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PDP15 System Development: Early 1971


The VT15 display eventually arrived in January, 1971. Quite a bit of time was wasted attempting to get the display through its acceptance tests. The standard repeatability test was to display a dot at one of the display corners followed by one near the centre of the display, one at another corner, another one near the centre and so on.. DEC had provided a photograph of a pattern of dots at the centre that was considered acceptable. Unfortunately, the engineer was unable to achieve this with the Atlas VT15. Eventually, after looking at the code, it was clear that the centre dots were meant to be on top of each other and the photo supplied by DEC showed a close cluster. the Atlas VT15 was sufficiently accurate that the displays were almost on top of each other. It was performing much better on the smooth Atlas 50-cycle supply than it had ever achieved on the 50-cycle supply in the factory ;-)

The VT15 was, in theory, capable of drawing full screen vectors between any two display positions. It also came with a set of fast line drawing commands that drew short vectors in eight basic directions. It arrived without the random vector generator.

The VT15 did have a number of other hardware faults that limited progress. Eventually, DEC agreed to replace the hardware in July/August 1971 while the machine was being moved into the 1906A machine room from G18. As compensation, DEC gave Atlas a sonic input tablet. This can be seen just under Maureen's elbow in the photograph below. This picture was taken after the move had taken place.

Maureen Smith pretending to use the VT15

Maureen Smith pretending to use the VT15
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The Fortran display package proved to be a big disappointment with some major bugs and omissions that inhibited progress. It was clear that some routines had never been tested. For example, the display package accepted interrupts from the pushbuttons but did not allow you to turn the button off once the interrupt was accepted. The display software required display jumps to be in the same store page while the software allowed the Fortran COMMON block that contained the display file to cross page boundaries.

Font Definer

By March, a rudimentary font definition system had been developed by Bob Hopgood. A mesh of points was displayed and the user indicated the set of lines that made up the character glyph using the lightpen.

Julian Gallop and Tony Burraston using the VT15

Julian Gallop and Tony Burraston using the VT15
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