Engineer and classic computing fanatic Steve Chamberlin has designed a useful machine for the classic Apple collector, changing the video output of traditional Macintosh machines to be used with a VGA monitor utilizing a Texas Devices LM1881 video sync separator.
“Within the late Nineteen Eighties when Apple launched its first Macintosh fashions supporting exterior colour displays, the corporate made some design selections that proceed to trigger hassle even right now,” Chamberlin explains. “Computer systems just like the Macintosh IIci supported 640×480 video decision, the identical decision because the VGA normal that was frequent within the PC world, however they used a distinct bodily connector for the monitor cable, a distinct vertical refresh charge, and a distinct methodology of encoding sync data. It’s these sync variations which have confirmed to be most problematic over time.”
This “huge mess o’ wires” is the beginning of a common VGA show adapter for classic Apple Macintosh machines. (📷: Steve Chamberlin)
Bodily changing between the two-rows-of-pins DA-15 port used on a Macintosh and the three-rows-of-pins DE-15 used on VGA displays ought to, in idea, be a simple job if it weren’t for distinction in signalling. The place VGA makes use of Pin 13 and 14 to offer horizontal and vertical sync alerts respectively, the Macintosh has horizontal, vertical, composite, and sync-on-green alerts — the latter two offering an XOR between horizontal and vertical sync and the identical sign blended into the inexperienced colour channel respectively.
“So what precisely is the Mac video sync normal? Does it output separate HSYNC and VSYNC, or CSYNC, or sync on inexperienced? The reply is sure,” Chamberlin explains, “the Mac generates all of these sync requirements at completely different instances, relying on what sort of monitor it thinks is related. But it surely doesn’t generate all of them the entire time. The video {hardware} checks the voltages on the connector’s three sense pins, which kind a 3-bit monitor ID for the related monitor sort.”
Whereas contemporaneous adapters which supply laborious guide configuration through DIP switches exist, Chamberlin determined to resolve the thorny mess a neater method: with a Texas Devices LM1881 video sync separator chip. “I got down to construct a Mac to VGA adapter with an built-in LM1881 that might allow my Macintosh IIci to work on [my] three fussy displays,” Chamberlin writes. “I’d use the LM1881 to extract VSYNC from CSYNC, passing the VSYNC to the VGA monitor, and I’d go CSYNC to HSYNC with my fingers crossed.”
The adapter labored with two of the three displays, although work is underway on bettering the image high quality. (📷: Steve Chamberlin)
The ensuing adapter proved suitable with two of the three displays Chamberlin had beforehand tried with the machine, exhibiting a good-quality picture on older CRTs however a “awful” picture on a extra fashionable LCD — and nothing in any respect on one other LCD. It is a good first effort, although, and Chamberlin already has plans for bettering the output and constructing every little thing right into a single adapter — which, he admits, will probably be “awkwardly giant.”
Chamberlin’s full write-up is on the market on his web site, Huge Mess o’ Wires.