Real time clock computer control

Real-time clock Every digital computer used for process control must have a real-time clock. This is the device that keeps track of the real world s time and allows the computer to schedule its functions at time intervals in coordination with the various needs of the real world. Thus it is the real-time clock that determines when the computer should take data from measuring sensors or change the values of manipulated variables. 

What is the real-time clock needed for How does it function Why is it very difficult to use a computer for process control without a real-time clock Do you have any suggestions on how you can count time elapsed without a real-time clock ... 

A coordinated time base is essential. This simple requirement is not necessarily trivial on-board ship. Many analytical instruments and recording devices keep time by counting electrical cycles. This timekeeping method uses the excellent frequency control commonly found in U.S. electric power systems. At sea, however, frequency drifts, thus, clocks drift too. If all laboratory data were logged immediately into a computer data base, it would be relatively easy to coordinate computer time and real time. However, the situation can be confused when signals first go to strip-chart recorders for subsequent evaluation and then transfer to the general files. Provision then must be made for regular time marks on the chart. 

THE EVENT-TRIGGERED (ET) model of computation is presented as a generalization of the time-triggered (TT) approach. It supports hard real-time and flexible soft real-time services. The ET model is built upon a number of key notions temporal firewalls, controlled objects, temporarily valid state data, and unidirectional communications between isolated subsystems. It uses the producer/consumer rather than client/server model of interaction. In addition to describing a systems model and computation model, this article considers issues of schedu-labiUty and fault tolerance. The ET model is not radically different from the TT approach (as in many systems most events will originate from clocks) but it does provide a more appropriate architecture for open adaptive applications. 

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