Monitor ID and Control: A number of different schemes are used to identify monitor capabilities to the PC. Misidentification can cause monitors to fail to work or to work in odd ways. Since the different schemes aren't very compatible, it is probably amazing that monitors are interchangeable at all.

The VGA standard allocates three wires (pins 11,12 and 4 in the 15 pin connector) to identifying monitor capabilities. These can use a simple basic capability code:

Some vendors including IBM and Compaq apparently use their own proprietary ID schemes. It is rumored that some manufacturers used the "No Connect" Pins 9 and 15 as well as the normal ID pins. IBM PS/2, and 63xx/85xx/98xx series monitors have been cited as examples of monitors that may require tinkering with IDs which is done by altering the cable or connecting through an ID box that provides ID signals and passes the actual video on to the monitor.

Apple has their own elaborate identification scheme. This entails six simple codes and 64 additional codes derived from pulsing each ID line individually and reading out the other two lines. Note that although standard VGA is a permitted monitor type, it is on an extended code that requires diodes in the ID matrix for proper detection. Using an Apple monitor on a PC or vice versa will probably require tinkering with ID lines unless the monitor is explicitly designed to work with both. Refer to the following for details:

"Plug and Play" monitors use one of at least two varieties of VESA defined Display Data Channel (DDC) coding. These are used to identify monitor models and capabilities and (possibly) with DDC2, to set monitor operating modes. There may be a two varieties of DDC2, but it is not clear what the difference is.

DDC1 adds an additional ID line on Pin15 and uses Pin 12 as a monitor to PC data line that is clocked by the Vertical synch signal on pin 14. Video cards may clock data at up to 25KHz which has a certain potential for damage if the clock is interpreted as vertical synch.

DDC2 provides a bidirectional data signal on Pin 12 and puts the clock on Pin 15. The protocol used is Phillips/Digital I2U -- ACCESS.BUS. In some cases, the interface power (5 volts at .3A) will be provided by the PC and a non-standard VGA connector will be used.

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