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distribution of processing, as implemented in the Kern DSRl. Processor l 
takes care of the real-time positioning functions, while the front end 
processor 2 communicates with the operator. The operator receives information 
from a flat video screen and coordinate counters located at the plotter 
console, and his control input is entered from special switches and a numeric 
keypad. A video terminal providing a direct link to the main micro- or 
minicomputer is optional. 
Software Control of Functions 
Most analytical plotter functions are controlled by software, however, they 
have to be initiated by the operator. Operator activated switches are then 
detected and interpreted to interrupt/resume or to abort an operation, to 
start an action and to select a mode of current operations. The program must 
anticipate these signals, otherwise the operator's requests would go 
unnoticed. This is done by the interrogation of relevant switch status words 
during all indefinite measuring cycles, expected to be terminated or modified 
by the operator. 
OPERATOR-SYSTEM INTERACTION 
The operator interacts with any on-line photogrammetric system in two ways: 
through his video terminal link to the computer and through the input/output 
options of the analytical plotter. The computer directed interaction has 
several components: 
- solicited input based on a dialog; it is conducted either as a 
prompt/answer conversation or is menu driven, offering a multiple choice at 
several recursive levels; 
- unsolicited input needed to interrupt/resume an operation, to break for 
another independent operation in a multitask environment and to abort the 
task; 
- computer output of intermediate or final results; 
- continuous display output presented as a time scheduled screen output. 
As shown in Figure 2, the operator-plotter interaction is based on two input 
channels and two output channels. The operator receives a continuous, dynamic 
feedback on the progress of the operation from the optical system. He can 
also monitor current coordinates or solution parameters in display counters 
available on the system console or through the video screen. Based on this 
feedback the operator manually controls his X, Y, Z input transmitted and 
processed in real time. In addition, there is a secondary, occasional 
interaction through the switches. In most cases the switch signals affect the 
program decision making, however, some of the switches control and modify the 
hardware performance, e.g., the information flow between registers. 
COMPUTER SIMULATION OF HARDWARE FUNCTIONS 
Any simulation must model the corresponding physical reality and accurately 
reflect its specific characteristics. For reasons of space limitation the 
following analysis will be less general and will address a system with the 
central processing of data. 
General Approach 
The operator-computer dialog and interaction is controlled by existing