The only changes are:1. The control system‟s 4-20 mA interface is replaced with one that “talks” FOUNDATION fieldbus. We called it a FOUNDATION Fieldbus Interface (FFI). This FFI could, in fact, be in a personal computer or a PLC.2. The analog field device is replaced with one thattalks FOUNDATION fieldbus.3. A terminator is added at the FFI end of the wirepair. Another terminator is added at the field deviceend of the wire pair. The FFI may have a built-interminator so that you don‟t have to add one.
Fieldbus Network With Additional Devices Added Fieldbus Network With Chained Devices
Repeaters are active bus powered, or non-bus powereddevices, used to extend a fieldbus network.A maximum of four (4) repeaters and/or active couplers can beused between any two devices on a fieldbus network Using fourrepeaters, the maximum distance between any two devices on thatnetwork is 9500 m.A Bridge is an active bus powered, or non-bus powereddevice, used to connect fieldbus segments of differentspeeds (and/or physical layers - e.g. wire, optical fiber,..)together to form a larger network.A Gateway is an active bus powered, or non-bus powereddevice, used to connect a fieldbus segment or segments toother types of communications protocols (e.g.Ethernet, RS232, ..).
Addition of a Device and aBridge to Fieldbus Network
PolarityThe Manchester signal used by fieldbus is an alternating voltage thatchanges polarity once or twice per bit.In unpowered networks only this alternating voltage exists.In powered networks the alternating voltage is superimposed onto the DCvoltage being used to power the devices.In either case, the fieldbus receive circuits look at only the alternatingvoltage.Positive voltage swings have one meaning, negative swings have theopposite meaning.Therefore, the fieldbus signal is polarized. Field devices must be connectedso that they all see the signal in correct polarity.If a field device is connected “backwards” it will see an inverted version ofthe alternating voltage and won‟t be able to communicate.
Short Circuit Protection Short circuit protection has logic that detects a short, removes the shorted circuit from the segment, and lights a LED. This prevents a short from affecting the segment.
If you have 2-wire field devices in your network, youhave to make sure they have enough voltage tooperate.Each device should have at least 9 volts.You need to know:1. The current consumption of each device.2. Its location on the network.3. The location of the power supply on the network.4. The resistance of each cable section.5. The power supply voltage.
TerminatorsA terminator is an impedancematching module used at or neareach end of a transmission line.There need to be two (and ONLYTWO) terminators per bus segment.The terminators prevent distortionand signal loss, and are typicallypurchased and installed as apreassembled, sealed module.The user/installer need not beconcerned about or assembleindividual electrical resistors andcapacitors.
Terminal BlocksTerminal blocks can be the same terminal blocks as used for 4-20 mA.The terminal blocks typically provide multiple bus connections, such that a devicecan be wired to any set of bus terminals.
Power Supply• Wide input range: 90-264 VAC (47-440 Hz) 127-367 VDC• 24 VDC, 1.5 A output.• Galvanically isolated• Failure indication and output
Power Conditioner •A fieldbus power conditioner prevents the high frequency communications signal from being shorted out by the DC voltage regulators. •Typical power conditioners make 350 to 500mA available on the bus.
Alternative earthing arrangement for improvedEMC performance
Multiple grounding with potential equalization
Wiring DOs and Don’tsNormal wiring procedures apply:• No loose connections• No exposed conductors• Water proof junction boxes• Signal wires not too close to power wires• No safety barriers in parallel
Basic Troubleshooting• Correct polarity• Correct tag and address• Integrity of the fieldbus network• Supply voltage is sufficient, min 9.5 V even duringcommunication.•Wiring errors, including wrong connections, open or shortcircuits, intermittent•Connections and reversed polarity• Too many or too few terminators on each segment• Faulty „out of the box‟ physical layer components orfieldbus instruments• Inadequate grounding, such as multiple grounds in field, orthe absence of anyclear grounding strategy
Communication Errors• Poor connections• Wrong or no terminator placement• Too low or unstable power supply• Too long or over-populated spurs• Wrong or no grounding• Water filling due to poor plugs and cable-glands
Periodic monitoring• Short-circuits between the fieldbus + or – and the cable shield.• The signal level of each participant on the bus. A minimum level isspecified by Foundation fieldbus specifications. Low or high levels on alldevices suggest incorrect bus termination, but if the faulty signal level isonly on one device, there is possibly a problem on a single spur.• DC voltage on the bus, indicating correct functioning of powersupply/conditioner.• Noise: A maximum level is specified by Fieldbus specification.• Retransmissions. This provides a good measurement of physical layerhealth; retries can obscure faulty device or network.
Redundancy•No provision is made within either fieldbusstandard for redundant segmentcommunications.•Various fieldbus vendors, including majorprocess control companies, have developedredundant fieldbus schemes that involvecomplete duplication of all equipment.