Steve Chamberlin Gets to Work on the Ultimate Vintage Macintosh to VGA Monitor Adapter

Engineer and vintage computing enthusiast Steve Chamberlin has designed a handy device for the vintage Apple collector, converting the video output of classic Macintosh machines for use with a VGA monitor using a Texas Instruments LM1881 video sync separator.

"In the late 1980s when Apple released its first Macintosh models supporting external color monitors, the company made some design choices that continue to cause trouble even today," Chamberlin explains. "Computers like the Macintosh IIci supported 640×480 video resolution, the same resolution as the VGA standard that was common in the PC world, but they used a different physical connector for the monitor cable, a different vertical refresh rate, and a different method of encoding sync information. It’s those sync differences that have proven to be most problematic over the years."

Physically converting between the two-rows-of-pins DA-15 port used on a Macintosh and the three-rows-of-pins DE-15 used on VGA monitors should, in theory, be a straightforward job if it weren't for difference in signalling. Where VGA uses Pin 13 and 14 to provide horizontal and vertical sync signals respectively, the Macintosh has horizontal, vertical, composite, and sync-on-green signals — the latter two providing an XOR between horizontal and vertical sync and the same signal mixed into the green color channel respectively.

"So what exactly is the Mac video sync standard? Does it output separate HSYNC and VSYNC, or CSYNC, or sync on green? The answer is yes," Chamberlin explains, "the Mac generates all of those sync standards at different times, depending on what type of monitor it thinks is connected. But it doesn’t generate all of them all of the time. The video hardware checks the voltages on the connector’s three sense pins, which form a 3-bit monitor ID for the connected monitor type."

While contemporaneous adapters which offer laborious manual configuration via DIP switches exist, Chamberlin decided to solve the thorny mess an easier way: with a Texas Instruments LM1881 video sync separator chip. "I set out to build a Mac to VGA adapter with an integrated LM1881 that would enable my Macintosh IIci to work on [my] three fussy monitors," Chamberlin writes. "I would use the LM1881 to extract VSYNC from CSYNC, passing the VSYNC to the VGA monitor, and I would pass CSYNC to HSYNC with my fingers crossed."

The resulting adapter proved compatible with two of the three monitors Chamberlin had previously tried with the machine, showing a good-quality image on older CRTs but a "lousy" image on a more modern LCD — and nothing at all on another LCD. It's a good first effort, though, and Chamberlin already has plans for improving the output and building everything into a single adapter — which, he admits, will likely be "awkwardly large."

Chamberlin's full write-up is available on his website, Big Mess o' Wires.