Monitor Adjustment Info


Im not a EE. I dont understand electronics. Ive been told by one person that my methods will break my equipment. That said it works for me. Ill tell you in the text if I think something is hazardous. But if you blow up your computer, house, or self, I dont want to know.

Basic Information

In theory, its easy to use a monitor with your computer. All you need to do is get your video hardware to produce some video signal that the monitor can understand. In practice this can be a real pain. You need to adjust several things until they match.

The horizontal and vertical scan frequencies must be compatible.

If you have a multisync monitor then it can accept some range of sync rates, however if you have an older fixed frequency monitor then it only responds to one set of horizontal and vertical timings. Note that I didnt say anything about resolution. It is possible that you could run two different resolutions, say 1024x768 and 832x624 such that they have similar frequncies. For instance I have an old HP workstation monitor that was designed for 1024x768 60Hz_Vertical 47.7kHz_Horizontal. My macintosh can output 832x624 75Hz_Vertical 49.7kHz_Horizontal. The horizontal frequency is fairly close, and I got lucky on the vertical. I am able to use this monitor at 832x624 even though it was designed for 1024x768. An additional note about this. I am running the vertical sync at a rate quite different from the monitor spec. Ive been told that you will shorten the life of your monitor and possibly cause smoke. You have been warned. Dont run your monitor outside of its intended range. In any case, your video card and/or monitor may have a limited signal set, and we need to find or create some overlap.

The sync signal must be compatible.

Some devices output the sync signal on seperate lines, some add the signal into the green video signal. Your video card and monitor need to be doing the same thing. In addition there is a question of sync polarity. That is, do we raise or lower the voltage when we want to sync. Some video cards give you a choice, some dont. Some monitors care, some dont.

Macintosh Sense Pins

Unlike PCs the macintosh trys to be user friendly. This can turn out to be a stumbling block. Your macintosh trys to be helpful by sensing what kind of monitor you have and limiting your video choices to a set compaitble with your monitor. In general this is a good thing since sending a "wrong" video signal to the monitor could actually cause damage. However, when we know what we are doing, and are sure that the connected monitor will work, it makes it impossible to set the resolution/sync rate we desire. In these cases its important to understand how the macintosh decides what kind of monitor you have. Three of the pins on your monitor cable (pins 4,7,10) are called sense pins. By checking the state of these three pins the macintosh determines what kind of monitor you have connected. So that if we wish to set a certain resolution/rate, we can arrange for the actual hardware pins to be set correctly and reboot the machine. The macintosh will decide that we have the kind of monitor we intended and allow the choice of resolution we were after. So now, how do we set these pins to the right thing. The simple answer is go buy one of those resolution adapter thingies that go between the monitor cable and your mac. I highly recommend it. I have the Liberty Adapter from Enhance, bought at Frys. The adapter is great, the tech support was extremely unhelpful. It took quite some time to extract information I needed. Griffin Technologies makes several competing products, which I havent used, but appear to be better/cheaper. In addition Paul Griffin himself has been extremely helpful to me via email, and Id recommend buying from his company based on my experience.

Griffin Technology - http://www.Nashville.Net/~griffin/
MacAdapt -
Enhance Technology -

So you dont want to buy an adapter? Heres a partial list of sense pin settings and the corresponding resolutions on the mac. In some cases you ground the pins, in others you connect them to each other, either directly, or via a diode. Ive found that instead of cutting the cable and connecting the right things, I can use wirewrap on the cable connector pins themselves. Since wirewrap wire is small it just sits down in the male connector and scrunches up when I plug in the cable. This turns out to be much easier than cutting and soldering, etc. if you happen to have the right supplies around. In addition the Centris/Quadra 610 video has a couple of video modes that arent reported in the official Apple chart. These resolutions were included to make connecting an older Radius Two-Page monitor possible. I dont think other machines support these modes, but don't know for sure.

Radius TPD    1152x870    71Hz Vertical / 66kHz Horizontal 
              Pin 4 connected to Pin 7 via diode, Pin 10 grounded
              Reverse the diode for Radius 20" Color at these rates.

Additional info available at these places:!Hardware/Monitor!Sense!Codes!Defined

Table 1  Supported Multiple Scan Screen Resolutions
Monitor Type         Screen Resolution     Frame Rate (Hz)
------------         -----------------     ---------------
Multiple Scan 14     640 x 480             66.67
                     832 x 624             74.55
Multiple Scan 16     640 x 480             66.67
                     832 x 624             74.55
                    1024 x 768             74.93
Multiple Scan 21     640 x 480             66.67
                     832 x 624             74.55
                    1024 x 768             74.93
                    1152 x 870             75.08
The three tables below summarize the contents of this tech note.
Table S-1  Sense Line Code Assignments
Monitor Type          Sense 2  Sense 1  Sense 0  Frame Rate (Hz)
------------          -------  -------  -------  ---------------
RGB 21"                 0        0        0         75.08
Full-Page (B&W 15")     0        0        1         75.08
RGB 12"                 0        1        0         60.15
Two-Page (B&W 21")      0        1        1         75.08
NTSC Monitor            1        0        0         59.94  
RGB 15"                 1        0        1         75.08
Hi-Res (12-14")         1        1        0         66.67
Multiple Scan 14" *     1        1        0        See Table 1
Multiple Scan 16" *     1        1        0        See Table 1
Multiple Scan 21" *     1        1        0        See Table 1
No Display Connected    1        1        1          NA
PAL Encoder *           1        1        1         50.00  
NTSC Encoder *          1        1        1         59.94  
VGA *                   1        1        1         59.94
Super VGA *             1        1        1         55.98
RGB 16" *               1        1        1         74.55
PAL Monitor *           1        1        1         50.00  
RGB 19" *               1        1        1         74.93

* These monitors require extended sense line support.
  Interlaced timing.

Note: The binary values in this table indicate the relative state of the sense pin
measured against monitor ground, pin 11, on the DB-15 connector depicted in Table
S-3 below.

Table S-2  Extended Sense Line Code Assignments
                   Sense 2 Low    Sense 1 Low    Sense 0 Low
Monitor Type           1 & 0          2 & 0          2 & 1
Multiple Scan 14" *     00             00             11
Multiple Scan 16" *     00             10             11
Multiple Scan 21" *     10             00             11
PAL Encoder             00             00             00
NTSC Encoder            01             01             00
VGA/Super VGA           01             01             11
RGB 16"                 10             11             01
PAL Monitor             11             00             00
RGB 19"                 11             10             10

Note: In this table, the column under "Sense 2 Low 1 & 0" indicates the values the
software is reading from monitor sense lines 1 and 0 while driving sense line 2
low. Suppose you have a PowerBook 180c and you want it to believe there is an
Apple 16" RGB monitor attached. The sense line code in Table S-2 for this monitor
is 10 11 01. As shown in Figure 3, this sense code equates to sense line 2 being
connected to sense line 0 with a jumper. Based on Table S-3, your tying pin 4 to
pin 10 will make the PowerBook think that the 16" monitor is attached. It's
important to recognize that pins 4 and 10 in this example must not be tied to
ground, otherwise a 12" RGB monitor would be indicated.

Modifying A Monitor

In some cases, we can't get our video hardware to output a signal that is compatible with the monitor we have on hand. If the video signal is close to something the monitor can handle, we might be able to make some adjustments to the monitor. Some fixed frequency monitors have an internal dial controlling the expected Horizontal Sync Frequency. Adjusting this dial will make the expected frequency a little higher or lower. You will need to remove the outside casing of the monitor, and possibly the inside grill over the tube. Look around for some pots (small dials). These may be inset behind some small holes in a plastic casing. One of them will be labeled "Horz. Freq." This is the one to turn. Don't force it, turn gently. I dont know for sure that all monitors have this. Mine did, it was helpful.

Warning - Monitor guts can kill you. Even unplugged.

A CRT is a giant capacitor, capable of delivering 27,000 volts. It stays charged even after you unplug the monitor. Please don't fry yourself. Please don't open up a monitor if you don't know what your doing.

My monitor had a plastic screwdriver mounted inside the case for use turning these pots. Don't use a metal object to do it. The dial in question was through a hole behind a piece of plastic, on the left side (facing the screen). This location probably varies with monitor.

A document describing this for a sun monitor was helpful for me.

Sync On Green

Some video devices think sync signals should be included on the green signal line. Some think a seperate sync line is useful, and still others believe horizontal and vertical syncs each deserve their own line. VGA typically uses hsync and vsync lines. Older macs used sync on green. My Apple High Performance Video Card outputs hsync, vsync as well as composite sync, but cant do sync on green. Newer multisync monitors typically will take anything. Many older RGB monitors want sync on green.

Another issue with syncs is polarity. You can either push the voltage up whenever there is a sync, or pull the voltage down. I think typically these are 0V and 5V. Some video cards allow you to specify +/- sync polarity. For instance using an ATI card with XFree under Linux, I needed to specify hsync/vsync as -/- to drive a certain monitor. Most newer monitors will take either. All mac video that I have encountered has been negative, meaning sit at 5V until a sync and then drop to 0V.

So a common situation is that you have seperate syncs on your video card, and you want to drive a sync on green monitor. The best answer is that you should buy a converter that adds it in for you. Griffin technologies sells one.

Griffin Technology - http://www.Nashville.Net/~griffin/

Or you could build a circuit yourself. The Sync-On-Green FAQ points to a circuit schematic. If you do this, pay attention to the polarity since it will affect the logic to obtain a composite sync.

Lastly, if you are exceptionally lazy, and have little regard for your equipment, I've had luck as follows. I have found that you can just tie the sync signal wires together to obtain a composite sync. Then in some cases I can just tie the composite sync into the green wire and everything works. In other cases I've needed to put a 1k resistor between the composite sync and the green line. In all of my experiments its been necessary to insure negative sync polarity. And its a really bad idea to tie hsync and vsync together if they have opposite polarity. Now I think its important to point out that Ive been told that you should never just tie these signal lines together. That it is virtually guaranteed to damage your video card and/or monitor over time. And in fact I have blown the green amplifier in a monitor, so that I no longer get any green signal. On the other hand I've used composite sync tied into the green line through a resistor for a number of months on another monitor. Of course I may be gradually ruining my video card, I dont know. For a discussion of some of this see the exchange of email below. One final note is that its really easy to tie these lines togther using BNC style RGB cables. Just get a couple of the T joints used commonly in thin-net wiring, and you can connect as you choose, with no cutting and soldering. I did have to cut something to insert the resister though.

Discussion of why this is bad.

Cable Information

To build a useful cable all you need to do is connect the red signal to the red signal, the red ground to the red ground, the hsync to hsync, etc. Pinouts for several interfaces are given.

Macintosh DB-15 Male

\ 1  2  3  4  5  6  7  8 /
 \ 9  10 11 12 13 14 15 / 

1 Red Video Ground
2 Red Video
3 Composite Sync.
4 Monitor ID, Bit 1
5 Green Video
6 Green Video Ground
7 Monitor ID, Bit 2
8 No Connection
9 Blue Video
10 Monitor ID, Bit 3
11 CSYNC and VSYNC Ground
12 Vertical Sync.
13 Blue Ground
14 HSYNC Ground
15 Horizontal Sync.
Shell Chassis Ground

PC VGA HD15 Male

 DB15-S Male                 
\   1  2  3  4  5    /     
 \ 6  7  8  9 10    /     
  \ 11 12 13 14 15 /     

DB9 (DE-9) Male    
\ 1 2 3 4 5 /    
 \ 6 7 8 9 /    

VGA DB15-S Female DB9 Female
15-pin  9-pin   assignment
1       1       Red
2       2       Green
3       3       Blue
4       -       Monitor ID bit 2
5       -       N/C
6       6       GND (red return)
7       7       GND (green return)
8       8       GND (blue return)
9       -       N/C
10      -       GND
11      -       Monitor ID bit 0
12      -       Minitor ID bit 1
13      4       Horizontal Sync
14      5       Vertical Sync
15      -       N/C

Monitor ID bit 0: reserved
Monitor ID bit 1: GND = mono, OPEN = color
Monochrome monitors use the green signal

SGI Indy 13w3

\   -    1   2   3   4   5   -   -   /
 \ [R] 6   7   8   9   10   [G] [B] /

1 mon. type 3
2 mon. type 0
3 composite sync
4 horizontal sync [active low]
5 vertical sync [active low]
6 mon. type 1
7 mon. type 2
8 gorund
9 ground
10 ground

Sun 13w3

\   -    1   2   3   4   5   -   -   /
 \ [R] 6   7   8   9   10   [G] [B] /

1 N/C
2 N/C
3 sense 2
4 sense Ret.
5 composite sync
6 N/C
7 N/C
8 sense 1
9 sense 0
10 composite sync return (ground)

RGB BNC Connecters

The pin is signal, the shell is ground.

Radius GS/C Card

This card output a 1152x882 72Hz signal, using a DB15 connector.

1  Ground
2  Red Signal
9  Blue Signal
11 Ground
12 Green Signal
13 Ground


Apple 15" at 1024x768

If you just plug your Apple 15" monitor into your mac, you'll be able to use only 640x480 and 832x624. However the monitor can support 1024x768, and some macintosh video cards can output suitable video signals, so why can't we use it? Aw.. but you can. The answer lies in the sense pins. The Apple 15" is wired to set the sense pins such that 1024x768 is not allowed. However the monitor can work at 1024x768_70Hz and the Apple HPV video card can output this signal. So if we go buy an adapter, and set it to VGA/SVGA. Or build a cable such that pins 7 and 10 on the mac connecter are tied together. Go to the monitors control panel. Hold the option key and hit the options dialog button. You'll see a list of resolutions. 1024x768_70Hz will work nicely on you Apple 15". 60Hz will also work fine. I tried 75Hz which is the normal mac 1024x768 and things were weird, but they did come in. I think this may be over driving the monitor so I don't recommend it. 70Hz is mentioned in the manual as OK.

Old Radius 19" Two-Page on Centris 610

I had an old Radius 19" Two Page Display. It doesn't use the standard mac 1152x870_75Hz, so I thought I was out of luck. However, it turns that the Centris/Quadra 610 has a video mode to support this monitor. You ground pin 10, and put a diode across pins 4 and 7. The direction of the diode deterines color/grey. Radius will sell you a grey scale cable for this, Part # 590-0057-011. In my experience though, only one in three tech support guys at Radius will be able to refer you to this part. The other two will tell you that no mac can drive that monitor without a Radius card. Many adapters will allow you to set this mode, although I'm not sure that any will have it in their documentation. Installing Radius Precision Color (A free init available on their site.) will allow you to have a 1152x882 resolution. I should also mention that some old radius TPDs may not be compatible with this, according to radius tech support. Also note that the radius nubus cards that were designed to drive this monitor used a different cable pinout than standard mac video, so even if you have a DB15 to RGB cable, if it worked with a Radius NuBus card, it might not be the right cable. If you have a "normal mac" to RGB cable, then you can probably add an adpater thingie onto the cable to set the right sense pins. My Liberty Adapter did it, (although Liberty tech support couldn't tell me how, I had to guess), and Griffin's adapter can do it, and they can tell you what to set. Some of the cheaper adapters might not be able to set the mode, since Apple doesn't recognize it as official.

HP98785A on a Mac 1024x768 or 800x600

This monitor is a fixed frequency monitor that was used on an HP workstation. It was designed to run at 1024x768 at 60Hz. Its got RGB connectors on the back, and wants sync on green. Now my HPV card, can output this resolution, so I set my adapter to VGA/SVGA. This comes up in 640x480, which this monitor can't sync to, so things are all garbled. I had to attach another monitor to see what I was doing. Holding the option key and clicking the options button in the control panel, brought up a complete list of choices and I selected 1024x768_60Hz. Still no picture. This is because the HPV card outputs seperate sync, and the monitor wants sync on green. I solved the problem by connecting the composite sync line to the green signal via a 1K resistor, but see the sync on green section above for more info. Now hooking this green/sync onto the green connector of the monitor produced a picture. It was slightly crooked, so I messed with the monitor adjustments (inside the case) until I got a picture I liked. It turns out that choosing 800x600_72Hz also produces a syncable resolution, and I can switch between the two with no change of cabling.

HP98785A on a Mac 832x768

This resolution didn't come in immediately on this monitor. However opening the case, and adjusting with the Horz. Freq. as discussed above in Modifying a Monitor brought in a clear picture. This works because the monitor was intended to run at horizontal sync of 47.7kHz. It happens that Mac 16" resolution is at 49.7kHz, which in this case was close enough. Mac 19" is 60.2kHz, and I wasn't able to achieve that. Note that these are Horz. sync rates. The vertical sync rate also matters, but in this case the horz. seemed to be the limiting factor. That was on my Centris610 which ouputs a sync on green signal. When I switched to a PPC6100, I got seperate syncs, and a non-working monitor. The trick above of tying in the sync signal did not produce a good picture. When I left out the resistor, I did get a good picture, but also promptly screwed up the monitor, so that the green signal no longer registered. The effect is as if you left the green line unconnected. This is an example of why you shouldn't tie the sync and green together by hand. There needs to be a circuit in there. See that section above for details.

HP98789A on a PC

This monitor was designed to run at 1280x1024_60Hz. It has RGBHV connectors on the back. I tried this on a friends machine. Apologies fo rlack of details, can't recall exactly. My video card outputs this resolution, so I got a cable at Fry's and hooked it up. Windows worked right away. I just picked the 1280x1024 driver and got a picture. Under linux I needed to set a few things up. Its straight forward to set the resolution and frequency. What I didn't realize right away is that you also need to tell linux the polarity of the sync lines. These are options that you configure in the same file as the resolution. After setting the polarity to be negative for both syncs, I got a picture. One of the misc pages pointed to from this page had an example of the appropriate sytax. Note that I dont get a picture in DOS or Linux console mode. The monitor just won't sync to that signal.

Other Info

Some other info regarding monitors can be found at these locations.

The Hitchhiker 's Guide to X386/XFree86 Video Timing
Sci.Electronics FAQ: Repair: Monitor Repair
Sci.Electronics FAQ: Repair: Sync-on-Green
Fixed Frequency PC Video FAQ
Use your old Workstation Monitors with Linux/XFree86
Hitachi 19" monitor hardware-HOWTO

James Davis -