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analytical plotter software which is fully preserved in simulations, except 
for a few routines servicing the computer-plotter interface, as explained 
below. 
The internal hardware structure must be represented by suitable program 
modules which simulate the individual hardware functions. All physical 
registers and switches are replaced by suitable memory locations functioning 
as pseudoregisters and pseudoswitches, and all original hardware functions are 
programmed as equivalent manipulations of data among their pseudo- 
counterparts. In other words, the analytical plotter is now represented by a 
new subroutine, PLOTTER, which is called from the main program in all its 
parts normally operating on interface registers and switches. Except for 
photo stages which are represented in the simulation just by currently 
considered photo coordinates, all remaining six plotter subblocks of Figure 2 
must have equivalent sections in the PLOTTER subroutine. They will be 
individually discussed below. 
The computer maintains a link with the analytical plotter through registers 
and switches. For registers it is a two-way communication supporting both 
read and write functions, while switches are only signal transmitters. All 
register- and switch servicing subroutines of the original program must be 
modified to substitute newly defined pseudoregisters and pseudoswitches for 
the real ones. This usually applies only to a few interface oriented 
subroutines of the main software and is easy to accomplish. 
The operator's link with the analytical plotter is more difficult to simulate 
and cannot be represented only by software. The simulated communication must 
be supported by some standard, general-purpose interactive medium connected to 
the computer as a substitute for the analytical plotter. It may be an 
interactive graphics terminal, a programmable tablet attached to the computer 
or just the available video terminal alone, if it is of a suitable type. With 
reference to Figure 2, the input to | ANALYTICAL PLOTTER | block through 
[SWITCHES], [INPUT XYZ] and output from [COUNTERS], [OPTICS] is simulated by 
interacting with the graphical representation of these functions, with the use 
of a light pen, stylus or screen cursor. 
Register Control 
All coordinates registers can receive, capture and transmit numerical 
information and this is done in a logical system of communication lines 
defined by the functions of any particular analytical plotter. In the 
following we will examine register functions of the NRC Anaplot.  Analogy can 
be used to analyze other systems. 
For a real-time operation the most important are stage registers involved in 
the servo loop control. Each stage coordinate has two registers, demand 
register D with input coming either from the operator or from the computer, 
and status register S reflecting the current position of photo stages from 
their encoders. In order to close a servo positioning loop stages are 
automatically driven to eliminate any difference between D and S which 
eventually assume the same contents, except for some control fluctuation of 
the lowest bits. 
In the manual input of X, Y the transfer of increments from handwheels to D- 
registers is directed under control from both the computer and operator. With 
the computer-set register switch M (for manual status) the handwheels 
increment both model and stage registers. When M is computer cleared the 
manual input goes only to model registers and stages receive corresponding