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real-time, computer controlled mode of stage positioning. It is obvious that 
these program sections already contain calls to interface servicing routines 
to read or write values from or to registers. Let the names of these existing 
routines be GETREG, PUTREG. 
To make sure that the simulation control is complete and all analytical 
plotter operations are properly represented in simulations, a call to the . 
PLOTTER subroutine must be placed from inside of existing GETREG and PUTREG 
routines. This approach will also accommodate a proper simulation display of 
any positioning operations needed, e.g., to drive to loading or centre 
positions of stages, to slew in specified point patterns and restore already 
recorded positions. The GETREG and PUTREG modules are part of the existing 
interface library which has to be modified anyway, to adapt all its functions 
due to the conversion from the real environment to its fictitious substitute. 
À great advantage of this approach is that the bulk of the analytical plotter 
software is not affected and can be used in simulations unchanged. 
The PLOTTER subroutine must, of course, also take care of the proper 
integration of the new interactive medium and this depends on the hardware 
used. To operate from an independent graphics terminal or tablet involves 
additional software, usually a library of supporting subroutines for that 
particular peripheral. In operating from the same video terminal one must 
implement, among others, an additional split screen manipulation of data to 
ensure a clear separation of the regular monitoring of analytical plotter 
operations from the interactive displays of the simulated functions on the 
same terminal. 
IMPLEMENTATION 
Experiments have been conducted to simulate the functions of the NRC Anaplot 
and to test the feasibility of analytical plotter simulations in two different 
configurations: with the use of a graphics terminal DEC GT-40 and with the 
use of a standard video terminal DEC VT-100. The host computer to the Anaplot 
is DEC PDP11/45. 
Graphics Terminal 
The GT-40 terminal allows one to interact with its screen information via a 
light-sensitive pen. Any of the displayed lines, figures or letters can be 
defined sensitive to the pen close pointing. Thus, the position of a 'hit' is 
identified and its significance or meaning interpreted by the program as an 
activation of a pseudoswitch or input from a pseudo-joystick of the simulated 
system. Figure 6 illustrates the screen configuration. The upper part 
represents output information from the fictitious plotter: two circular 
optical fields with a measuring dot in the centre, displaced crosses 
representing the optical details, a square frame to show the current positions 
within the area of both photographs (L, R) and corresponding photo-coordinate 
counters, Displayed in the lower part are Anaplot control switches and two 
joystick arrays with coordinate counters to control operator's input in 
planimetry and elevations. Any of the boxed switch letters or numbers is 
sensitive to the light pen and, when touched, they start blinking to indicate 
activation. With the light pen hit of the joystick areas a small circular 
shaped cursor appears on the screen and can be moved around to select the 
speed of X, Y, Z incrementing as in the operation with a pressure-sensitive 
joystick.