„7 m 
effects on the accuracies. Designs using linear encoders lend them- 
selves well to this separation and allow for easy adherence to Abbe's 
principle. The universality is limited by the size of the carriage and 
the design of the optics. The first limits the maximal format of the 
images and the second one influences the flexibility of the procedures 
in man-machine systems and defines the geometrical characteristics of 
the images that can be processed. It should be noted that the latter 
limitations are actually not the consequence of the inability of the 
analytical instrument to cope with a particular image geometry but 
solely due to the inability of the operator to accommodate larger 
differences in the orientation of epipolar lines and scale differences 
between conjugate images. In other words, an analytical instrument that 
has the four basic positioning devices will be able to correctly address 
the corresponding conjugate points in any type of images (as long as 
their geometry is defined), but the operator will not be able in some 
cases to fuse the related image segments into a stereoscopic model. 
Consequently the interface of a fully universal man-machine analytical 
system has eight channels for control of four carriage positioning 
devices and four positioning devices for optical magnification and 
rotation of images (e.g. Zoom-optics and Dove prisms). Considering the 
high precision required, the control of the first four channels must 
have a closed feed-back loop. Each of these feed-back loops compares 
the desired and the actual state of the device, e.g. an input and a 
status register. These two registers are compared at short time- 
intervals and the difference between their contents will determine the 
action of the motors [2]. To approximate as closely as possible the 
desired deterministic behavior of these devices their response time At 
has to be shorter than the cycling time AT of the real-time programs. 
It is evident that AT < At has no practical value, and that state S of 
a channel at time t + At depends only on input I and state S at time t, 
i.e. S, PALE e(1, ; S,); (the knowledge of the state at time t - At 
is irrelevant at time t + At, and the input at t + At will not influence 
the state at t + At). 
The other four channels do not require controls as stringent 
as the first four ones, since about 10% difference in image scales, and 
about 1° difference in image rotation may be tolerated by the operator. 
Consequently they may be designed as open loop channels. The time for 
transfer of information between the computer and the registers of all 
eight channels can be easily kept below 100 usec, thanks to the facility 
for fast communication between the computer and the peripherals inherent 
in the architecture of most mini-computers. This short transfer time 
helps considerably when writing real-time programs. It is worth noting 
that the design of interfaces is completely independent from the mathe- 
matical model of a particular transformation performed by a real-time 
program. This characteristic of the interface design contributes to the 
universality of analytical instruments since as long as a functional 
relationship between the variables can be mathematically defined, that 
is as long as a real-time program can be coded, the positioning devices 
will simulate the underlying geometry. Additional channels for the 
control of auxiliary devices (e.g. plotting table) are normally included 
in the interface. Their number, which depends on the general configura- 
tion of the system, is limited by the required overall response time of 
the system.