With the ever-growing demand for plant-wide information and, new products and concepts do achieve this appearing almost daily, adopting a standard communication structure for one's plant can be a tricky and frustrating business. Most of the driving force between the different solutions has been from the control systems vendors themselves. Depending on what level of control one is looking at (PLC, DCS, scada etc), as well as what side of the Atlantic one's favourite vendor's development team resided, would in many cases determine what fieldbus and network topology one finished up with.
The question of "Will it be able to interface with my existing XYZ instrument?" is usually met with a resounding "No problem!" And I guess the truth of the matter is that anything is achievable if there is enough money in it for someone.
Ethernet, in its industrial guise, has forced its way onto the factory floor. But that tends to be at a higher level. A number of Ethernet I/O solutions are now coming to the market. But will Ethernet ever be cost-effective and rugged enough to get down to an individual point basis? Only time will tell. Even Ethernet falls foul of the seven-layer ISO structure, with many still believing that it is as simple as connecting ABC TCP/IP to XYZ TCP/IP and off you go.
In many cases it is not that any one network, fieldbus, protocol, is better than any other. It is more a question of which one fits the actual specific requirement. And to do that, it is just necessary to understand what the benefits of each are.
Below is a glossary of some of the more often commonly used terms in the area.
* Modem: Transforms and packs digital data into signals, which are defined for the media that is to transmit the data (four-wire, telephone, fibre-optic cables etc). A modem operates on Layer 1 of the OSI model.
* Repeater: Amplifies and restores signals for long-distance transmission. RS422 and RS485 allow connections to be made to a maximum of 1200 m with a maximum of 32 nodes.
* By installing a repeater, you can add a further 1200 m and 31 node segments to the network.
* Multiplexer: Used to save on wiring. For example, instead of installing eight connections with modems and cables, the same function can be obtained using two multiplexers and one line. The multiplexer re-creates the eight channels. Each channel can communicate as if it were an independent permanent connection with full-duplex transmission and an optional transmission speed.
* Isolator: A unit that maintains a signal path while simultaneously providing galvanic isolation/insulation to protect connected devices from damage should one of them develop a fault, or experience a power surge. (This is often accomplished via optical transmission). An isolator/insulator does not function as a modem.
* Interface converter: Used to enable devices with different interfaces to communicate with each other, eg RS422/485 to V.24 (RS232) or from fibre-optic cables to RS485.
* Router: Connects local networks with different standards (eg Ethernet with Token Ring or Profibus). The router transforms and re-packs the electrical signals and data for the different standards. A router can also be used to separate different segments in a network to improve performance and reliability.
* Bridge: An intelligent connection between two local networks with the same standard but with different types of cables. Example: Ethernet Coaxial to AUI.
* Gateway: An intelligent connection between a local network and external networks with completely different structures. Examples: Modbus Plus to Profibus or DeviceNet to Modbus TCP/IP.
Bob Petrie, Throughput Technologies
011 705 2497
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