Identifying connections on unknown or cut monitor cables

From: (Sam Goldwasser)
Date: 29 Apr 1996 16:32:32 GMT

Obviously, this is best done with a schematic. However, since such a luxury may not be possible, how can you go about figuring out where all the wires go? Easy answer - very carefully.

For the following, I assume a VGA/SVGA monitor. You need to identify the grounds, video signals, H and V sync, and monitor sense lines. The procedure is described with respec to a cut cable but if you are trying to identify an unknown connector type on the monitor, the same comments apply to the wiring **inside** the monitor.

First identify the grounds. Use an ohmmeter between each wire and the shell of the video connector on the monitor. Resistance will be less than an ohm for the ground wires. These will often be colored black. The shields of the RGB coaxes will also be connected to ground.

The high bandwidth video signals will always use individual coaxial cables. These may even be color coded red, green, and blue. If not, you can determine which is which later on. If there are only three such coaxes, they are the video signals. If there are four, the extra one may be the H sync. If there are five, the extra two may be the H and V syncs. Testing between these wires and ground with an ohmmeter should measure 75 ohms for the video terminations.

Display a lively screen on your PC at a resolution you know the monitor should support (remember, trying to drive a monitor of unknown scan rate specifications beyond its ratings is like playing Russian Roulette.) When in doubt, VGA (640x480, 31.4 KHz H, 60 Hz V) should be safe.

Turn up the brightness and contrast on the monitor. If you are lucky, even without any sync, there will be a visible raster. Set it to be just visible. If there is none, then it should appear once there is valid sync.

You will need to bring out wires from the video connector on your PC.

Connect the ground of your video card to the ground wires you already identified on the monitor cable.

Attach a wire in series with a 200-500 ohm resistor to H sync (pin 13) on the VGA connector.

Momentarily touch the end of this wire to each of the remaining unidentified wires (including the coaxes if you have 4 or 5 of these and it is not obvious which are the video signals) on the monitor. When you find the H sync input, the raster should lock in and probably brighten up. If the monitor was originally whining due to lack of sync, it should quiet down.

Once you have located H sync, you can remove the resistor and connect the wire up directly.

Now, attach the video signals. It is likely that you will now have a picture but it will be rolling on the screen. Some monitors, however, will not unblank until they receive both valid H and V sync. Use your resistor with the V sync output of the video card (Pin 14) on the remaining unidentified wires. Once you find the V sync input, the display should lock in solid.

The only remaining unknowns are the monitor sense lines. For older monitors - those without the ACCESS.bus interface, you can just wire up the sense lines to the appropriate levels (Color: ID0 (Pin 11) to ground, ID1 (Pin 12) NC.

See the document "Pinouts for various connectors in Real Life(tm)" for detailed hookup information". Replacement VGA connectors are readily available.