On-line and Realtime Systems
An on-line system means that input and output devices are directly connected to the CPU irrespective of geographical location. Thus the user can directly interact with the computer system through the terminal interface.
A realtime system is defined as one where response (feedback) from the system is fast enough to affect the on-going process. The responses time varies from situation to situation. For example, in launching a satellite the allowed feedback time may be in microseconds, for airline reservation system, the acceptable response time may be 20 seconds, while for a Production Planning and Control System the response time of a few minutes may be acceptable. All the above illustrations are of realtime systems, even though response time varies.
A realtime system is necessarily on-line but an on-line system may not be realtime.