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computed values from the computer. The operator also affects the X, Y input 
by footswitch P (for parallax status). When P is activated the input is 
transferred only to D-registers of one of the stages currently defined by the 
state of another manually operated L/R (left/right) switch and bypasses model 
registers. The Z-wheel generated input is always simply directed to the Z- 
register. 
  
  
  
  
  
  
  
  
  
  
  
+ x y Legend: P - parallax mode 
M - manual mode 
Y L,R - left, right stage 
  
Fig. 5 Register incrementation from handwheel input in NRC Anaplot 
The above hardware logic must be fully reflected by the corresponding program 
logic in simulations. Hardware generated pulses are simulated by coordinate 
increments from the 'INPUT XYZ' module, directed to and accumulated in 
pseudoregisters, as controlled by memory located pseudoswitches M, P, L, R. 
The flowchart in Figure 5 reflects the above functions. 
Stage Servo Control 
The hardware implementing the control of the servo loop operates on D- and S- 
photo registers. Depending on the type of the analytical plotter the servo 
loop is closed gradually by generating suitable voltages for servo motors, 
while applying some mechanical acceleration and velocity control to achieve a 
smooth and vibration free homing-in of photo stages. A program simulating 
this process can exactly replicate the closing function by iteratively 
adjusting the content of S-pseudoregisters to that of D-pseudoregisters. 
However, because this program is alternately run for all four photo 
coordinates and is dependent on a higher programming loop containing the 
interface servicing routine from which it is called, the simulated homing-in 
may be time consuming for longer distances to be travelled. It is then 
appropriate to simplify the simulation process in order to achieve a response 
time which is commensurate with the timing of the original hardware. In the 
extreme case the S-pseudoregisters can be directly assigned the current value 
of D-pseudoregisters to close the loop immediately. 
Hardware Control 
Switch Control. Hardware switches of an analytical plotter are connected 
through interface in such a way that their manual activation immediately 
results in setting or clearing flag bits of memory located switch status 
words. The switch control part of the PLOTTER routine takes care of 
identifying the position of any corresponding pseudoswitch in the graphical 
representation of the switch control on the interactive screen. Once such a 
position is selected and identified, e.g., with the use of an interactive 
light pen, the corresponding flag bit setting and clearing is accomplished by