3 
l \ 
GENERAL PRINCIPLES 
The different approaches to digital type equipment have led to diverse system con 
figurations and operational characteristics. A classification of such equipment is 
shown in fig. 1. 
Fig. 1 
Classification of systems employing digital components 
The on-line systems may be classified into: conventional plotters supplemented by 
a computer (e.g. mini- or table-top), hybrid systems, and fully digital type systems. 
The first and the last categories represent the limiting cases of the hybrid systems. 
The conventional plotters can be interfaced with a computer serially [ 1 ] or by 
means of one or more feedback loops [ 2 ] [ 3 ] . 
According to the extent to which a digital computer is involved, a distinction can be 
made between different levels of hybrid systems. Those most appropriate appear to 
be the optimal hybrid systems which have well balanced analogue and digital compo 
nents [ 4 ] [ 5 ] . 
i 
f 
The fully digital type systems (i. e. analytical plotters) implement digital transfor 
mations of locations in model space into those on photo-planes, or the reverse. Thus 
a minimum of mechanical components can be employed. Regarding the involvement of