The FDX 80 Column Graphics Card

Introduction

The 80 Column (80C) board in the FDX is built around a Motorola MC6845 Cathode Ray Tube Controller (CRTC). The 6845 was designed to provide the interface between a microprocessor and raster-scan CRT displays, it generates the necessary signals to interface with the display, but does not generate the pixels themselves. It generates horizontal and vertical sync signals and supplies the memory address from where the next pixel should be generated. 

The notes on these pages were written in the context of my trying to understand the design of the Memotech FDX 80 column card to help me find a fault on one of mine. Most of the CRTC notes should be common to other systems using the 6845, but some points are specific to the Memotech implementation. The details on these pages are only intended to provide an overview of the 6845 operation, for an in depth discussion of the chip, refer to the MC 8645 datasheet. The information on these pages is likely to apply equally well to the display generation portion of the combined 80 Column and serial card used in MTX512 Series 2 computers.

Originally intended to be a single page summarising the FDX 80 Column card design, the content has grown to the point where it is better split across a number of pages, use the navigation links at the bottom of the pages to move between related pages.

Raster Scan Displays (See the MTX Video Page for more detail)

In a raster scan display, the screen is divided into a number of horizontal scan lines and each pixel on a given line is illuminated, or  not, depending on the characters being progressively built up. Only 1 pixel is actively illuminated at any point in time, other pixels may appear to be illuminated due to the persistence of the phosphor display; until the electron beam returns to the same coordinates, the brightness of the pixel will gradually reduce until the pixel is refreshed, but this is usually not noticeable by the user.

One line of picture elements (pixels) is drawn from left to right in turn, the display is built up as subsequent lines are drawn from the top to the bottom of the screen.

The horizontal synchronisation pulse (HSYNC) separates the scan lines. The horizontal sync signal is a single short pulse that usually indicates the start of the line and the rest of the is drawn after it. Horizontal retrace means that the electron beam has reached the right hand end of a row – then it must return to the beginning of the next scan line.

The vertical synchronisation pulse (VSYNC) separates the video fields (frames). Vertical retrace means that the electron beam has reached the lower right hand corner of a row – then it must return to the upper left corner. The frequency of the vertical sync can therefore be much lower than that of the horizontal sync. Usually, the frame rate is matched to the power line frequency (50Hz or 60Hz) which prevents the display from "weaving".

Probably the most common sync systems are separate sync and composite sync. As its name suggests, separate sync uses separate wires for horizontal and vertical synchronization. When used in RGB connections, five separate signals are sent (Red, Green, Blue, H-Sync, V-Sync). Composite sync combines the horizontal and vertical synchronization signals onto one pair of wires. When used in RGB connections, four separate signals are sent (Red, Green, Blue, Sync).

The maximum rate that a monitor can refresh the screen is measured in Hertz (cycles/second) and is called the vertical refresh rate (or vertical scan rate). The horizontal scan rate is the number of times that the monitor can move the electron beam horizontally across the screen, then back to the beginning of the next scan line in one second. Most early monitors were fixed frequency e.g, the IBM CGA 5153 monitor had a horizontal sync rate of 15.85 kHz and a vertical refresh rate of 60.5 Hz, these frequencies are compatible with the FDX 80 Column card output.

Raster Scanning Example

For example, consider a system where each ASCII character is displayed using an 8 x 8 matrix of pixels. To allow for white space between the characters and rows, the character set could be formed from a matrix of 7 x 6 pixels. A portion of 1 character line from such a system, comprising of 8 scan lines is illustrated below :

If "0" is used to represent a pixel being OFF and "1" for it being ON, it can be visualised as shown :

Colour Displays

This neat diagram from Wikipedia, shows in graphical form, how 8 colours, or rather, 7 + Black, can be produced from the three primary colours (Red, Green and Blue). As the image shows, on an 8 colour RGB system, Cyan for example is generated by combining Blue and Green. To display colour images on a raster scan CRT, three electron beams are required for each pixel, one for each of the RGB colours and the perceived colour of the pixel results from additive colour mixing.

The coloured dots do not overlap on the CRT, but are very close to each other. When viewed at normal viewing distance, the human eye cannot distinguish them and the three dots are perceived as a mixture of the three colours. The FDX 80C card is limited to the 8 colours available from mixing the three primary colours as shown above. The IBM Colour Graphics Adapter (CGA) could set each colour beam to two brightness levels (in addition to off), which enabled CGA to support 16 colours, i.e., the 8 colours at two levels of brightness.

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