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Chapter 25

The DEC 338 display computer


The C(display; 'DEC 338) is a C('DEC PDP-8) with a P.display which can connect to T(#1:8; CRT; display; area: 9.375 X 9.375 in.2). The PMS structure is shown in Fig. 1, Chap. 5, describing the PDP-8. The Pc ISP is given in Appendix 1 of Chap. 5.

The C('338), although designed to stand alone, is generally used as a satellite to a larger C, via an L(Dataphone). The rationale for using a C as a T is based on the bandwidth and storage requirements needed to maintain graphical picture displays. A human being manipulating pictures (rotation, scale change, and conversion of internal linked data structure to a picture structure) requires short response time; this requirement places high processing demands on larger C's. Thus this C(display) is a preprocessor for larger, more general C's.

The actual T(CRT) is a 16-inch CRT with a 93/8-inch square viewing area covered by 1,024 X 1,024 (XY) points. The diameter of the points is ~0.015 inch. The spot is magnetically deflected and focused. All eight T(CRT)'s can be driven together or used

Fig. 1. DEC 338 instruction-interpretation state diagram.

independently. A photomultiplier connected through a fiber-optic bundle link is used as a light pen (a photosensitive sensor) to detect spots on the T. The light pen allows the P.display to detect whether a user has "pointed to" a displayed spot.

Pc and P.display access the same Mp; the total data rate available from Mp is one 12-bit word/1.5 microseconds. The instruction times of P.display are a function of the point plotting times of the T(CRT):0.3 microsecond to the next incremental unintensified point (approximately 0.010 inch away); 1.2 microseconds to an incremental intensified point; and 35 microseconds to a point plotted at a random position.

The state (registers) of C.display is given in the ISP description of Appendix 1 of this chapter. There are four parts of the state: the control registers for Program Flow State, the Picture State (or position of beam), Console and Light-pen State, and Mp State. The instruction interpreter is fairly simple and is best described by the state diagram (Fig. 1). The instructions are given in Tables 1 and 2. The remainder of the chapter discusses the P.display instructions and the Pc instructions for communicating with P.display.

Principle of operation

The actual picture is held stationary by repeatedly displaying (intensifying) a particular point, line, etc. The number of times a figure has to be displayed so that it appears stationary and does not flicker depends on the CRT phosphor, the figure, and environmental parameters. The generally accepted range is a plotting rate of 20 ~ 50 plots/second; thus a complete picture has to be drawn in 50 ~ 20 milliseconds. If we assume a 30-Hz plot rate, about 28,000 points can be plotted in vector mode (or 280 ~ 1120 inches, depending on the spacing). About 1,000 characters can be displayed in 30 milliseconds using character mode.

When the light pen is used, a display program is required to "track" the pen. The pen's position is determined by displaying known points. The pen, of course, detects the points when it is present at the displayed points position; therefore the program knows the location of the pen.

The parameters of interest for a display vary, depending on the application. However, the general parameters are:


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