Monochrome monitors displayed either green, amber, or white characters on a black background. A monitor that displayed green did so because it used green "P1" phosphor to light up each pixel on the display. Old monitors tended to have a very low refresh rate (the speed at which the entire screen's contents were refreshed with new content based on data coming from the display card). Green phosphor had the longest "afterglow" so remains lit up on the screen for longer between refreshes, and green is the brightest type of phosphor. This made it a cheaper monitor to build.

The various monochrome PC display colours

Amber monitors came a little later, and used "P3" phosphor. It was considered easier on the eyes for business use but required a faster refresh rate and so was more expensive to manufacture.

Black and white monitors displayed white or grey characters on a black background, and used "P4" phosphor. These were sometimes referred to a "paper white" displays.

Strangely, when purchasing a PC in the 1980s and early 1990s that had a monochrome display, you were rarely informed of the "colour" you would receive. Instead, it was simply referred to as a mono (or monochrome) monitor.

CGA and EGA Monitors

CGA monitors arrived on the scene in 1981, coinciding with IBM's launch of the Color Graphics Adapter (CGA) card. This permitted up to 4 colours simultaneously displayed on-screen (not including black) in the low resolution of 320 x 200 pixels. It also provided the option of monochrome in 640 x 200, mimicking the MDA format but with a slightly smaller character cell size.

The EGA standard, introduced in 1984, expanded on this with up to 16 colours at a resolution of 640 x 350. Once launched, an inexpensive PC clone that supported EGA graphics could produce better graphics than rivals at the time including the Commodore 64 and Apple II.

Both CGA and EGA send their signals as digital TTL, just like MDA and Hercules. The previously unused pins in the same 9-pin DSUB that was used by MDA/Hercules were now employed with CGA and EGA to transmit Red, Green and Blue colour and intensity information to the supporting monitor:

A CGA or EGA card's female 9-pin DSUB output connector and its pinouts

Note: Red 0, Red 1 and Green 1 are the "Primary" colour signals, whilst Red 0, Grn 0 and Blue 0 are the "Secondary" colour signals. The primary signals provide the actual colour whilst the secondary signals provide the "intensity" of the colour.

If you connect an EGA card to a CGA monitor it should work if the EGA card is outputting at the same scan rate as CGA (15.75 kHz) and the monitor isn't running pin two to ground which would short out the EGA card and potentially damage it. All EGA modes that support 200 lines operate at this lower scan frequency of 15.75 kHz. Most EGA cards have DIP switches on the side to set the monitor type.

VGA and Beyond

With the advent of the VGA graphics standard, the output of such graphics cards moved to analogue in order to support a seemingly infinite number of colours for display. If your graphics card has a 15-pin D-SUB female connector, it's outputting analogue RGB (Red, Green, Blue) signals. More specifically, they carry RGBHV (Red, Green, Blue, Horizontal Sync, Vertical Sync) signals. These connect to an "analogue" monitor, which accept these analogue RGB signals and convert them back into digital signals for display on-screen.

It's worth mentioning that mono VGA is different - it only needs 8 or 9 wires but the signalling is completely different to MDA (Mono TTL is 5V digital whereas VGA is 1V analogue).

A VGA monitor's female 15-pin DSUB output connector and its pinouts

Monitors and Connectors

Monitors typically accept either digital or analogue signals - not both!

An exception to this was some of the early multisync monitors including the NEC Multisync 3D. This could switch between analogue and digital and so was able to support MDA, CGA, EGA (all digital) as well as VGA (analogue) signals coming in. Later multisync monitors supported only analogue [VGA] signals - the purpose of these was to display different screen resolutions at different refresh rates (frequencies).

Some digital TTL monitors have a DIN socket, so a cable with a 9-pin D-SUB [male] on the graphics card end goes to a DIN plug on the other.

The table below provides a list of the specifications of each display type and what the expected monitor's capabilities need to be:

TTL stands for "Transistor-Transistor Logic" - basically a digital signalling system which sends and receives -5V or +5V signals to indicate the logic level of 0 or 1.

RGBI stands for "Red, Green, Blue and Intensity" - these define the colour palette available, based on the 3-bit palette of RGB alone, but with an added intensity bit (dark or bright) which gives 16 colours in total. If a standard RGB monitor is used with a CGA card it will only display a maximum of 8 colours, as the Intensity bit is not supported.

Notes:
1) This extended CGA graphics mode is not very common. It was used primarily in games where the number of colours was far more advantageous than the screen resolution. PakuPaku, a Pac-Man clone, was one such game.

Multisync Monitors

Before multisync (or multiscan, same thing) CRT monitors existed, a monitor displayed just one resolution at one refresh rate. It simply wasn't changeable. An EGA monitor could only display 640 x 350 at 60 Hz. If you tried to display 320 x 200, it would display it in the middle of the monitor surrounded by black. Any higher resolution than the monitor could handle (or different refresh rate) would simply fail and possibly damage the monitor.

A multisync monitor can display different resolutions at different refresh rates (it supports multiple synchronisation rates). Early multisync monitors were not only auto-switching for sync frequencies, they could also switch between analog and digital. The NEC Multisync 3D (around 1988-1989) was one of the last monitors to support analog/digital switching. Most of these monitors had a 'Mode' switch somewhere to switch between analog and digital. Later monitors are all analog-only, but of course do support different resolutions and refresh rates.

9-pin MDA/CGA to 15-pin Monitor-end

Below is the cable wiring required to adapt a 9-pin D-SUB (MDA, CGA, EGA, and PGA/PGC) to the 15-pin D-SUB of a multisync monitor that supports digital as well as analog signals. As mentioned above, only early multisync monitors support digital signals as well as analog, so please check in your monitor's documentation before attempting to connect a digital signal display card to your monitor!

I've tested this layout with both CGA and MDA cards and it works perfectly. The only special thing to note is that when using MDA the "MODE" switch on the front panel of the monitor must be set to "ON", for all others it is set to "OFF"

For convenience, below are a number of user manuals for multisync CRT monitors:

Frequently Asked Questions

Q) Will this monitor X work with my graphics card Y ?

A) This depends on several factors. The first, most important, question is: Is the monitor analogue or digital? Remember, almost all monitors can only accept either digital signals or analogue signals. Get this wrong and you'll likely permanently damage the graphics card or the monitor. If you know this, the next question to ask is: does the monitor have the horizontal scan frequency that the graphics card is outputting? Check the table above for details, and compare it to your monitor's specifications in tbe back of its manual.