Input/Output

Overview

This chapter covers the input/output between a node and the controlled object. It starts with a discussion on the dual role of time at the I/O interface: time can act as a control signal causing the immediate activation of a computational activity, and time can be treated as data that records the occurrence of an external event on the time line. If time is treated as data, then the temporal control structure within the computer is not affected by the occurrence of the external event. In many situations, the I/O interface can be simplified by treating time as data and not as a control signal. Section 9.2 introduces the notions of raw data, measured data, and agreed data. It then continues with a discussion about the different types of agreement, syntactic agreement and semantic agreement.

The differences between sampling, polling, and interrupts are the topic of Section 9.3. From the functional point of view, sampling and polling are identical. However, from the robustness point of view, sampling is superior to polling. Section 9.4 is devoted to a discussion of interrupts. An interrupt is a powerful, and therefore potentially dangerous, mechanism that interferes with the temporal control structure within a node.

Sensors and actuators are the topic of Section 9.5. The concept of the intelligent instrumentation that hides the concrete world interface and interacts with the computer by a standard, more abstract message interface is elaborated. The notion of fault-tolerant actuators and fault tolerant sensors is introduced. Some hints concerning the physical installation of the I/O system are given in Section 9.6.