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Wrangling an Apple ADC Studio Display onto a G3 Powermac DVI (Part 1)


Veteran Member
Apr 24, 2009
Canberra, Australia
One of Apple's worst-ever design cul-de-sacs was the Apple Digital Connector (ADC). Apparently their design teams sit around a kitchen table to brainstorm, and sometimes take too much coffee or other stimulants.

After the industry standard for Digital Video Interface (DVI) was well established, Apple decided it was not good enough for Apple. The full DVI-I standard has 24 pins in a 8x3 matrix, plus 4 pins grouped around a crossed blade slot. The blade is common ground, the 4 adjacent pins are analog RGB and horizontal sync. The 8x3 matrix carries two sets of TMDS DV signals plus clock and various sensing signals.

Apple wanted their Display series monitors to sit alone on a desktop, with the boxy host somewhere out of sight. Keyboard and mouse would connect via the monitor's USB ports. The USB hub also identifies the monitor itself as a USB device and enables video controls via buttons located on the monitor. 24volt power and the USB signals were to be crammed into a single connector and cable, to reduce desktop clutter in pursuit of Apple purity.

ADC looks like a DVI connector but has six extra pins and reassigns some other pins.

24v DC power is fed to pins 11 and 12, with DC grounds adjacent on 1 and 2.

USB data is on 21 and 22, USB GND on 23.

Even though DVI includes a +5V pin (14), ADC has no USB +5vdc, because the monitor supplies power to its onboard USB hub stepped down from the 24v monitor power supply.

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Monitors up to 17" require only one TMDS signal set. Apple AGP/ADC Cinema Displays at 20" and above used both TMDS signal sets. I'm not aware of any product that ever used the analog signal channels on the ADC connector. (Please correct me if you know better.)

Apple dropped the ADC connector midway through its G5 range and reverted to standard DVI. Owners of ADC displays were outraged because good and expensive monitors were orphaned when they upgraded their hosts. You can't just swap in an ADC video card to the standard AGP slot, because the ADC power and USB signals were fed to the card from the motherboard through unique extensions to the AGP slot.

Apple put out a DVI>ADC converter that cost more than a generic DVI monitor and left a tangle of cables, yokes and adapter boxes on the desktop worse than if they had never messed with the DVI standard in the first place.


I found myself with a perfectly nice G3 PowerMac with AGP DVI video card, plus a couple of abandoned Studio Display monitors, 15" and 17", in good condition. I wanted to mate these to the G3 because they look happy together. As usual I started by searching the relevant keywords plus "DIY". I found several blogs by people who had built their own adapters. Usually they stuffed the adapter board into a mint tin and had a desktop littered with odd cables and at least two adapter boxes.

I'm no Jony Ive but I wanted a cleaner result. The G3 PowerMac tower has plenty of room inside. I decided to built my adapter to go inside the tower. It then made sense to mount the adapter on a PCI card mounting strip (not using the PCI bus). The ADC connector faces out, the inputs to the adapter are inside the tower case along with the 24v power source and most of the connecting cables. Here is the result:

View attachment 10222

This is how the back panel of the G3 looks, with the adapter in place:

View attachment 10224

This is the adapter, mounted over a PCI slot but connected only to the 3 cables for power, USB and DVI.

View attachment 10225

Component-side view of the completed adapter:

View attachment 10226

See Part 2 for the construction issues and solutions...
Wow - so it pushed >100watts through that ADC connector and cable. No reference to whether that CRT Studio display also carried the USB hub and ports that the LCD models had. I wonder how long they kept it in production.