Point Plot Display
Raster Display
| Point Plot Display | Raster Display (Image from PC link above) |
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Raster scan displays come in two types, character and bit mapped. For character one memory location holds the character for each display location and hardware converts that to the correct rows of dots to display. This is the method used in the simple text terminals like the DEC VT52/VT100 and the original IBM monochrome display adapter. For a 24x80 character screen 1920 bytes or memory are required. This method the limitation of not being able to display graphics like a curved line or pictures. For bit mapped display each pixel on the screen is represented by 1 or more bit of memory which are used to generate the rows of pixels to drive the monitor. For a 640x480 bitmap black and white display 38k bytes of memory are required. Since the display was made up of an array of square dots at lower resolution
The point plotter display allowed the generation of graphical displays like the bitmap display without requiring large amounts of memory. The program on the PDP-8 would generate a X,Y point and display a dot on the screen under program control and then generate the next until the entire display was drawn. This greatly reduced the memory requirements but had several drawbacks which is why it is no longer used. The major ones were the display is much slower to go between random X,Y locations than to sequentially scan the screen. This particular display could only draw 50,00 dots per second for full screen beam movement, more for smaller changes. The VC8E was capable of driving faster monitors such as the Tektronix 602 at 166,666 dots per second. If the time taken to draw all the points is greater than a 1/60 seconds then the display will flicker so only about a 1000-2700 points could be used on the screen for good display quality. This does not give you very complex displays. Also the program had to continuously redraw all the dots at 60 Hz which used a lot of the processor capacity.
More complex displays of this type were made that could draw lines, arcs, and characters and used a method called display lists which contained the drawing information so the hardware could refresh the display without using the main processor. These could make very good looking displays (when properly working) since they didn't show any of the stair step artifacts you get on lines with low resolution raster displays. Since raster scan displays and the point plotter use an array of small dots at lower resolution such as 640x480 these dots are visable and lead to angled lines having bumps that look like staircases and similar effect on the edges of characters.
DEC sold a system introduced 1967 called the DISPLAY-8 or type 338 programmed buffered display which was another PDP-8 which handled all the display processing and could draw 300,000 points, 600 inches of vectors, or 700 characters flicker free at the same time.
Tektronix has a series of terminals (4010/4014 etc) which used a storage tube which was a CRT that doesn't need to be periodically refreshed which solved the flicker problem on complex displays. The entire screen had to be erased and redrawn for any changes which was its drawback. These terminals were used on PDP-8 systems and in the online software library are routines for driving them. Information on Tektronix Graphics language (picture from raster emulation so shows stair-steps on lines)
Feel free to contact me, David Gesswein djg@pdp8online.com with any questions, comments on the web site, or if you have related equipment, documentation, software etc. you are willing to part with. I am interested in anything PDP-8 related, computers, peripherals used with them, DEC or third party, or documentation.
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