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Dale Adams on Centris & Quadra 800 Video Capabilities

Built-In Video on the Macintosh Centris 610/650 and Quadra 800

There will probably be a number of questions which arise concerning the built-in video capabilities of the new Macintosh Centris 610 and 650, and the Quadra 800. In anticipation of these questions, here is an article (similar to that which I posted for the earlier Quadra machines) which provides an description of these video capabilities. This article discusses a few general video topics, details how to wire the video connector sense pins to access all the supported video modes of these machines, and describes the memory configurations necessary to support each of the video modes at specific pixel depths. (I am the designer of the video hardware for the Quadra 700/900/950/800 and the Centris 610/650, you can be reasonably sure this information is accurate.)

General Video Design Philosophy

The video hardware design of the Centris 610/650 and the Quadra 800 is highly derivative from the earlier Quadra machines. However, one of the main goals of these CPUs (and perhaps the #1 goal) was to reduce the cost of the machines relative to the Quadra 700 and 950, while still providing the same (or better!) performance. While it was not possible to significantly reduce cost while maintaining the exact feature set of the earlier Quadras, most of the Quadra video features were carried through to the Centris 610/650 and Quadra 800. The main exceptions are support for Apple convolution (flicker reduction) for NTSC and PAL, and support for 24 bits per pixel (bpp).

The video hardware for all three CPUs, the Centris 610 (C610), Centris 650 (C650) and Quadra 800 (Q800), is virtually identical. The only exception is that the C610 only requires 100 ns VRAM, while the C650 and Q800 require 80 ns VRAM.

The maximum supported pixel depth is 16 bpp. This is not a matter of the amount of VRAM in the machine - it is a hardware limitation The RAM/DAC used by all three machines simply does not include the hardware required to do 24 bpp on any display. The same is true for the lack of support for Apple convolution - the hardware necessary to do this is not present in the machine.

24 bpp support was dropped for a number of reasons:

  1. Cost reduction. It is still relatively expensive to provide the 24 bpp support offered by the Q700 and Q950. It would not have been possible to hit the price point of the C610 while still offering 24 bpp.

  2. Marketing research data has shown that a very large percentage of Quadra users do not use the onboard video, but rather use an accelerated video card capable of driving a 2-page display at 24 bpp. Since it was not reasonable to burden the price of every C610, C650 and Q800 with the cost of a 2-page 24 bpp frame buffer, the 24 bpp feature was dropped altogether (with a net result of a substantial decrease in cost).

  3. 16 bpp offers most of the advantages of 24 bpp, without much of the cost and at a higher level of performance. QuickTime MooV playback is optimized for 16 bpp. Also, for casual browsing of image data (e.g., scanned images or PhotoCD pictures) 16 bpp is quite adequate and offers better performance with fewer system resource requirements than 24 bpp.

Convolution support was dropped primarily for cost reasons, and also because it is very rarely used. NTSC and PAL timing support are still offered, however. The C610, C650, and Q800 do support all the monitor types supported by the Q950. This includes support for a 1024 x 768 resolution on 19-inch displays (which was not provided by the Q700).

One area that was positively impacted was performance. At the same processor clock speed, the video section of these new machines outperforms the video section of the older Quadras. (I.e., video performance on the C650 is better than the Q700, and on the Q800 is better than the Q950.) Due to an improved video memory controller design, one wait state was removed from many of the frame buffer access cycles. This results in reduced memory access time overall, and therefore improved performance. The graphics tests in Speedometer (version 3.11) show an improvement of roughly 6-10% over the earlier Quadra machines (each running System 7.1).

Supported Display Configurations and Monitor ID Codes

The Centris/Quadra frame buffer determines what type of display is attached to the video connector by examining the state of 3 sense line pins. The following chart details how these three pins must be wired for each of the supported display types. For each supported display, the screen resolution (horiz. pixels X vertical pixels), dot clock frequency, and the vertical and horizontal scan rates are listed.

Basically, the Centris 610/650 and Quadra 800 support any display, whether from Apple or from another vendor, that meets one of the following specifications:


                     Sense pins       Hor x Vert    Dot      Vert     Horiz
Display              10   7    4      Pixels        Clock    Refrsh   Refrsh
-----------          -----------      ----------    -----    ------   ------

Apple 21S Color      0    0    0     1152 x 870     100        75     68.7

Apple Portrait       0    0    1      640 x 870     57.2832    75     68.9

12" Apple RGB        0    1    0      512 x 384     15.6672    60.15  24.48

Apple Two-Page Mono. 0    1    1     1152 x 870     100        75     68.7

NTSC                 1    0    0  underscan-512x384 12.2727    59.94  15.7
                     1    0    0  overscan- 640x480 12.2727    59.94  15.7
(To produce a color NTSC signal, a RGB-to-NTSC converter is required.)

12" AppleMonochrome  1    1    0      640 x 480    30.24      66.7    35.0

13" Apple RGB        1    1    0      640 x 480    30.24      66.7    35.0

Extended sense codes will be examined if the following sense code is 
                     1    1    1
NOTE 1 on above monitors: A sense pin value of 0 means that the pin should be grounded to the C&V SYNC.GND signal; a value of 1 means do not connect the pin.

NOTE 2 on above monitors: sense pins 4, 7, and 10 are referred to as SENSE0, SENSE1, and SENSE2 in pinout tables for the video connectors.

NOTE 3: The terms 'underscan' and 'overscan' are used to describe the active video resolution for NTSC and PAL modes. Underscan means that the active video area appears in a rectangle centered on the screen with a black surrounding area. This ensures that the entire active video area always is displayed on all monitors. Overscan utilizes the entire possible video area for NTSC or PAL. However, most monitors or televisions will cause some of this video to be lost beyond the edges of the display, so the entire image will not be seen.


NOTE for extended sense codes: A sense pin pair value of 0 means those pins should be tied together (as opposed to grounding the pins to pin 11); a value of 1 means do not connect the pins. Do _not_ wire any of these pins to ground.

                   Sense pins      Hor x Vert      Dot      Vert     Horiz
Display           4-10 10-7 7-4      Pixels        Clock    Refrsh   Refrsh
-----------       -------------    ----------      -----    ------   ------

16" Color          0    1    1      832 x 624      57.2832    75     49.7

PAL has two wiring options, using the extended sense pin configuration.  
To produce a color PAL signal, an RGB-to-PAL converter is required.

PAL Option 1       0    0    0   underscan-640x480  14.75     50     15.625
                                 overscan-768x576   14.75     50     15.625

PAL Option 2       1    1    0   underscan-640x480  14.75     50     15.625
                                 overscan-768x576   14.75     50     15.625
Note: This sense code also requires a diode between sense pins 10 & 7, 
with anode towards pin 7, cathode towards pin 10. 

VGA                1    0    1       640 x 480      25.175    59.95   31.47

SVGA               1    0    1       800 x 600      36         56     35.16
To enable SVGA, after configuring and connecting the monitor for VGA, open 
the Monitors control panel and select Options.  Choose Super VGA from the 
dialog and reboot your system.

19" Color          1    1    0      1024 x 768      80         75     60.24

No external monitor (video halted)
                   1    1    1

Here are the video connector pinouts:

Pin    Signal          Description
-----  -----------     ----------------------------------------
1      RED.GND         Red Video Ground
2      RED.VID         Red Video
3      CYSNC~          Composite Sync
4      MON.ID1         Monitor ID, Bit 1 (also known as SENSE0)
5      GRN.VID         Green Video
6      GRN.GND         Green Video Ground
7      MON.ID2         Monitor ID, Bit 2 (also known as SENSE1)
8      nc              (no connection)
9      BLU.VID         Blue Video
10     MON.ID3         Monitor ID, Bit 3 (also known as SENSE2)
11     C&V SYNC.GND    CSYNC & VSYNC Ground
12     VSYNC~          Vertical Sync
13     BLU.GND         Blue Video Ground
14     HSYNC.GND       HSYNC Ground
15     HSYNC~          Horizontal Sync
Shell  CHASSIS.GND     Chassis Ground
If your monitor is a VGA type, you can try the following cable pinouts.
Macintosh Video                         VGA Connector
-------------                           --------------

2  ------------------- Red Video ------------ 1
1  ------------------- Red Ground ----------- 6
9  ------------------- Blue Video ----------- 3
13 ------------------- Blue Ground ---------- 8
5  ------------------- Green Video ---------- 2
6  ------------------- Green Ground --------- 7
15 ------------------- Hsync ---------------- 13
12 ------------------- Vsync ---------------- 14
14 ------------------- Sync Ground ---------- 10
10 ------------------|
7  ------------------| Connect 7 and 10 so the sense pin ID will equal VGA

There are a few issues to keep in mind with VGA monitors:

Most NTSC devices use an RCA-type phono-connector and the following diagram uses that as a reference point. A cable wired as follows may allow many different brands of NTSC monitors to work on a Macintosh Centris or Quadra. I would advise you to test the monitor on one of these machines prior to purchase to see if it meets your expectations.

Adjust the phono-connector side to whatever type of connector is used (RCA, BNC, etc.). "Tip" is the pin in the center of the connector (the signal); the sleeve is flange around the outer edges of the connector (the chassis ground).

   Card Connector                     RCA-Type Phono-Connector
   --------------                     ------------------------
   4      MON.ID1  (sense0) --|
   7      MON.ID2  (sense1) --|
   11     C&V SYNC.GND --------|

   5      GRN.VID  -----------------> Tip (signal)
   Shell  CHASSIS.GND --------------> Sleeve (ground)
By grounding pin 4 and pin 7 to pin 11, the Macintosh Centris and Quadra CPUs are told that an NTSC monitor is attached. The actual black and white video signal is on pin 5 and connects to the center (Tip) of the phono-plug. The shell of the card connector connects to the sleeve of the phono-plug.

To acquire a color NTSC signal from a Centris or Quadra (or any Apple Macintosh display card), an RGB-to-composite video converter is required.

VRAM Requirements for Supported Display Configurations

The frame buffers on the new Centris and Quadra machines support a variety of pixel depths, from 1 to 16 bits per pixel (bpp). The supported pixel depths (1, 2, 4, 8, or 16 bpp) depend on the display resolution and the amount of VRAM present. The fully expanded capability of all three machines is the same - 1 MB of VRAM. As with the Quadra 950, these machines can be expanded using 256K (i.e., 128K x 16) 80 nS VRAM SIMMs (although the C610 only requires 100 ns VRAM).

The following chart lists the Centris 610/650 and Quadra 800 built-in video's maximum pixel depth supported depending upon the VRAM configuration:

Display size                        512K VRAM     1MB VRAM
-----------------                   ---------     --------
12-inch landscape
384 x 512                           16 bpp         16 bpp

12-inch Monochrome
640 x 480                            8 bpp          8 bpp

13-inch RGB & VGA
640 x 480                            8 bpp         16 bpp 

800 x 600                            8 bpp         16 bpp 

15-inch Portrait (b/w)
640 x 870                            4 bpp          8 bpp

16" Color,
832 x 624                            8 bpp         16 bpp

19" Color,
1024 x 768                           4 bpp          8 bpp

2-Page Display (b/w)
1152 x 870                           4 bpp          8 bpp 

21" Color
1152 x 870                           4 bpp          8 bpp 

underscan-640x480                    8 bpp         16 bpp
overscan-768x576                     8 bpp         16 bpp 

underscan-512x384                    8 bpp         16 bpp
overscan- 640x480                    8 bpp         16 bpp
- Dale Adams

Netscape HTML Checked! November 16, 1993 - Robert Lentz (

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