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COMPUTER BITS By JENNIFER BONNITCHA Display adapters - from monochrome to thunderdome Early personal computers used to blank the screen while they were performing operations but modern computers don't because they have separate circuitry including dedicated memory to drive the video monitor. This is why we have monochrome display adaptors, colour graphics adaptors and so on. The smallest IBM PC ever made had 16K of RAM. The operating system fitted into working storage but there wasn't much left for anything else. You could write simple BASIC programs but sophisticated applications such as spreadsheets, word processing, graphics and accounting simply would not fit in such a small memory area along with the operating system. During the reign of the 8-bit Apple, Tandy and Commodore computers, 64K of memory seemed ample to run many software products, A Monochrome Display adapter (MDA) drives a monochrome display in text mode using special character generator circuitry contained in the card. 82 SILICON CHIP and run them well. The announcement of IBM's 16-bit personal computer in 1981 changed all that. The IBM PC heralded a new age in both computing and the amount of memory required to run a program. As the applications became more complex, the memory requirements increased, although not at a proportional rate. Increased memory was primarily required for programs utilising colour and producing graphics (and not to mention lazy programmers writing volumes of program code). The need to produce better images brought about a rash of third party add-ons ranging from memory boards through to monitors and graphics boards. Although many types of display adapters can be used on the PC, two adapters represent a large majority of those already in use - the IBM monochrome display and printer adapter card and the colour/graphics monitor adapter. Demand by users has seen the enhanced graphics adapter become the defacto standard for screen display. The monochrome monitor supplied with the original IBM PC was a high-resolution, long-persistence green phosphor display. Longpersistence phosphors tend to prevent any flickering of the image on the screen however they also tend to prevent use of a light pen. The screen was a nominal 30cm diagonally, capable of displaying 25 lines of 80 characters using a 9 x 14 dot matrix to form the characters. An Enhanced Graphics Adapter (EGA) card supports the monochrome display, the colour display and the enhanced colour display. It can support 640 x 350 pixel resolution and display up to 64 colours. Although the position of the characters on the screen changes, their size and shape are fixed. To use the monochrome display, a Monochrome Adapter had to be installed into one of the system expansion slots. The adapter has a 9-pin video connector to which the cable from the monochrome display is attached. A second 25-pin connector. enables the connection of a printer with parallel interface to the system unit. Using connections from the monitor to the electrical outlet at the rear of the system reduces the requirements for power points to power the entire computer system and simplifies the power on/off procedure - just switch the computer on and the monitor is on too. Monochrome Display Adapter The Monochrome Display Adapter (MDA for short) drives the monochrome display in text mode using special character generator circuitry contained in the card. It contains 4000 (4K) 8-bit bytes of onboard display memory. Since the computer uses the card's memory to display screen information, the computer's Random Access Memory (RAM - the computer's thinking and working space) remains unaffected by the display of information. · Although the card allows a limited capability for drawing line and block characters, such as when Borland Sidekick is used, no true graphics capability is present. So if you want to see your work in living, breathing colour, you need the Colour Graphics Adapter (CGA). Colour Graphics Adapter The CGA card has several video interfaces as well as a light pen connector for those who cannot live without one. You can have either a 9-pin "D'' type shell RGB (Red, Green, Blue) connector for a TTL colour monitor or a composite video output via an RCA phono socket to feed a monochrome monitor. Behind the retaining bracket sits a 4-pin Berg strip for connection to an RF modulator. The modulator could then, in turn, be connected to the trusty TV however the lower resolution and bandwidth of the TV normally limits the display to 40 characters per line rather than 80 characters. These days though, few serious users would be using an IBM computer or compatible with video display via a normal TV. The CGA card permits black-andwhite (monochrome) or colour operation via a TTL colour monitor. The two basic modes of operation of the card are text (alphanumeric) and graphics (All Points Addressable - APA). Within each mode, several display character widths or pixel sizes are available for selection. Text mode differs somewhat from that of the monochrome adapter. It also contains circuitry to produce text characters on the screen, this time in colour or black and white. As already noted, you can operate the monitor in 40-character by 25-line mode if you have a low resolution monitor or TV. With a high resolution monitor, the PC can operate in an BO-character by 25-line mode. Text mode enables character blinking, highlighting and reverse video under program control. The actual characters are formed within an 8 x 8 dot box where each character is 5 x 7 dots with a 1-dot descender (lower part of letters such as "g"). The letters in this mode are different in shape and fineness from those produced by the monochrome adapter; generally the letter quality of the monochrome adapter is far superior. In total, 16 foreground and 8 background colours for each character, together with individual character blinking under program control, are suppqrted by the adapter. The CGA card contains 16,384 bytes (16K) of RAM storage. A 40-character by 25-line screen display uses 1K to store character information, while an additional 1K contains the attribute/colour information required by the screen. Since 2K of memory is required to display a 40 x 25 screen, up to eight pages or screens can be stored in the card's memory. The PC's processor allows direct addressability of the colour/ graphics card thus permitting a large degree of software flexibility for managing data on the screen AUGUST 1989 83 The Video Graphics Array (VGA) provides an analog RGB output to the monitor and can support 256 simultaneous colours from a choice of 262,144. It offers BIDS level compatibility with MDA, CGA & EGA. or a large degree of anguish for some early "clone-makers" . In graphics mode, three resolutions are available - low resolution 160 x 100 pixels (picture elements), 320 x 200 pixels (medium resolution) and 640 x 200 pixels (high resolution). The lowest resolution 160 x 100 pixels is seldom used; the medium resolution 320 x 200 mode enables each pixel to use one of four colours. One of 16 background colours may be selected and then three other colours from one of two palettes (cyan, magenta and white or green, red and brown). However the medium and high resolution modes differ in their utilisation of colour due to the fixed amount of memory available in the CGA card. In higher resolution 640 x 200 mode, the card requires all available memory to define the on or off state of each pixel and thus the display is in two colours only black and white. Differences in the way the signal is sent to the monitor and in the monitor resolution itself mean that not all colour monitors will have all colours available. Enhanced Graphics Adapter The Enhanced Graphics Adapter (EGA) card supports the monochrome display, the colour display and the enhanced colour display. In monochrome and enhanced colour display modes, the card supports 84 SILICON CHIP 640 x 350 pixel resolution with the capability to display up to 64 colours in enhanced colour display mode. Typically the EGA card may consist of several modules, depending on the manufacturer. In the case of IBM, the board consists of several modules: the extended graphics adapter (EGA) which is a full-slot board containing 64K of RAM; a graphics memory expansion card (GMEC) containing 64K of RAM which piggybacks onto the EGA; and finally a graphics memory module kit that adds an additional 128K of RAM to fill out the GMEC. The GMEC and memory module kit permit the display of 16 colours simultaneously in the enhanced resolution mode compared to four in basic EGA mode. Professional Graphics Adapter Squeezed in at this point is the Professional Graphics Adapter (PGA), a card producing analog RGB signals for the monitor which IBM released in order to allow for the display of a wider range of colours. Until this time, the MDA, CGA and EGA cards had been designed for connection only to TTL monitors. Therefore, to use a PGA card, you need a PGA monitor which will accept analog RGB signals. These are available, at a price, and are generally compatible with EGA cards too. With the release of the PS/2 range of computers, IBM introduced with them a new video display adapter, the Multi-Colour Graphics Array (MCGA - a variant of VGA) and Video Graphics Array (VGA). The immediate difference between these "standards" and their predecessors is that the graphics circuitry and associated video BIOS [Basic Input Output System) is actually present on the motherboard using custom gate array chips. In addition, new extended video modes and analog output enable support for up to 256 simultaneous colours from a choice of 262,144. The MCGA offers BIOS level compatibility with CGA monitors while VGA is jack-of-all-trades and offers BIOS level compatibility with the MDA, CGA and EGA. MCGA has four additional operating modes in addition to CGA compatibility; the first is a 320 x 200 pixel graphic mode with 256 colours from a palette of 256K colours. This is double scanned on the monitor to produce an image that is 320 dots wide by 400 high, with each of the 200 vertical dots appearing twice - one beneath the other. The second is the 640 x 480 pixel mode where the first and second graphics modes have a unity aspect ratio. This means that in the case of say a circle, the image appears circular and not elliptical as would be the case on CGA. MCGA's two text modes provide either 40 columns x 25 rows in 16 colours or 80 columns by 25 rows in 16 colours. The character size for both modes is 8 x 16 dots. VGA is basically a superset of EGA which supports many of the VGA-specific display modes such as 640 x 480 pixels in 16 colours when driven at higher frequencies. VGA also provides compatibility with the past while providing a glimpse of the future. Take a digital to analog converter chip to help drive the new analog monitors, some extra logic and a little work on the video ROM [Read Only Memory) BIOS and, hey presto, software aimed at the VGA could be adapted to suit the enhanced available EGA hardcontinued on page 99 Polystyrene capacitors YES Made here in Australia? YES Made Special to Type? features two control or alarm setpoint relays, an isolated 4-20mA recorder output, and an optional RS232 or RS485 serial data output. For further information, contact Amalgamated Instrument Co Pty Ltd, PO Box 134, Terrey Hills, NSW 2084. Phone (02) 450 1744. Cordless soldering iron from Scope YES Where From? Allied Capacitors Australia Alli~~ Capacitors Australia specialises in custom made good quality, high stability polystyrene capacitors. Why design circuitry which requires additional components to achieve a specific capacitance; we can wind exactly the capacitance you need to match your design criteria within the ranges of 1OpF to 1µ,F and up to 10,000VDC. Capacitors are manufactured to 0.25%, 0 .5%, 1.0%, 2.0% , 2.5% , 5%, 10% and 20% and are priced according to tolerance . All capacitors are tested to 2.5 times the rated voltage. Interested? Call us now on: (02) 938 4690 OR Cut out and post this coupon to Post Office Box 740 Brookvale, N.S.W. 2100 This fancy looking cordless soldering iron is designed and made in Australia. It has a 60 watt rating and is powered from two nickel cadmium cells. On a full charge, it is capable of making between 40 and 140 solder joints. For further information, including prices and charger options, contact Scope Laboratories, PO Box 63, Niddrie, Vic. 3042. Phone (03) 338 1566. Portable hard disc plugs into your PC Many businesses must be concerned that so many of their valuable company records are locked up in the hard disc of one of their computers. It would be so easy for a ------------I 1 Yes, I am interested in your capacitors. Please post me details of the following values: I ... /.. .VDC... /. I Name I Add ress VDC ... / ... VDC ... /. VDC (Please print) -------------L~~~~~~~~~-~J ware, provided software commands were routed through the operating system. In text mode, the character size for VGA is 9 x 16 dots, thus producing a resolution similar to that of enhanced EGA cards, but not in the same way nor using the same range of colours. Using, for example, the PS/2 Display Adapter, a PC or AT can use all of the features of the VGA. While VGA is not as advanced as perhaps expected or wanted, it does provide good compatibility at the BIOS level with previous display adapters. The advances include a greater range of colours, however performance has not increased substantially. As with monitors, the evolution continues. ~ thief to break in and steal the machine and then all the confidential records would be either lost (if not fully backed up every day) or possibly made available to competitors. Now there is a viable solution available with Portadisk. This is a standard NEC 40 megabyte hard disc that fits into the normal 5¼-inch spacing on any PC XT, AT or 386 computer. When you go home, you just use the key to unlock the disc and unplug it - just like the removeable car radios now available from some manufacturers. The Portadisk comes with keylock and cabling and is easy to fit. The hard disc controller on most computers can support a second drive. Portadisk comes with a 3-month warranty and a 14-day money back guarantee. The introductory price is $1090 including sales tax. For further information contact Electronic Solutions, PO Box 426, Gladesville NSW 2111. Phone (02) 906 6666. Computer Bits ctd from page 84 AUGUST 1989 99