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Mac Tech: Displays

Warning

  • Don’t operate a display with parameters (resolution, scan rate, etc.) it wasn’t designed for.
  • Don’t open an analog display unless you have training on discharging the CRT inside.

Be careful. Improper electrical work can damage electronics, or you.

Contents

Summary

Pinout

Description DA-15 VGA
Red signal 2 1
Red ground 1 6
Green signal 5 2
Green ground 6 7
Blue signal 9 3
Blue ground 13 8
HSync signal 15 13
HSync ground 14 5
VSync signal 12 14
CSync signal 3 NC
VSync/CSync ground 11 10
Sense 0 4 NC
Sense 1 7 NC
Sense 2 10 NC
Unused 8 NC
Chassis ground Shell Shell

I’ve seen schematics with all the grounds (pins 1, 6, 11, 13, and 14) wired to the shell, and observed this in an adapter in practice.

Configurations

Code Mode Resolution Wiring Example
N/A 8” 512×342 N/A Macintosh 128k internal
000 21” Color 1152×870 G=4=7=10  
001 Apple Portrait 640×870 G=7=10 Macintosh Portrait Display
010 12” 512×384 G=4=10 Macintosh 12″ RGB Display
011 21” Mono 1152×870 G=10  
100 NTSC Monitor 512×384 G=4=7 American TV [1]
101 Color Portrait 640×870 G=7 Radius Full Page Display
110 13” 640×480 G=4 AppleColor High-Resolution RGB Monitor
111-000000 PAL Encoder 512×384 4=7=10 European TV [3]
111-010100 NTSC Encoder 512×384 7=10, 10→4 American TV [1]
111-010111 VGA 800×600 [2] 7=10 Non-Apple monitor
111-101101 16” 832×624 4=10  
111-110000 PAL Monitor 512×384 4=7, 10→4 European TV [3]
111-111010 19” 1024×768 4=7  
110-101011 Up to 13” Up to 640×480 G=4 (same as 13” above)
110-000011 Up to 14” Up to 800×600 G=4, 7=10 Apple Multiple Scan 14 Display
110-001011 Up to 17” Up to 1024×768 G=4, 7→10 Apple Multiple Scan 15 Display
110-100011 Up to 21” Up to 1152×870? G=4, 10→7 Apple Multiple Scan 20 Display
111-111111 No display   None  
[1](1, 2) Overscan 640×480; underscan 512×384. I don’t know what the difference between a “NTSC Encoder” and a “NTSC Monitor” is.
[2]Defaults to 640×480; change to 800×600 and restart.
[3](1, 2) Overscan 640×480; underscan 512×384. I don’t know what the difference between a “PAL Encoder” and a “PAL Monitor” is.

Details

Compact Mac

The original Macintosh had a single resolution: 512×342. This resolution remained unchanged on later black and white Macs: the Plus, SE, SE/30, and Classic. (The Color Classic is 512×384)

Size 8”
Visible area 512×342
Total area 704×370
Scan rate 60.15 Hz
Line rate 22.25 kHz
Dot clock 15.6672 MHz [4]
Width 512px
Total width 704px
HBlank 192px
Front porch 14px
HSync 288px
Back porch -110px [5]
Height 342px
Total height 370px
VBlank 28px
Front porch 0px
VSync 4px
Back porch 24px
[4]15.6672 MHz is twice the clock rate of the original Macintosh, or 68 * 32 * 7200 Hz.
[5]The HSync pulse is longer than the the HBlank interval, so it overlaps the visible part of the scan line, and there is no back porch.

Sense codes

The Macintosh II required an external display, and connected to it through a DA-15 video port. Aside from the signal and ground pins, three sense pins were used so that the computer would know what kind of display was connected. Any or all of them could be grounded, identifying 8 possible configurations.

These configurations are identified by a 3-bit sense code ABC:

  • A=0 if sense 2 (pin 10) is grounded; A=1 if floating
  • B=0 if sense 1 (pin 7) is grounded; B=1 if floating
  • C=0 if sense 0 (pin 4) is grounded; C=1 if floating
Size 12” 13” [6] Portrait 21”
Sense codes 010 110 101, 001 000, 011
Visible area 512×384 640×480 640×870 1152×870
Total area 640×407 864×525 832×918 1456×915
Scan rate 60.15 Hz 66.67 Hz 75 Hz 75 Hz
Line rate 24.48 kHz [7] 35.00 kHz 68.9 kHz 68.68 kHz
Dot clock 15.6672 MHz 30.24 MHz 57.2832 MHz 100 MHz
Width 512px 640px 640px 1152px
Total width 640px 864px 832px 1456
HBlank 128px 224px 192px 304px
Front porch 16px 64px 32px 32px
HSync 32px 64px 80px 128px
Back porch 80px 96px 80px 144px
Height 384px 480px 870px 870px
Total height 407px 525px 918px 915px
VBlank 23px 45px 48px 45px
Front porch 1px 3px 3px 3px
VSync 3px 3px 3px 3px
Back porch 19px 39px 42px 39px
[6]Some machines with low amounts of VRAM support a “640×400” mode, allowing 16-bit color at the cost of screen space. The parameters are the same as 640×480, letterboxing it by adding 40px each to the front and back porch.
[7]While this resolution shares the overall scan rate (60.15 Hz) and dot clock (15.6672) with the Compact 8” resolution, the line rate differs. Despite having the same total pixel size, the total area is more squarish. Reusing the 8” screen’s parameters would have been impossible, because its total height is less than 384px.

Extended Type-7 sense codes

With more resolutions, new sense codes were needed. In order to prevent older computers from detecting these newer displays and trying to display to them, the three sense pins were left ungrounded (which indicates “no display attached” to older computers), and some combination of the sense pins were tied together, either directly or with diodes. To detect the display, the computer would:

  1. Check if any sense pins are grounded (indicating a non-extended code)
  2. Ground each sense pin in turn, checking which other pins were pulled low in response.

These configurations are identified by a 9-bit sense code 111-ABCDEF:

  • 111, indicating the basic sense code, which is 7 in binary
  • A=0 if grounding sense 2 (pin 10) would pull sense 1 (pin 7) low
  • B=0 if grounding sense 2 would pull sense 0 (pin 4) low
  • C=0 if grounding sense 1 would pull sense 2 low
  • D=0 if grounding sense 1 would pull sense 0 low
  • E=0 if grounding sense 0 would pull sense 2 low
  • F=0 if grounding sense 0 would pull sense 1 low

(note that there are 29 possible Type-7 sense codes, not 64, and most are unassigned)

Size 16” 19”
Sense codes 111-101101 111-111010
Visible area 832×624 1024×768
Total area 1152×667 1328×804
Scan rate 75 Hz 75 Hz
Line rate 49.73 kHz 60.24 kHz
Dot clock 57.2832 MHz 80 MHz
Width 832px 1024px
Total width 1152px 1328px
HBlank 320px 304px
Front porch 32px 32px
HSync 64px 96px
Back porch 224px 176px
Height 624px 768px
Total height 667px 804px
VBlank 43px 36px
Front porch 1px 3px
VSync 3px 3px
Back porch 39px 30px

Extended Type-6 sense codes

Eventually displays became able to support multiple resolutions. The minimum resolution supported by such displays was 640×480, so the 13” sense code 110 (grounding pin 4) became the baseline for multiple-resolution displays. Older computers would detect multiple scan monitors as 640×480 displays. For larger resolutions, pins 7 and 10 were connected:

  1. Directly for 14” (max 832×624) [8]
  2. With a diode from 7 to 10 for 17” (max 1024×768)
  3. With a diode from 10 to 7 for 21” (max 1152×870)

These configurations are identified by a 9-bit sense code 110-ABCDEF:

  • 110, indicating the basic sense code, which is 6 in binary
  • ABCDEF using the same method as Type-7 sense codes
[8]VGA adapters may handle this by connecting a diode in both directions. This is fine. It’s convenient when such adapters already have dip switches to diodes for the larger resolutions.

Appendix A: Apple IIe

Machines that can host an Apple IIe card are capable of outputting 560×384, which is double the IIe’s 280×192 “Hi Resolution” graphics mode. It shares its vertical parameters with the standard Macintosh 512×384 mode, but runs with a faster dot clock.

Size 12” Quad Hi-Res
Visible area 512×384 560×384
Total area 640×407 704×407
Scan rate 60.15 Hz 60.15 Hz
Line rate 24.48 kHz 24.48 kHz
Dot clock 15.6672 MHz 17.2340 MHz
Width 512px 560px
Total width 640px 704px
HBlank 128px 144px
Front porch 16px 16px
HSync 32px 48px
Back porch 80px 80px
Height 384px 384px
Total height 407px 407px
VBlank 23px 23px
Front porch 1px 1px
VSync 3px 3px
Back porch 19px 19px

Appendix B: VGA Timing

For comparison, here are the standard timings for VGA resolutions at 60 Hz:

Size VGA SVGA XGA
Visible area 640×480 800×600 1024×768
Total area 800×525 1056×628 1344×806
Scan rate 60 Hz 60 Hz 60 Hz
Line rate 31.46875 kHz 37.8787… kHz 48.3630… kHz
Dot clock 25.175 MHz 40 MHz 65 MHz
Width 640px 800px 1024px
Total width 800px 1056px 1344px
HBlank 160px 256px 320px
Front porch 16px 40px 24px
HSync 96px 128px 136px
Back porch 48px 88px 160px
Height 480px 600px 768px
Total height 525px 628px 806px
VBlank 45px 28px 38px
Front porch 10px 1px 3px
VSync 2px 4px 6px
Back porch 33px 23px 29px

Adapters

Generic adapter

For a VGA adapter:

  1. Omit the dip switches and diodes.
  2. Wire Sense1 and Sense2 (DA-15 pins 7 and 10) together directly.

For a multi-scan adapter:

  1. Omit the dip switches.
  2. Wire VGAGnd and Sense0 (D-15 pin 4 and ground) together directly.
  3. Connect Sense1 and Sense2 (DA-15 pins 7 and 10) according to the maximum resolution of the display:
    • 1152×870: diode with cathode on Sense1 (DA-15 pin 7)
    • 1024×768: diode with cathode on Sense2 (DA-15 pin 10)
    • 832×624: wire pins together directly
    • 640×480: no diodes

See Also