Apples and the IBM 6091 monitor

The following is a mail I got from Ian P Mccullough <ipm+ andrew.cmu.edu>. He would not let me keep the text about "no success with plain apple stuff". :-)
I have marked up the text little with HTML otherwise it is the original mail.

I currently have a 6091 on a standard PowerMac 9600. The video card is the standard one shipped by Apple with the 9600 the IMS(IMS is now ixMicro) TwinTurbo M4A.

I have it operating in 1152x870 @ 75Hz. The monitor is set to Mode 3. I get the feeling that I am overdriving the dot clock a little because the convergence goes way out if I turn the brightness up all the way. I also had to open it up and adjust the H size, V size and H phase to get this to an accetable aspect ratio. In other words, it looks like a letterboxed movie because if you stretch the V size all the way to the edges of the screen, the icons look very rectangular and vertically stretched.

The details of the cable are as follows: RGB signal and ground are straight though as should be obvious, and I am using composite sync.

mac video pin                    4 BNC Cable

1  Red Ground                    Red BNC Shield
2  Red Signal                    Red BNC Pin(Signal)
6  Green Ground                  Green BNC Shield
5  Green Signal                  Green BNC Pin(Signal)
13 Blue Ground                   Blue BNC Shield
9  Blue Signal                   Blue BNC Pin(Signal)
11 Comp. Sync Ground             White BNC Shield
3  Composite Sync                White BNC Pin(Signal)
4  Sense Pin 0 --|        
7  Sense Pin 1 --|
10 Sense Pin 2 --|
                 |
Shield Ground  --|

That Sense Pin configuration tells the video card that it is connected to a Apple 21" RGB Monitor, and to output 1152x870 @ 75Hz. I originally tried tying the sense lines to the Composite sync ground but this seemed to cause a little noise in the picture. Tying it to the shield of the mac video connector seemed to help.

A few more notes on cable making: I have made many mac video cables and this is far from the first Fixed Freq monitor I've used on a mac. Mac video cables that are made by apple (in other words chopped off mac monitors) are usually pretty good and it doesnt matter where you put the splice. Most third-party mac monitor cables are of very mediocre quality and I recommend making the connection as close to the Mac end as possible. I cant stress enough the need for good cable. In my earlier days as a Tech Support worker, I've had people complain about poor picture only to find that thier cable is all but unshielded, and that replacing it with a higher quality cable, solved their problems.

Other pages of interest to Mac Fixed Frequency Enthusiasts:

http://devworld.apple.com/dev/technotes/hw/hw_30.html -- This explains a lot of things about mac video useful to the cable splicer.

http://www.nashville.net/~griffin/monitor.html -- This is an on-line database of Fixed Freq monitor and how they work with a Mac. Sadly the 6091/19 doesnt appear to be in there.

Lastly I'd like to say the following: on your web page you say "No success as far as I know using plain Apple stuff." I feel confident in saying that any Apple Video that supports the Apple 21" RGB monitor will support the 6091 in the configuration I have illustrated above. I would assume that this would include all the PowerMacs and most of the Quadras. But I cannot guarantee that as I have not tested each one. If you want, I will try this cable with different macs as I get a chance to and let you know how it goes.

One final note: I would have liked to have run the monitor at 1024x768 @ 75hz so I could get more brightness without losing convergence, but I was unable to get it to sync at any frequency at 1024x768.

Hope this makes your Web Page more complete,

Ian P. McCullough
Happy IBM 6091/19 owner

I just thought I would let you know that I found my Quadra 700 onboard
video drives the IBM 6091 very well at 2-page resolution, via a
dip-switching adaptor or cable, albeit at 256 colours.  The old Apple 4*8
vid card does the same thing, with the same settings.  I don't really
understand the technology here but I have all 5 BNC leads plugged in, but
the monitor works only when the adaptor is set to 'sync on green'.

But it works even better with one of the Villagetronic MacPicasso cards,
which gives 'millions of colours' at either 1152x870 x 75Hz, or even
1280x1024 x 60Hz (which hurts, if you see what I mean - I couldn't work
with that for very long).  Like yourself, I haven't managed to get it to
run at any lower resolutions yet.

Power Mac 6500
Performa 6400
Power Mac 7500

All work just fine at 1152x870x75Hzx16K colors with their standard video
and a cable that has the monitor sense pin jumpered for a 21" Mac monitor.