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.

• Some older machines provide a radio frequency (RF) signal or composite video output (see below) for use with a television receiver, but the picture quality is invariably very poor.

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.

• Note that the entries shown in the Code columns in the tables below aren’t universally accepted. However, a consistent code has been used in each table to identify a wire that conveys the same kind of information.

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:-

and includes the following analogue connections:-

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.

• An ADC feed can be connected to a DVI monitor by using an ADC to DVI adaptor. Other adaptors can be used to connect a DVI feed to an ADC monitor (see below).
• To convey an ADC signal over a long distance you must use an ADC repeater box.
• The ADC standard shouldn’t be confused with the DigitalConnect system used for projectors, which conveys DVI and analogue signals via a single connector, sometimes accompanied by USB circuits for projector control.
• Apple’s LCD monitors usually incorporate a USB hub wired to the USB+ and USB- circuits.

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:-

• A DFP output can also be used with a DVI monitor (see below) by using a special DFP to DVI adaptor cable.

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:-

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:-

• A DVI output can be connected to an ADC-equipped monitor via a DVI to ADC adaptor cable, one version of which is known as the DVIator, which also includes two USB plugs that connect the monitor’s USB hub. Alternatively, you can use a DVI to ADC conversion box, which also incorporates a suitable USB connection.
• A DVI output can be used with a DFP monitor via a DVI to DFP adaptor or via a standard DFP connecting cable plugged into an inline adaptor, which has a DVI plug at one and a DFP socket at the other.
• A DVI-I output can be connected to an VGA-equipped monitor via a DVI to VGA adaptor cable.
• Portable computers are sometimes fitted with a mini-DVI connector, requiring you to use a mini-DVI to DVI adaptor, a mini-DV to VGA adaptor or another suitable adaptor.
• Apple’s 30-inch LCD monitor, which has a resolution of 2560 × 1600, requires more data than can be conveyed over a standard DVI circuit, requiring a special video card and a pair of DVI cables.

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.

• Some video cards also have a S-Video output for use with matching equipment.

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:-

whilst other monitors are normally wired as follows:-

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:-

whilst modern Sun workstations are wired as follows:-

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:-

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.

• Don’t worry if some of the pins seem to be missing from the plug on your video cable. This is perfectly normal, since these correspond to circuits that aren’t used.
• Some laptop computers have 7-pole mini-VGA connector, which can be connected to a VGA monitor via a mini-VGA to VGA adaptor cable. The mini-VGA outlet also provides a composite video output, requiring a different adaptor.
• A VGA or SVGA output can be connected to an ADC-equipped monitor via a VGA to ADC adaptor cable or box. This has a HD15 plug for connecting the computer and a USB port for controlling the monitor. However, such an adaptor doesn’t give such a good picture quality as that provided by an ADC-equipped video card or computer.

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

which is wired as follows:-

Classic Apple Connector

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

which is wired as follows:-

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.

• Some older Mac OS computers can be made to produce different signals by fitting links inside the plug that engages with the computer. These are wired in various combinations to the Monitor ID pins (see above).

• Modern video cards often don’t provide composite synchronisation on the green circuit, as required by an older Apple monitor. Sometimes an alternative cable fixes the problem, otherwise you’ll need a different video card.

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:-

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.

• To avoid damaging equipment you should switch everything off before making a connection.
• Not all adaptors contain the adjustable links for setting the mode of a Mac OS computer’s video circuitry, whilst others have fixed links that give odd results with some video cards. Also note that you must restart the computer to force the video circuitry into the new mode after changing the links in an adaptor.

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.

• TTL monitors, as used with a PC, employ interlaced scanning, also known as interleaved scanning. A Mac OS machine, however, requires non-interlaced (NI) scanning, resulting in a jittery image on such a monitor.

MDA, CGA and EGA

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:-

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:-

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.

• All three interfaces are compatible with different types of monitors, although a monochrome monitor may not work with some RGB sources and vice versa.

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.

• The signals in video equipment normally employ interlaced scanning for compatibility with older analogue devices, whilst those generated by a computer are often non-interlaced or use progressive scanning. Supplying a non-interlaced signal to a device that needs an interlaced signal often results in unpleasant image jitter.
• Some devices, such as a plasma display screen or video projector, may require a non-interlaced feed.

Reference

ePanorama website at www.epanorama.net

©Ray White 2004.