End Mac group on Facebook and elsewhere online, those who had one loved what it did for video gaming.

iMac Gets SCSI

The German company Formac developed iPro RAID, a $100 Ultra Wide SCSI card for the mezzanine slot. It used an uncommon slim SCSI 3 connector because of the limited space in the iMac and the small opening in the side of the computer.

Formac also produced iPro RAID TV, a $149 card with SCSI, a TV tuner, and S-video output.

Not Quite Mezzanine Cards

Griffin iPort

The little Griffin iPort gave the iMac a traditional Apple serial port and video out. It doesn’t use the mezzanine connector but it does use the opening on the iMac that’s used by mezzanine cards.

Sonnet HARMONi and Other CPU Upgrades

Sonnet was a force to be reconned with in the Mac upgrade market, and its HARMONi not only gave tray-loading iMacs a faster CPU – 500 MHz and 600 MHz G3 options – but added a FireWire port as well. The FireWire port poked through the slot where something in the mezzanine slot would, but it doesn’t use the mezzanine slot and is compatible with all four revisions of the original iMac design.

FastMac, Newer Tech, and PowerLogix also made CPU upgrades for tray-loading iMacs, and several of those included G4 CPUs at speeds up to 500 MHz. All of these companies required you to send them your CPU daughter card either so they could create the upgrade or after you did the upgrade so they could build one for the next customer.

That’s All Folks!

According to the sources I have been able to locate, these were the only three cards made for the mezzanine/PERCH slot in the 233 MHz iMacs.

All of these cards are quite rare and usually sell for premium collector prices on eBay.

There have been other cards for the PCI Mezzanine Card slot, but they have all been designed to work in x86 PCs, not PowerPC Macs, so they are not usable in these iMacs.

* No, it isn’t a typo. Compleat is a legitimate, albeit archaic, spelling for complete. As Kenneth G. Wilson says in The Columbia Guide to Standard American English: “This obsolete spelling of the adjective complete suggests an air of antiquity that seems to please some of those who name things….” We find that fitting for Low End Mac’s Compleat Guides to “obsolete” hardware and software.


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