Monitor Ports

To use any type of display, you must have matching video hardware in your computer. Fortunately, most portable and desktop computers have built-in hardware and a monitor socket. Other machines often come with a video card already installed in an available AGP, PCI, NuBus or other type of expansion slot. Your monitor can then be plugged into the video connector on the card.

The connection between a computer and monitor can be digital or analogue. A digital connection may be necessary for an Liquid Crystal Display (LCD) monitor, whilst an analogue interface is required for a Cathode Ray Tube (CRT) display. And, although some LCD devices can work with analogue signals, you’ll always get better results with a digital interface.

Display Data Channel (DDC)

Some video interfaces carry extra data between the computer and monitor to control the display. The Display Data Channel (DDC) mechanism is the most common system, often including support for Extended Display Identification Data (EDID).

DDC uses the Inter-Integrated Circuit Bus, more commonly known as the I2C bus, which was originally devised by Philips. This 8-bit bidirectional interface, in which the most significant bit (MSB) is sent first, requires only two wires, a data line and a clock circuit. It runs at 100 kbit/s in standard mode, 400 kbit/s in fast mode and 3.4 Mbit/s in high-speed mode.

Digital Interfaces

If you have an LCD monitor you’ll get best results by using a digital connection, assuming your machine or card provides this kind of output and that the display itself is designed for a digital input.

This kind of connection is really the obvious partner for an LCD screen: the computer simply sends digital instructions to the display, telling it to set the brightness and colour of specific pixels to particular values. This means that the display doesn’t exhibit any of the usual artifacts associated with an analogue connection, such as patterning, jitter or flicker.

The various types of digital connection are described in the following sections.

Apple Display Connector (ADC)

This proprietary interface, now abandoned, is really a non-standard implementation of DVI (see below). It provides digital signals for an LCD monitor, analogue signals for a CRT display, USB circuits and power. It accommodates almost any type of display, optionally controlled via USB or powered from the supply in the computer. The video data travels over six data pairs, with DDC data sent over separate circuits.

The ADC port appears on a special 35-way socket, as shown below.

This is wired as follows:-

1+25V+25 ​V ​Display ​Supply
2+25V+25 ​V ​Display ​Supply
3LEDMonitor ​LED ​Feed
4TX0-Channel ​0 ​Data ​-
5TX0+Channel ​0 ​Data ​+
6SHLD0/5Channels ​0 ​and ​5 ​Shield
7TX5-Channel ​5 ​Data ​-
8TX5+Channel ​5 ​Data ​+
9SDADDC ​Data ​(bidirectional)
10VSYNCAnalogue ​Vertical ​Sync
11SPLY_GNDDisplay ​Supply ​Ground
12SPLY_GNDDisplay ​Supply ​Ground
13PWRNSoft ​Power ​On
14TX1-Channel ​1 ​Data ​-
15TX1+Channel ​1 ​Data ​+
16SHLD1/3Channels ​1 ​and ​3 ​Shield
17TX3-Channel ​3 ​Data ​-
18TX3+Channel ​3 ​Data ​+
19SCLDDC ​Clock
20SCL_GNDDDC ​Clock ​Ground
21USB+USB ​Data ​+
22USB-USB ​Data ​-
23USB_GNDUSB ​Ground
24TX2-Channel ​2 ​Data ​-
25TX2+Channel ​2 ​Data ​+
26SHLD2/4Channels ​2 ​and ​4 ​Shield
27TX4-Channel ​4 ​Data ​-
28TX4+Channel ​4 ​Data ​+
29TXC+Clock ​+
30TXC-Clock ​-

and includes the following analogue connections:-

PinCodeAnalogue Function
C4HSYNCHorizontal Sync

The interface provides a +25 V feed for powering the display, although any self-powered monitor can also be used. In addition, the PWRN circuit allows the monitor to incorporate a (Power) button, which is used to start the computer. Similarly, the LED circuit can feed an indicator on the display, showing that the machine is actually running.

Digital Flat Panel (DFP)

This digital interface, also known as PanelLink, is for LCD screens. The circuitry, in common with ADC (see above) uses TMDS, this time with four data pairs, plus separate wires for DDC information, the latter also supporting EDID.

The 20-way Mini D Ribbon (MDR-20) socket, as fitted to a computer, is shown below.

This is wired as follows:-

1TX1+Channel ​1 ​Data ​+
2TX1-Channel ​1 ​Data ​-
3SHLD1Channel ​1 ​Shield
4SHLDCClock ​Shield
5TXC+Clock ​+
6TXC-Clock ​-
7GNDLogic ​Ground
8+5V+5 ​V ​from ​interface
9NCNot ​connected
10NCNot ​connected
11TX2+Channel ​2 ​Data ​+
12TX2-Channel ​2 ​Data ​-
13SHLD2Channel ​2 ​Shield
14SHLD0Channel ​0 ​Shield
15TX0+Channel ​0 ​Data ​+
16TX0-Channel ​0 ​Data ​-
17NCNot ​connected
18HPDHot ​Plug ​Detection ​*
19SDADDC ​Data ​(bidirectional)
20SCLDDC ​Clock
* Linked to pin 8 in monitor to indicate monitor is connected, although it may not be powered

Digital Visual Interface (DVI)

This type of port, fitted in recent Mac OS machines and PCs, is primarily designed for a DVI display that has a matching DVI socket. It employs Transmission Minimised Differential Signalling (TMDS), with six data pairs for the video material. Separate wires carry the DDC information between the monitor and computer, which can be used to control the monitor.

The DVI port appears on a special 29-way socket, as shown below.

This is wired as follows:-

1TX2-Channel 2 Data -
2TX2+Channel 2 Data +
3SHLD2/4Channels 2 and 4 ​Shield
4TX4-Channel 4 Data -
5TX4+Channel 4 Data +
7SDADDC Data ​(bidirectional)
8VSYNCAnalogue Vertical ​Sync •
9TX1-Channel 1 Data -
10TX1+Channel 1 Data +
11SHLD1/3Channels 1 and 3 ​Shield
12TX3-Channel 3 Data -
13TX3+Channel 3 Data +
14+5V+5 V from interface
155V_GNDGround for +5 V
16HPDHot Plug ​Detection *
17TX0-Channel 0 Data -
18TX0+Channel 0 Data +
19SHLD0/5Channels 0 and 5 ​Shield
20TX5-Channel 5 Data -
21TX5+Channel 5 Data +
22SCL_GNDDDC Clock Ground
23TXC+Clock +
24TXC-Clock -

Only used on a DVI-I port, which also conveys analogue signals

* Linked to pin 14 in monitor to indicate monitor is connected, although it may not be powered

Some devices have separate DVI-Digital (DVI-D) and DVI-I ports, the DVI-D port providing standard DVI digital signals, as shown above, whilst the DVI-I variation also supplies SVGA analogue signals, wired to the following additional pins:-

PinCodeAnalogue Function
C4HSYNCHorizontal Sync

Analogue Interfaces

The connections for a CRT monitor are usually analogue, since the display is an analogue device. The circuits are often similar to those in audiovisual equipment, conveying three one-volt signals, representing the intensity of red, green and blue (RGB), as well as synchronisation (synch) signals that keep the monitor’s scanning in step with the computer.

Modern video interfaces, including older non-Apple video cards, generate signals at various refresh rates, resolutions and pixel counts, most of which are accommodated by modern multi-scan or multi-synch monitors. Unfortunately, older types of Apple monitor work at a fixed refresh rate and pixel count, so they’re often unsuitable for use with modern hardware.

Fortunately, most analogue monitors work with almost any suitable video connection, although some older monitors can’t cope with the faster signals produced by newer equipment. Most multi-scan monitors comply to the VGA or SVGA standards, originally designed for used with a PC.

Common analogue interfaces are described in the following sections.

Silicon Graphics Inc (SGI)

This appears on SGI computers and SGI-compatible video cards. It’s similar to the more common SVGA interface (see below) but appears on the computer as a 13-way plus 3-way D (13W3) socket, as shown below.

The R, G and B coaxial connections are always used for the Red, Green and Blue circuits respectively, although a greyscale monitor only uses the G connection, this being used for the ‘grey’ signal. The use of the other pins varies, although they always carry signals at TTL levels. A device that supports DDC information is usually connected as follows:-

2SDADDC Data ​(bidirectional)
3CSYNCComposite Sync
4HSYNCHorizontal Synch
5VSYNCVertical Sync
6+5V+5 V from ​interface
8GNDLogic Ground
9GNDLogic Ground
10GNDLogic Ground

whilst other monitors are normally wired as follows:-

1ID3Monitor ID Bit 3
2ID0Monitor ID Bit 0
3CSYNCComposite Sync *
4HSYNCHorizontal Sync •
5VSYNCVertical Sync •
6ID1Monitor ID Bit 1
7ID2Monitor ID Bit 2
8GNDLogic Ground
9GNDLogic Ground
10SYNC_GNDSync Ground

* Active Low

Active High

Related Standards

Other computers, including some PowerPC-based machines and Sun workstations, also use the 13W3 connector, again with the R, G and B circuits on the same pins, but with various assignments to the other pins. The PowerPC connections are:-

1ID2Monitor ID Bit 2
2ID3Monitor ID Bit 3
3TESTSelf Test
4GNDLogic Ground
5HSYNCHorizontal Sync
6ID0Monitor ID Bit 0
7ID1Monitor ID Bit 1
8NCNot connected
9VSYNCVertical Sync
10GNDLogic Ground

whilst modern Sun workstations are wired as follows:-

1NCNot connected
2NCNot connected
3ID2Monitor ID Bit 2
4GNDLogic Ground
5CSYNCComposite Sync
6NCNot connected
7NCNot connected
8ID1Monitor ID Bit 1
9ID0Monitor ID Bit 0
10CS_GNDComposite Sync ​Ground

The latter normally supports monitors of 1152 × 900 or 1280 × 1024 pixels, running at vertical scan rates of 66 or 76 Hz.

Super Versatile Graphics Array (SVGA)

Many modern computers and video cards are fitted with a standard VGA/SVGA 15-way high-density D (HD15) socket. This connector, which has three rows of pins, shouldn’t be confused with the older 15 way D (DB15) plug that only has two rows, as used on earlier Apple computers and monitors. The modern HD15 socket is illustrated below.

This socket matches the plug fitted to nearly all VGA and SVGA monitors. Hence you can usually connect a monitor using a cable fitted with an HD15 plug and socket.

The connector is commonly wired as follows:-

4ID2Monitor ID 2 ‡
5NCNot connected ‡
6R_GNDRed Ground
7G_GNDGreen Ground
8B_GNDBlue Ground
9-No pin (key) #
10SYNC_GNDSync Ground
11ID0Monitor ID 0 * ‡
12ID1Monitor ID 1 •
13HSYNCHorizontal Sync
14VSYNCVertical Sync
15NCNot connected †

Commonly connected to ground

# Used for +5 V supply from interface in some Apple hardware

* Optional circuit connected to ground to indicate colour in older colour monitors

Connected to ground in some mono monitors, later used for DDC data (SDA)

Used for DDC clock (SCL) in later hardware

Note that only the G and G_GND circuits are used in a monochrome monitor. SVGA and XGA hardware normally supports line rates of 31.5 to 117 kHz and frame rates of 40 to 100 Hz, although VGA supports only 31.5 kHz and 60 to 70 Hz.

VGA circuits can also appear on a 9-way D socket (DB9), as shown below:-

which is wired as follows:-

4HSYNCHorizontal Sync *
5VSYNCVertical Sync •
6R_GNDRed Ground
7G_GNDGreen Ground
8B_GNDBlue Ground
9SYNC_GNDSync Ground

* Composite Sync (CS) on IBM PGC

Not connected on IBM PGC

Classic Apple Connector

Older Apple monitors and computers have a 15-way D (DB15) socket, as illustrated below:

which is wired as follows:-

1R_GNDRed Ground
3CSComposite Sync
4ID0Monitor ID 0
6G_GNDGreen Ground
7ID1Monitor ID 1
8NCNot connected
10ID2Monitor ID 2
11CS/VS_GNDComposite/​Vertical Sync ​Ground
12VSYNCVertical Sync
13B_GNDBlue Ground
14HS_GNDHorizontal Sync ​Ground
15HSYNCHorizontal Sync

The computer and monitor are normally connected via a plug-to-plug cable. However, some third-party video cards have a D socket with integral coaxial contacts or another connector, requiring an adaptor cable or an inline adaptor.

If your monitor has separate BNC coaxial sockets for red, green and blue circuits, and sometimes for synchronisation as well, you’ll need a suitable adaptor cable. Sometimes only three plugs are used, with composite synchronisation sent over the green circuit. In other cases the synchronisation is sent on its own, sometimes separated into horizontal synchronisation and vertical synchronisation signals, demanding a total of five or six plugs.

To connect a VGA or SVGA monitor to an Apple DB15 port you’ll need an adaptor cable or inline adaptor, fitted with a DB15 plug for the computer or video card and an HD15 socket for the monitor, wired as follows:-

Red Ground16
Green Ground67
Blue Ground138
Horizontal Sync1513
Vertical Sync1214
Horiz Sync Ground1410
Composite/Vertical ​Sync Ground114
With some types of Apple computer or video card you must link pins 7 and 10 at the DB15 connector to persuade the video circuitry to produce an SVGA signal.

If you don’t want to make your own adaptor cable you can buy an inline adaptor from your Apple dealer. Just push the adaptor into the back of your computer and then plug in a standard SVGA cable.

Obsolete Interfaces

Even if you can make them work, most older monitors give inferior results when connected to a modern computer. The original PC and other older computers generate logical video signals that are incompatible with the analogue signals required by a modern monitor. These outdated RGB interfaces employ 5 volt transistor-transistor logic (TTL) signals. In a simple TTL interface only eight shades of colour are produced, since each primary colour is simply on or off.

You can’t use a TTL monitor with a modern machine unless you do one of the following:-

  1. Fit a special video interface card onto a suitable computer’s motherboard and connect a cable directly between the card and the TTL monitor.
  2. Install a video card that supports a TTL monitor, if you can find one.

The results will be ghastly compared to a modern display, although old TTL monitors can be obtained very cheaply. In addition, most monitors of this kind are designed for low resolution (LR) video formats. Remember, a TTL monitor simply can’t cope with the signal from a modern video card.

To summarise, all TTL monitors should be consigned to the dust-cart of history.


The original PC accepted a Monochrome Display Adaptor (MDA), Colour Graphics Adaptor (CGA) or Enhanced Graphics Adaptor (EGA) card in an expansion slot. Each adaptor works in two basic modes. In a text mode (T) each character is simply placed on the screen, but in a graphics mode (G) each item is drawn pixel-by-pixel. The standard video modes are:-

0T140 ​char25 ​line-CGA ​EGA+
1T1640 ​char25 ​line-CGA ​EGA+
2T180 ​char25 ​line-CGA ​EGA+
3T1680 ​char25 ​line-CGA ​EGA+
4G4320 ​px200 ​pxMedCGA ​EGA
5G1320 ​px200 ​pxMedCGA ​EGA
6G2640 ​px200 ​pxHighCGA ​EGA
7T180 ​char25 ​line-MDA
8G16160 ​px200 ​pxLowSpecial
9G16320 ​px200 ​pxMedSpecial
10G4640 ​px200 ​pxHighSpecial
13G16320 ​px200 ​pxMedEGA
14G16640 ​px200 ​pxHighEGA
15G1640 ​px200 ​pxHighEGA
16G64640 ​px200 ​pxHighEGA
+ EGA can provide 43 lines of text in these modes

Note that 1-bit video can only produce monochrome images or text, with an increasing number of bits accommodating more colours.

Besides these modes, an MDA or Hercules Graphics card can be used for monochrome graphics at 720 by 350 pixels. Other products may produce monochrome or colour graphics at 720 × 348 pixels. Unfortunately, all the above graphics modes use interlaced scanning.

In the MDA and CGA text modes a character box of 8 × 13 dots is used, although each character only occupies a box of 7 × 13, allowing for spaces between characters. CGA can also use 8 × 8 characters, although these look absolutely dreadful.

EGA is a replacement for MDA and CGA. It creates graphics at up to 640 × 350 pixels and in text mode produces reasonable characters of 8 × 14 dots. Super EGA (SEGA) is similar to EGA but provides graphics at up to 640 × 480 pixels.

TTL Monitor Connections

The MDA, CGA, EGA and SEGA interfaces use a 9-way D socket (DB9), as shown below:-

This connector is wired as follows:-

2GroundGroundSecondary ​Red
3-RedPrimary ​Red
4-GreenPrimary ​Green
5-BluePrimary ​Blue
6IntIntSecondary ​Green ​or ​Int
7Mono-Secondary ​Blue ​or ​Mono
8Horiz ​SyncHoriz ​SyncHoriz ​Sync
9Vert ​SyncVert ​SyncHoriz ​Sync

The intensity circuit (Int) in CGA creates 16 shades by switching the display’s intensity from to bright to dim. If this circuit is missing the display still works but is limited to eight shades. EGA and SEGA provide 64 shades by using three intensity circuits, also known as secondary colour circuits.

Connecting Audiovisual Devices

The interfaces described in the main part of this document are specifically designed for connecting a computer to a high-quality monitor, giving pixel-by-pixel resolution. Hence they employ specific technologies that are usually incompatible with the circuitry used for standard video or audiovisual equipment.

In some instances, however, you may want to connect the monitor output of your computer to another device. For example, you could send computer-generated images to a video projector or to several video monitors, so as to present your material to a larger audience, or you could transfer the content to a video cassette recorder (VCR) or similar video recording device.

Connecting via RGB

The standard family of VGA or XGA interfaces (see above) can’t normally be connected directly to video devices. However, a connection may be possible if:-

  1. The video device has a SCART socket that accepts RGB signals.
  2. Your computer monitor output can generate a full NTSC or PAL image.
  3. The signal from the computer is fully interlaced.
  4. You have obtained a suitable adaptor cable.

Composite Video, RF and S-Video

A composite video signal conveys all of the information about a moving image over a single screened wire, which is usually connected via an RCA phono plug, also known as a PIN plug.

Unfortunately, using composite connections can degrade picture quality. The S-Video interface overcomes this by separating the signal into its components, which are then conveyed via multiway cable and a special 4-pin mini-DIN connector.

Some computers provide useful video outputs. For example, some Apple portables have a 4-pole 3.5 mm jack socket that provides an audio output and composite video connection, some PowerBook models have a combined S-Video and composite output socket at the rear of the machine and others have a DVI output that provides a composite signal via a special adaptor.

If your computer doesn’t have a suitable video output you can use a gadget known as a video presenter. This is wired to the normal VGA/SVGA port on your computer, converting the signal into suitable video formats, usually composite video and S-Video, with a SCART connector option. Unfortunately, the results aren’t always impressive, since the digital signals created by computers are significantly different to those produced by conventional video technology.


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©Ray White 2004.