In digital communication, as we have seen, it is important to both physical and i through which communication is established, as the characteristics of the information itself. At this point it is important to note that digital instrumentation equipment differ in the type of protocol used to communicate, being necessary to review the characteristics of them.
The protocol is a set of rules and conventions that establish reliable communication between two entities of a process.
.. And what are the advantages of DIGITAL COMMUNICATION
Digital communication links consist of two basic levels: the physical link or means of connection between the digital system modules and protocol. The medium may be coaxial cable, a radio signal, a fiber optic cable, a modem pool on a telephone line, a satellite link, a cable of 25 conductors from the digital system to a modem or a combination of these media.
Some physical links are designed strictly for connecting only two devices such as RS-232 and RS-422. Some are designed for use with multiple devices that communicate over a single physical link (such as a local area network or LAN), such as RS-485 interface.
When two or more modules of interconnected digital systems into a single communications circuit or network, they must be able to support a network protocol, these protocols use different mechanisms to ensure a reliable and robust communication. Many of them are owners that is, for use only in a particular brand of equipment requiring special drivers or software interfaces and often hardware.
Examples of these protocols are those used in communication with PLCs, such as Data Highway + Alien-Bradley or Modicon Modbus +.
There is now a growing trend towards connectivity between different manufacturers' equipment through standardized versions of sophisticated industrial networking protocols, the main reasons for this are:
- Open Systems: It is difficult and expensive instrumentation integrating systems from different manufacturers due to different protocols used ios. Standard protocols, devices from different backgrounds can coexist in the same network and communicate.
- Reduction in the cost of wiring: Many systems still use OS 4 to 20 mA analog instrumentation, requiring a lot of point to point wiring. Wiring for a muitidrop means a reduction in installation costs.
- Needs more information: Currently, companies need to have more information about their processes and the instrumentation connected to them. The traditional instrumentation provides only a value, e! of the process variable. In a digital network, you can provide information tools for maintenance and diagnostics to better understand the performance of the instruments.
- Smart devices, equipment manufacturers are putting more and more intelligence into their devices to meet the growing demand for functionality at low cost. Most information in a digital network is necessary to capitalize on improved capabilities made possible by the presence of intelligence in devices.
The digital information provided by a smart transmitter is transmitted according to a particular protocol, which determines the way it is coded and super-imposed on the signal line. As in the case of proprietary systems, each manufacturer has its own protocol. One of the most used is the HART (Highway Addressable Remote Transmitter), which signal information is contained in a digital FSK signal superimposed on an analog 4 to 20 mA.
Figure: Signal HART
Here is the HART PROTOCOL ...
Outside his breakthrough as a de-facto standard, this protocol still has some way to go before the advent of an open market. In fact, all interconnected devices are fully compatible, interchangeable and interoperable with a single terminal scheduled. The reason for this is the application layer. The HART command set has three classes of commands:
Commands are universal understood by all field devices.
common practice commands that are recognized by most ios but not necessarily all field devices.
Special commands that are unique to each device.
To operate a transmitter, all the commands required for a particular application should be implemented in the main computer and in the field universal programmer. To meet this requirement is necessary to submit a diskette device description, together with the transmitter, to be read and maintained by the computer. The portable terminal can then be fed by the information required by the application you are running. This description is known as DDs (Device Descriptions) and each manufacturer has a description for each instrument and can not be shared for other instrument of a different brand. A very good hart is HandHeld In theory this seems a good solution, but in practice, even simple devices most used commands are device specific. This means that they can not be properly operated with the device descriptions of another transmitter. Therefore, for applications in systems made up of devices from different manufacturers, you should aim to standardize on a common user interface for configuration and interrogation of transmitters. Therefore, HART is not considered as a solution to the needs of open systems and only a transition protocol that goal.
The following is a summary of the HART protocol features:
