21 Pin SCART (EURO) Connector 

(Syndicat des Constructeurs d'Appareils Radiorécepteurs et Téléviseurs)

As used on many TVs, Videos and computer games.

(pictured looking at solder side of plug)

The following notes are based upon empirical evidence, and aren't meant to imply and kind of rock solid implementation standard.

Note 1: (Pin 8 usage) 
On many TV's taking pin 8 high will auto select the SCART input. With a voltage range of 4.5V to 7.0V a compatible set will select AV input in 16x9 mode. With a voltage range of 9.5v to 12.0v the set will select AV input in 4x3 mode. 

It has come to my attention that certain newer sets (notably those containing Sony CXA2069A chipset) implement a third intermediate switching level. Details are sketchy, but it would appear to select a letterbox format rather than full 4:3 or full 16:9.

Rumor has it that pin 8 on certain VHS machines can be used to start a recording, which could be used by an electronic program guide on a set top box for instance.

Note 2 : (Pin 16 usage) 
On some devices  pin 16 is used to select between composite or RGB input modes using the same SCART.
With a voltage of 1-3V DC (with respect to pin 18) RGBS input mode is selected, reversing the usual function of pins 7,11,15 from output to input. Pin 20 is used to input composite syncs (N.B. as the green input on pin 11 and the syncs input on pin 20 are both terminated, a simple loop through arrangement can't be used to connect a 'syncs on green'  feed). 
The switching signal needs to be able to source upto 20mA into pin 16 @ 3V for switching to take place. With a voltage range of 0-0.4V composite mode is selected and video is taken via pin 20 as normal.

Note 3: (YC input)
A further variation on the theme allows the use of a YC input. In this case pin 20 becomes the luma input and pin 15 becomes the chroma input. Selecting this mode is usually done through menus, or sometimes automatically if a signal is detected on both pin 15 and 20 simultaneously (RGB and YC inputs aren't normally be implemented using one SCART). If your picture is unexpectedly black and white, you may be looking at a composite input with the set to YC mode! YC connections more commonly appear as mini DIN connectors.

A quick note about PC monitors and TV monitors:

I've had quite a few messages from people wanting to use their spare PC monitor as a TV monitor to watch DVDs, VHSs etc or for use as a security camera monitor. Similarly it would seem many people would like to use their spare TV as a screen for their computer.

Firstly, it is technically possible to make one device do the job of the other, but it's  a very difficult task. It's not just a case of grafting a SCART onto your VDU lead. The bottom line is that if you need to ask if and how it can be done you are a very long way off having the requisite knowledge to do so. In short, it can't really be done economically,  effectively or safely (you're taking your life into your hands when you take the back off a telly or a monitor -you've been warned!).

It's like having a spare combine harvester and asking if you can make it into a small family car. It certainly has an engine and wheels, so it's a bit like a car -but only the fool hardy would entertain the conversion!

There are a number of scan converter boxes available to do the job, but try before you buy. The results are very variable!