Fig. 1 - 52. Scheme of intrinsically safe I / O on the basis of IS RIO

Pay attention to the digital bus (IO BUS), connecting the controller and the nodes IS RIO. Some of the tire is in hazardous areas, so for her, as for the field signals to provide a reliable spark protection. This is done by dividing the barriers to digital bus (Bus Couplers), installed in the safe zone. For example, such a barrier can send pictures Profibus DP protocol for the special bus, built on the basis of physical-level IS RS-485 and which is intrinsically a modification of conventional RS-485. Some systems IS RIO supports connection to an optical digital networks and, naturally, does not require any barriers bus (fiber in principle can not be spark dangerous). Such systems are available, for example, the German company Stahl.

Nodes IS RIO are not fastened to the wall in an unprotected form, and are installed in a special box (boxes) for hazardous areas. Shells intended for zone 1, are produced in accordance with explosion protection type EEx e (increased safety) and have a degree of protection not less than IP65. These shells are robust rubber seals, and cable connections are made with screw connections and securely sealed. For Zone 2 safety requirements more democratic. Fig. 1 - 53 shown mounted closet system IS RIO:

Fig. 1 - 53. Example of mounted shelf of system IS RIO

The advantages of using the system IS RIO are obvious:

1. Reducing of cable routes. In contrast to the barriers, IS RIO units can be installed close to the sensors and actuators, and digital signal input / output are transmitted to the upper level of a two-wire digital bus.

2. Minimization of equipment. Each module I / O system IS RIO, in fact, serves two functions: a direct input / output field signals and providing INDICATORS connected to the field circuits.

However, it would be unfair not to mention one serious drawback: need a sufficiently large amount of work to configure the network interface between controllers and hosts a top-level IS RIO, which are often in relation to the controller, a third-party devices. In general, the practice shows that the use of IS RIO justified at high density of intrinsically safe signal input / output, when they accounted for less than 30% of the total number of signals, in which case the cost of organization-level I / O may be reduced by 20 -- 25% compared with the decision based on the classical barriers.

12. Alarm and archiving which are in distributed control systems

Archiving process parameters - the inherent requirement for all modern systems of governance. In fact, advanced system backup - this is a characteristic that distinguishes CSF from numerous full-featured, so to speak, "analogues".

Why do need archiving process parameters? For three very simple reasons:

1. Archive of values of technological parameters (in technical jargon - the history, history) to evaluate the quality and effectiveness of the control system. This is done through a retrospective analysis of key process parameters, which gives an idea about the extent to which achieved the goal of management.

2. Archiving allows experts to assess the dynamics of technological change over a long period of time, which is extremely useful for understanding the behavior of the process in different (including emergency) situations, and, consequently, the acquisition of knowledge about a particular technology.

3. Archive can contain information that helps to establish the causes of various accidents and emergencies. Thus, having carefully studied the log of actions of the operator, a technician can determine what kind of transaction manager has led to a deviation from the regulations or by accident (we can assume that the awareness operators that their every action is logged the system, largely of their disciplines).

What information is archived (written in history)? In general, all data that is stored in the archive can be divided into three groups:

1. Process variables (process values).

2. Alarm (alarms).

3. Actions of operators, technologists (operator actions).

Variable Process

Under the phrase "process variable" refers to the numerical parameters defining the current state of the process. For process-related variables include the signals input / output parameters of the functional blocks, local and global flags (variables), tags, SCADA, etc.

Process variables are divided into discrete and analog. A discrete variable can take a finite number of values from a fairly narrow range. In practice, a discrete variable usually involve the value of a Boolean type (binary), indicating one of two possible states of the object (or control signal), although, formally speaking, it is not entirely correct. In the general case of a discrete variable is similar to the type of enumeration language C.

Analog variable can take any value from the limited continuous range. By type representation of the analog variable is the real number.

How Process variables are written to the archive? There are two techniques of their registration in the archive:

1. Cyclical record (cyclic archiving) implies a periodic record of the current value of process variable via user-defined intervals, regardless of the value and rate of change of this variable (see Fig. 1 - 54). Although this technique is not very economical, it is often used for archiving of analog variables. Frequency loop recording for each variable can be adjusted individually and as a rule, lies in the range from 0.5 to 10 min. As for discrete variables, and the rapidly changing analog variables, such an approach to the archive records clearly not optimal.

Fig. 1 - 54. Cyclical recording of variable process in the archive

2. Backing to change the variable (delta-archiving). This approach involves recording the variable in the archive only when a change in its value, compared with the previous recorded (absolute difference), reaches a certain value - delta (Fig. 1 - 55). The value of the delta set by the user and can be expressed in absolute units, and as a percentage of the scale. Of course, this technique is more economical than the cyclical record, as it adapts to the rate of change of archived values. For discrete variables - this approach is indispensable. Suppose we have a discrete variable, which varies, say, once per hour. Why did it back up every second or minute? It is much more logical to write down the value of the archive only in those moments when the value goes from 1 to 0 or vice versa.