The sensing of monitor type is through pins 4, 7, and 10. The sensing pins should be grounded or left unconnected. You can ground a pin by connecting it to Pin 11.
Display Pixels Hz Pins grounded
Apple Hi Res Monochrome 480 x 640 67 Hz 4
Apple Hi Res RGB 480 x 640 67 Hz 4
Apple 12" RGB 512 x 384 67 Hz 4, 10
Apple 15" Portrait 640 x 870 75 Hz 7, 10
Apple 2-Page Monochrome 1152 x 870 75 Hz 10
VGA 480 x 640 10
RS-170A (interlaced) 480 x 640 60 Hz 4, 7
Sources:
Apple Macintosh Technical Note 144: Macintosh Color Monitor Connections.
Apple Macintosh Developer Notes: Macintosh Display Card 8•24 GC.
Message: #27362, S/2 Mac Tech Questions
Date: Tue, May 12, 1992 8:21:16 PM
Subject: LC technical question
From: Kevin Norris [AsstSysop] 75410,1674
To: Ken Settle 76556,753 (received)
Reply: #27441 (1 reply)
Mac LC video connector pinouts:
Pin Number Signal Name Signal Description
1 RED.GND Red ground
2 RED.VID
Red video signal
3,15 /CSYNC or /HSYNC Composite synchronization signal if
Apple monitor. Horizontal
synchronization signal if VGA
monitor.
4 SENSE0 Monitor sense signal 0
5 GRN.VID Green video signal
6 GRN.GND Green ground
7 SENSE1 Monitor sense signal 1 (grounded
internally)
8 n.c. Not connected
9 BLU.VID Blue video signal
10 SENSE2 Monitor sense signal 2
11 C&VSYNC.GND Ground for CSYNC and VSYNC
12 /VSYNC Vertical synchronization signal
13 BLU.GND Blue ground
14 HSYNC.GND HSYNC ground
Shell CHASSIS.GND Chassis ground
A "/" at the beginning of a signal name indicates that the signal is active low.
The Macintosh LC does not supply vertical synchronization with the Green video signal (pin~5). The vertical synchronization signal is supplied on pin 12.
The Macintosh LC requires that pin 4 (SENSE0) be connected to Ground to signal the connection of a 640 x 480 monitor. The Macintosh LC requires that pin 4 and 10 (SENSE0 and SENSE2) be connected to Ground to signal the connection of a 512 x 384 monitor (i.e., the Macintosh 12" RGB Display). The Macintosh LC requires that pin 10 (SENSE2) be connected to Ground to signal the connection of a VGA monitor. Pin 7 (SENSE1) is grounded in the Macintosh LC.