Data Acquisition

Grounding has safety and noise connotations. 8802.3 states that the shield conductor ofeach coaxial cable shall make electrical contact with an effective earth reference at onepoint only.The single point earth reference for an Ethernet system is usually located at one of theterminators. Most terminators for Ethernet have a screw terminal to which a ground lugcan be attached using a braided cable preferably to ensure good earthing.Ensure that all other splices, taps, or terminators are jacketed so that no contact can bemade with any metal objects. Insulating boots or sleeves should be used on all inlinecoaxial connectors to avoid unintended earth contacts.9.8 FieldbusesThere are currently several hybrid analogue and digital standards available forcommunication between field devices and also between field devices and a mastersystem. But only a fully compatible digital communication standard will provide themaximum benefits to end users and one such standard, currently being proposed, isFoundation FieldBusIt would be helpful to ask, why is there considerable effort, time and money beinginvested in searching for a ‘perfect’ digital communication network? Why are thereseveral approaches and not just one unified effort? Aren’t there enough standards andwhat is wrong with the one’s we have?The current approach to cabling of a typical control system is shown in Figure 9.10.Note that each instrument and actuator is connected back to the instrument room (to acontroller) with an individual pair of wires.The strategy espoused today is to replace this with a communication cable, whichconnects all the instruments and actuators together, and has several significant advantageslisted below. Each instrument and actuator now becomes an ‘intelligent device’. Anintelligent device can be considered a computer-controlled device that takes analog data(e.g. flow meter), performs an operation on it (e.g. square root extraction), and sends thisup a communication network to another device(s), which requires this data. Similarly, anintelligent actuator can control a valve to a specific position with a data value sent downthe communications network from another device.The answer then lies in the potential benefits, and to a lesser degree, in the realizationthat the older standards were developed for specific reasons that may not be applicable tothe demands placed on present day communications systems.There are real benefits to be gained from emerging networks, including:• Greatly reduced wiring costs• Reduced installation and start-up time• Improved on-line monitoring and diagnostics• Easier change-out and expansion of devices• Improved local intelligence in the devices• Improved interoperability between manufacturers