Old monitor = no boot on G4?

[Bunsen]

Got a little video related mystery for you all:

I've been testing out my miniature CRT monitors on my QS G4 (OS X 10.4.11, Geforce 4MX AGP video card, VGA + ADC).

Plugging in the "Esteem" brand colour 10" VGA (1991, model cd-1035) works fine (although it's a bit .. meh, as a monitor.)

However, plugging in the greyscale 9" CRT (1994, TUV Product Service GS [unreadable], part no. MA-0933) results in a chime, but no boot, even though there's an ADC monitor present on the other port. I've tested this with two identical monitors with the same result. The monitors (or at least one from the same batch) have previously been shown to work OK on some other system (probably a P4-era or earlier PC/Windows - it was a while ago).

I headed over to pinouts.ru and prepared a table comparing the pins present on each monitor:

Monitor cable end.

Pin  Name       Grey  Colour  Description

1    RED                X     Red Video (75 ohm, 0.7 V p-p)
2    GREEN        X     X     Green Video (75 ohm, 0.7 V p-p)
3    BLUE               X     Blue Video (75 ohm, 0.7 V p-p)
4    ID2                      Monitor ID Bit 2
5    GND          X     X     Ground

6    RGND         X     X     Red Ground
7    GGND         X     X     Green Ground
8    BGND               X     Blue Ground
9    KEY                      Key (No pin)
10   SGND         X     X     Sync Ground

11   ID0                X     Monitor ID Bit 0
12   ID1/SDA      X           Monitor ID Bit 1
13   HSYNC/CSYNC  X     X     Horizontal Sync (or Composite Sync)
14   VSYNC        X     X     Vertical Sync
15   ID3/SCL                  Monitor ID Bit 3

Any thoughts, people?

 

[Charlieman]

Sense line problem? Have you tried booting with the known working VGA monitor, then swapping over to the greyscale 9"? With the Geforce card, is there a utility that allows you to specify which port is used for the primary monitor?

 

[Gorgonops]

First thought I'd suggest:

Look at the cable for the monochrome monitor and see if pin 9 is present. On the original VGA that was an unused "key" line, but on cards supporting VESA DDC2 that line is a +5v power line. It might be possible that the mono monitor has pin 9 wired to ground, which would short that power supply.

Second thought:

The sense line on Pin 12 should be grounded in a fixed-frequency VGA monitor. Pin 12 was reused as the i2c serial bus line on VESA DDC. (which means that DDC and monochrome are exclusionary.) Perhaps the Mac's video card freaks out at having that line pulled to ground while the BIOSes on the PC cards still acknowledge the pre-DDC usage of the sense lines and adjust accordingly.

 

[Bunsen]

Have you tried booting with the known working VGA monitor, then swapping over to the greyscale 9"?

Nope - got scared something was shorting out - also sleep isn't working and I didn't feel like hot swapping would be a good idea.

With the Geforce card, is there a utility that allows you to specify which port is used for the primary monitor?

Just the standard OS X Displays prefpane

the cable for the monochrome monitor and see if pin 9 is present.

Nope - as marked on the table above.

Second thought:
The sense line on Pin 12 should be grounded in a fixed-frequency VGA monitor. Pin 12 was reused as the i2c serial bus line on VESA DDC. / Perhaps the Mac's video card freaks out at having that line pulled to ground

Ahhh - you might be on to something here - the greyscale does have pin 12, and the colour one does not. I'm resisting the temptation to snap that pin out and going to wait till I can make up an adapter to test this theory.

 

[Gorgonops]

If you disable pin 12 just remember you'll encounter a few minor issues:

A. If mono autodetection doesn't work and your video card outputs a color display it won't be like looking at a B&W TV... what you'll see is only a luminance representation of the green component. Red and blue elements will be completely dropped out. To get a sensible display you'll have to force the OS to render in grayscale

(Back in the DOS era there were programs that would hit the VGA registers directly and ignore the state of the sense lines. If you had a mono monitor, like I did, the result was most annoying.)

B. I remember reading a discussion somewhere speculating whether it could damage a video card to run with one or two of the analog color lines active but disconnected. (an old-fashioned VGA card supposedly shuts down red and blue when a mono monitors active.) I think the consensus was "probably not, but if you're really paranoid you could hang some resistance off them". Personally I wouldn't worry about it.

 

[Bunsen]

Sounds like the optimum solution to both problems is to sum the R G and B lines to the G input, via resistors. Adapter box it is then.

 

[Osgeld]

yea that would work

 

[Gorgonops]

Sounds like the optimum solution to both problems is to sum the R G and B lines to the G input, via resistors. Adapter box it is then.

You know, this gnawed on the back of my head for a while and I've finally realized what's wrong with that.

A color monitor is of course designed to work with the cone cells in your eyes, with the three color guns corresponding to the three basic colors of light that said cones perceive. (And on the output end on the monitor the three colors are presented as a tiny little triad of subpixels.) If you just sum all three together and stuff them out as additive luminance on a monochrome monitor then you're also going to make for a pretty bizarre display. For example bright white will be three times brighter than, say, bright green, which is not how our eyes perceive a normal color monitor. Black-and-White displays are designed solely to understand luminance, and what you'll be feeding it is a three-way compound of luminance, chroma, and hue. So, yeah... it will look really weird.

I'd say just wire it up as it is, with the luminance line on blue, and force OS X to monochrome. (You can do that via the "Universal Access" settings control panel.)

 

[Bunsen]

force OS X to monochrome. (You can do that via the "Universal Access" settings control panel.)

Well, that I did not know. Thanks - that will make things a lot easier, at least in this application. I may still want to try a patch box for future uses.

If you just sum all three together / then you're also going to make for a pretty bizarre display. For example bright white will be three times brighter than, say, bright green

The circuit I found here seems to compensate for that somewhat, if I understand it correctly:

                        ____
RED    1 >-+-----------|____|----+
          |  ____     82 ohm    |
          +-|____|-+            |
           180 ohm |            |
*RGND  6 -----------+            |
                       ____     |
GREEN  2 >-------------|____|----+----------------> VIDEO OUT
                      22 ohm    |
                       ____     |
BLUE   3 >-+-----------|____|----+
          |  ____     240 ohm   
          +-|____|-+            
           100 ohm |            
BGND   8 -----------+

GGND   7 ------------------------------------------ VIDEO GROUND

HSYNC 13 >----------------------------------------> HSYNC

VSYNC 14 >----------------------------------------> VSYNC 

 GND  5 ----------------------------------------- GROUND

*incorrectly marked as BGND 6 on source page

I'd say just wire it up as it is, with the luminance line on blue

:?: There's no "luminance line" on VGA AFAICT - and the only connected color line is green

 

[Gorgonops]

:?: There's no "luminance line" on VGA AFAICT - and the only connected color line is green

There's no line labeled "luminance" on the VGA card's pinout, but by definition whatever color line is connected to the monochrome monitor thus becomes "the luminance line", since luminance is all the Mono monitor understands. (And on a VGA monitor's pinout that line will probably be labeled "luminance".)

I always forget which of the three lines is the one the mono monitor connects to... it's been twenty years, cut me some slack. :^b One of the reasons I'm skeptical about the contention that leaving the lines hanging will hurt anything is I actually made my own mono VGA cable once (it was for a grayscale NEC Multisync GS that had a nonstandard 9 pin connector on the monitor side.) and I would *swear* that when I was fooling around with it (I was young and stupid and willing to simply stuff stripped wires into holes to test if I had it right) that when I had the mono select pin grounded I could connect the luminance line from the monitor to *any* of the three color guns and get the same picture. If that memory is true it implies that at least the old Trident VGA card I was working with didn't bother downing the extra lines, it just locked all three palettes together.

The Linux on Obsolete Displays webpage shows an interesting exercise in using the three color lines as *separate* luminance signals. Scroll down to the "Poor Man's Triple Headed Display". He does it using TVs, but he notes in principle you could do it just as easily (well, more easily) with three mono VGA monitors.

 

[H3NRY]

Bunsen, your mixed RGB circuit should be the best choice. You want mostly green in a grey-scale display, then some red and less blue. Second choice is just to hook the video input to the green output. Another source of problems might be the sync lines. Some monitors use composite sync on green, some use separate sync, etc. If one of the sync outputs is grounded, that might prevent video from working, but I'm guessing about that. I think you're just going to have to use Ohmmeter and trial and error. There's not much voltage or current in video outputs (ADC excepted!) therefore not much chance of BIG smoke.

 

[Osgeld]

aye 1 volt peak

I see what Gorgonops is saying, but its old school, and not likely encountered in this day and age unless its a relic

personally every one I have tried it on sums the RGB values in house, course every one I have seen are missing pins 9 and 12 for the reasons given

 

[Bunsen]

it's been twenty years, cut me some slack.:^b

Heh I got what you meant eventually - I was mostly checking in case I'd misunderstood something myself.

"Poor Man's Triple Headed Display"

Oh yes, I've long been impressed with that simple and obvious-only-in-restrospect hack. It would be great to be able to use all three lines as real workspace, but, y'know, drivers ... :-/ I know a few people who could probably pull that off pretty quickly in an open OS, but Mac/Win... possibly not so much.

I'm skeptical about the contention that leaving the lines hanging will hurt anything

You're probably right, but I reckon I'll err on the side of caution with my main machine. Besides, making up a little video patchbox will come in handy for future tomfoolery.

 

[Bunsen]

If one of the sync outputs is grounded, that might prevent video from working

Righto - I'll check that. Thanks for the tip.

not likely encountered in this day and age unless its a relic

1994?

fixed-frequency

I don't think they're fixed freq, in as much as I seem to remember being able to get more than 640x480 out of them on an old PC. But I could be mixing up my terms here.

 

[Osgeld]

1994?

far as vga monitors go technology wise, thats practically new :lol:

 

[Gorgonops]

I see what Gorgonops is saying, but its old school, and not likely encountered in this day and age unless its a relic
personally every one I have tried it on sums the RGB values in house, course every one I have seen are missing pins 9 and 12 for the reasons given

I've never seen a mono monitor that does that, but I also don't think I've actually worked with any that had a manufacturing date later than 1990. ;^b I suppose I don't doubt that such animals exist, as they were at least making the POS-size mono monitors until, what, the early 20-aughts? (Never mind, I googled, you can still buy them new), and after the mid-90s or so you probably couldn't count on video cards recognizing the old sense line layout... but I've never seen one.

I don't think they're fixed freq, in as much as I seem to remember being able to get more than 640x480 out of them on an old PC. But I could be mixing up my terms here.

They did/do make mono VGA monitors capable of higher-than-640x480 but they were rare as hen's teeth back in the era when people actually bought mono monitors because they were cheaper than color ones. Most were plain-old VGA. (Which technically isn't quite "fixed frequency", they actually support three different frame sizes of 480, 400, and 350 lines, but they're not "multisync".) I'd be really surprised if your 9 inch sense-pin equipped monitor does better than that.