https://patents.google.com/patent/GB2294135A/en?q=ibm+%22monitor+id+bits%22&oq=ibm+%22monitor+id+bits%22

is When the IBM Video Graphics Array (VGA) and the 8514/A adapter card
were introduced, a range of monitors was offered, both monochrome and
colour. one of the monitors supported an additional mode, available only
from the 8514/A adapter card. The other monitors did not support this
mode. The mechanical connections of the monitors were common, and it was intended that any monitor should work with any adapter, without the user needing to change any switches, or answer any questions asked by an installation program. in addition, applications did not have to know
whether a monochrome or colour monitor was attached, the signals
provided by the VGA being adapted by the personal computer to provide
readable displays on either monochrome or colour displays without user intervention.

This was achieved by the use of ID bits in the interface connector.
Inside the connector attaching the monitor to the personal computer, the
three ID pins were either open circuit, or connected to a ground signal.
A sensing circuit in the personal computer could then determine whether
a monitor was plugged in and if so, whether the monitor was monochrome
or colour, and whether it supported the higher resolution capabilities
of the 8514/A adapter card.

For example, ID bit 0 was grounded for a monochrome monitor, otherwise
open circuit. ID bit 1 was grounded for a colour monitor, otherwise open circuit. ID bit 2 was grounded if the monitor supported the higher
resolution capabilities of the 8514/A adapter card.

Later, when the number of monitor types proliferated, a fourth ID pin
was added, and in addition to the pins being connected to a ground
signal or open circuit, some connectors had the pins internally
connected to either the vertical or horizontal synchronising signal.
With the advent of multiple mode monitors, capable of supporting more
than one of these combinations, switches on the monitor, or software to manually override the auto-detected monitor type were required. Either
of these solutions to the problem of identification of the monitor type connected involved the manual setting of the monitor type.

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