Introduction

Real-time electronic distributed control systems are an important development of the technological evolution. Electronics are employed to control and monitor the most safety-critical applications from flight decks to hospital operating rooms. As these real-time systems become increasingly prevalent and advanced, so does the demand to physically distribute the control in strict real-time. Thus there is a need for control network protocols that can support the application's stringent real-time requirements. Real-time networks must provide a guarantee of service so that they will consistently operate deterministically and correctly.

Ethernet, as defined in IEEE 802.3, is non-deterministic and thus it is unsuitable for hard real-time applications. The media access control protocol, CSMA/CD with its truncated binary exponential backoff algorithm, does not allow the network to support hard real-time communication as it incorporates random delays and allows for the possibility of transmission failure.

Decreasing costs and increasing demand for a single network type, from boardroom to plant-floor, have led to the development of Industrial Ethernet. The desire to incorporate a real-time element into this increasingly popular single-network solution has led to the development of different real-time Industrial Ethernet solutions. Fieldbus networking standards have failed to deliver an integrated solution. Typically the emerging real-time Industrial Ethernet solutions complement the fieldbus standards, for example by using common user layers. This course covers an introduction to real-time systems along with a study of Ethernet with emphasis on its suitability as a real-time network. Module 402 provides a study of the real-time Industrial Ethernet solutions available today.

Real-Time Introduction

Real-Time (RT) systems are becoming increasingly important, as industries focus on distributed computing in automation, see Figure 1. As computing costs decrease, and computing power increases, industry has become more dependent on distributed computers to deliver efficiency and increased yield to the production lines. Real-Time does not automatically mean faster execution but rather that a process is dependent on the progression of time for valid execution.

Figure 1 — Distributed Real-Time Processing

RT systems are those whose correct execution depends not solely on the logical validity of data but also on its timeliness. A correct RT system will guarantee the successful operation of a system — so far as its timely execution is concerned. RT systems are generally broken into two main sub-categories: hard and soft.

Hard Real-Time (HRT) systems are those in which incorrect operation can lead to catastrophic events. Errors in HRT systems can cause accidents or even death. Such systems are typically found in flight control or train control systems, where an error could potentially incur loss of life.

Soft Real-Time (SRT) systems, on the other hand, are not as brittle. An error in a SRT system, while not encouraged or appreciated, will not cause loss of property or life. SRT systems are not as safety-critical as HRT systems, and should not be employed in a safety-critical situation. Examples of SRT systems would be online reservation systems, or streaming multimedia applications where slight occasional delays might cause small inconvenience but will not have serious impact.

Jobs are the real-time system's building blocks. Each real-time job has certain temporal quantities (Figure 2) associated with them:

1. Release Time, 2. Ready Time, 3. Execution Time, 4. Response Time, 5. Deadline.

Figure 2

The Release Time of a job is when the job becomes available to the system. The Execution Time is the time it takes for a job to be completely processed. The Response Time is the interval between the release time and the completion of the execution. The Ready Time is the earliest time the job can start executing (always greater or equal to the Release Time). The Deadline is the time by which execution must be finished. If execution is not complete by the deadline, the job is late. A job's deadline can be either hard or soft, indicating the job's temporal dependence. As mentioned earlier, a missed hard deadline can have serious consequences for correct system operation. All real time systems have a certain level of jitter. Jitter is a variance on the actual timing of the above times. In a real-time system, jitter should be measurable within a +/– interval so that the system performance can still be guaranteed. For textbook information on real-time systems, refer to [1].

To develop a real-time distributed system, where computers are interconnected, it is vital to employ a network that can provide communication between the various distributed computers in a reliable and timely fashion. Distributed processors running real-time applications must be able to inter-communicate via a real-time protocol, otherwise the temporal quality of their work is lost. Real-Time Communication networks are like any real-time system. They can be hard or soft, depending on the system requirements and their 'jobs' include message transmission, propagation, and reception. There are a number of real-time control networks available and employed in industry, but none have the popularity or bandwidth capabilities of Ethernet. In the following section, we will discuss the demand for a real-time Ethernet solution.