13 
V System configurations 
Regarding the way and extent of involvement of digital components in 
a system, several feasible configurations can be drawn up. A principal dis- 
tinction can be made between hybrid systems and computer supported con- 
ventional instruments. Systems of both classes can be highly modular. 
In hybrid systems the analogue components are insufficient for correct 
(independent) functioning, i.e. without a computer support. The model-to- 
photo transformation is assisted by a digital computer. Hybrid systems can 
be imitated by computer-supported conventional instruments (3), (4), (6). 
In such a case the capabilities of a conventional instrument are only partly 
exploited. The unexploited capabilities are supplemented and extended by 
means of digital components. 
The configurations of hybrid systems may differ considerably; they 
may have a digital or an analogue emphasis, or the two component types may 
be in balance (7), (8). 
The configurations of computer supported conventional instruments 
can be differentiated according to: 
— uni-directional or bi-directional flow of information between the 
analogue and digital components; 
— single or multiple systems. 
Recording of final (or intermediate) data is computer controlled. Data 
may be recorded in digital form, in the form of symbolised graphs,or photo- 
graphically. Accordingly a distinction can be made between the different 
automatic recording devices. Some of these devices function in real-time or 
nearly real-time, while others operate time delayed. Real-time operating 
systems require adequate interface units. In off-line systems (with time 
delayed operation) these units are replaced by equipment for intermediate 
recording of data and for data entry. 
In the following consideration will first be given to open loop systems 
(i.e. with uni-directional flow of information) and then to closed loop 
systems (i.e. with bi-directional flow). For each of these the single and 
multiple systems will be considered.