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Quadra 950: Apple Multiple Scan 720

I'd purchased an Apple Multiple Sync 720 (17") CRT along with the PowerPC 7200 and they worked fine together. I've since gotten rid of the PPC and have tried to get this monitor to work on the Quadra 950. On first plugging in, Mac OS 8 reported that only 640x480 was available. I know it can do up to 1280x1024, so I dug deeper.

From a brief google I couldn't work out if this monitor was supported officially or not. The resolution available indicated that the monitor was not correctly detected; but was this a fault of the monitor or an issue with my macintosh/rom/software/firmware? Or just the fact that the monitor was newer than the Quadra and was never going to be correctly detected?

A little digging indicated that Apple monitors used 'sense pins' to tell the Macintosh what was connected and then what resolutions should be displayed. This monitor uses 'extended sense pins' and I wasn't sure if the Quadra 950 understood these.

Sense Pins and related IDs

There are some very interesting articles online relating to Apple video hardware. I found this email to the comp.sys.mac.hardware usegroup from Dale Adams who was actually one of the engineers who created the specifications/hardware. In it he describes the technology and the 'pinouts' of the sense pins and associated monitors. I've reproduced this here for easy reference.

Monitor Sense Pin 0 (4) Sense Pin 1 (7) Sense Pin 2 (10) Resolution
Apple 21S Color 0 0 0 1152 x 870
Apple Portrait 0 0 1 640 x 870
12" Apple RGB 0 1 0 512 x 384
Apple Two-Page Mono. 0 1 1 1152 x 870
NTSC 1 0 0 underscan - 512x384
overscan - 640x480
12" AppleMonochrome 1 1 0 640 x 480
13" Apple RGB 1 1 0 640 x 480
Extended sense code monitor 1 1 1

From the table above, I can tell you that my monitor is of the 'extended' variety and provides '1' on each sense pin. When the 'extended' mode is found, the Macintosh BIOS is then meant to send voltages to each line and determine what the value of the other pins are. I couldn't actually find out when this logic was supported in what Macintosh ROM/BIOS and so I assumed that the Quadra 950 didn't know how to do this. A little bit more reading through Dale's post indicated that if I could ground the wires then I could fake a monitor code and test other variants.

Faking codes via the sense pins

I started with aluminium foil, folding it down to a thin strip and punching a hole in one end. These strips were then slid over the three pins that needed to be grounded to fake an Apple 21S Color monitor. This was fiddly work and took quite a few attempts. Foil isn't strong when punching it with a pin and isn't easy to manipulate. It also moves as you plug the monitor into the port, so it was a very one-shot affair.

After getting a successful connection, I started the mac and ... shit ... it just worked. The resolution was already set to the only fixed resolution that this monitor could handle. From the information page at everymac you can see that it can support 1152x870, but I had assumed that this was the max and that I could set any resolution up to that. Turns out that the 21S is a vintage monitor and only does one resolution; but I'm sure it does it well!

But I want to fake a multi-sync monitor!

It turns out that you can't. To do this you need 1,1,1 on the pins, of which my monitor is already outputting! Therefore I was back to zero. I kept reading posts/documents online and stumbled across the Video Compatibility reference article where I see it mentioned that, if you mate my monitor up to the Quadra 950, you can output the correct resolution. Why doesn't mine work then? Reading Monitor Adjustment Info by James Davis tells me that if the BIOS doesn't support the extended sense pins then the monitor will be seen as a 12" RGB. This seems to be the case, as I can only choose 640x480 when the monitor is plugged in as usual. But then again, that contradicts the first page.

Installing the correct drivers...

After a little more googling, it seemed that I'd needed enabling software called "Apple Multiple Scan Software". This was mentioned in this tidbit on hooking up foreign monitors and in the manual from my actual monitor. This wasn't easy to find but a lot of digging produced a copy over at

I initially tried with my PowerPC card enabled and the install software told me that my machine didn't need it. So I rebooted to 68k and it installed... but.... as it was installing it told me that the files on disk were already newer than the files being copied. Whatever... I copied them anyway. After a reboot there was no change.


I managed to find this version (and have made SwitchRes v2.1 available here) and tinkered. I had assumed it would allow magical resolutions to be set... it really did nothing but cause problems.


A fellow vintage Macintosh enthusiast in a forum post over at the 68k Macintosh Liberation Army entertained me with the following user manual for the Mutliple Scan 720. I assumed it would lead to another dead-end... but as I was reading through I noticed that it indicated that if DDC was enabled then specific machines (PPC9600, PCs, etc..) would behave differently. What if it happened to be enabled and my poor Macintosh was getting confused?

I got home and booted the machine into its glorious array of 640x480 pixels. Flicking through the onscreen display on the monitor itself, I navigated to information and then DDC. It was set to 2B. I wonder what those codes even stand for... anyway, I knew that DDC wasn't what the Quadra spoke, so I turned it off.

Low-and-behold after a reboot the standard Monitors and Sound control panel allowed me to select right up to 16-bit 1024x768. Not quite the 1280 or 1152 that I was after, but nearly twice as good as what I had before. Moral of the story? Don't use newer tech on older machinery!