Product category:
Fieldbus systems, Fibre-optic systems
News Release from: Moore Industries International | Subject: Trunkguard TG200
Edited by the Processingtalk Editorial
Team on 31 July 2006
Trunkguard for fieldbus short-circuit
protection
Trunkguard TG200 Series fieldbus device couplers prevent segment failure caused by single device faults, cure common commissioning headaches, and handle both Foundation fieldbus and Profibus systems
Trunkguard TG200 Series fieldbus device couplers prevent segment failure caused by single device faults, cure common commissioning headaches, and handle both Foundation fieldbus and Profibus systems The TG200 Trunkguard Series Fieldbus Device Couplers from MooreHawke, a division of Moore Industries, use a unique "fold-back" technique to disconnect a shorted instrument from the fieldbus segment, thus preventing complete segment failure
This article was originally published on Processingtalk on 28 Jul 2008 at 8.00am (UK)
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Other fieldbus device couplers use a "current-limiting" technique, which holds a fault on the segment; this adds up to 60mA of current, which may deprive other instruments on the segment of power, overload the segment power supply, and cause an entire segment to have catastrophic failures.
The fold-back technique used in the TG200 has a logic circuit on each "spur" that detects a short in an instrument, disconnects that spur from the segment, and illuminates an LED that can be seen by maintenance personnel.
By removing shorts from the fieldbus segment, users have confidence that wiring and instrument mishaps will not jeopardize their fieldbus segment reliability and availability.
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No other fieldbus coupler has this fold-back feature.
The TG200 Device Coupler also simplifies commissioning and startup through use of a patented Automatic Segment Termination feature that eliminates problems caused by over or under-termination.
The Automatic Segment Termination feature in the TG200 series of device couplers automatically activates when the device coupler determines that it is the last fieldbus junction device in the segment; if it is, it terminates the segment correctly.
If it is not the last device, it does not terminate the segment, since the downstream device coupler will assume that responsibility.
No action - such as setting DIP switches - is necessary by the installation person to terminate a segment properly.
If a device coupler is disconnected from the segment accidentally or for maintenance, the Automatic Segment Termination detects the change, and terminates the segment at the proper coupler.
This allows the remaining devices on the segment to continue operation.
No other fieldbus device coupler has this automatic segment termination feature.
TG200 device couplers are available with four, eight or ten fieldbus spurs, and work with Foundation fieldbus H1 and Profibus PA interfaces.
Couplers can be DIN-rail mounted or enclosed in IP66-rated glass-reinforced polyester (GRP) or stainless steel enclosures.
Up to 32 fieldbus devices can be accommodated in a single segment.
The TG200 Trunkguard has ATEX/FM approvals for use in Zone 2 and Division 2 hazardous areas.
Other Trunkguard models are available for other hazardous area classifications or other protection concepts.
HOW FIELDBUS WORKS.
A fieldbus segment - Foundation fieldbus or Profibus - is a single twisted pair wire carrying a digital signal and power that connects multiple fieldbus devices, such as temperature, flow, level and pressure transmitters, smart valves, actuators, and other instruments to a DCS or similar control system.
Foundation fieldbus can support up to 32 devices; Profibus PA can have up to 128 "slaves." The fieldbus segment begins at an interface device at the control system.
On a Foundation fieldbus system, the interface is called an H1 interface; on a Profibus system, it is a Profibus DP/PA segment coupler.
In terms of signal wiring and power requirements, Foundation fieldbus H1 and Profibus PA are identical.
Each segment is driven by a power conditioner that provides 350-500mA at up to 32 V over a 1,900m distance.
Each fieldbus device typically requires 10-30mA, but some devices can draw more current.
Generally, power conditioners are used in redundant pairs for an entire segment.
Each fieldbus device connects via a "drop" on the fieldbus segment.
The simplest connection is a "T." The problem with a "T" connection is that if any one of the devices short out, it takes down the entire segment.
Another way to connect fieldbus devices is via fieldbus junction boxes, often referred to as "device couplers" - such as the MooreHawke TG200 Trunkguard - that allow multiple fieldbus devices to connect at one time.
Typically, users will install a device coupler in a field enclosure, and connect multiple instruments to it.
The fieldbus cable may continue onward to another device coupler.
A 32-instrument segment may have one or more device couplers.
The advantage of a coupler is that each device connects via a "spur" - like a "T," but with protection against shorts.
If a fieldbus device shorts, only that spur is affected.
Other fieldbus couplers use a "current-limiting" technique, which holds a fault on the segment; this adds up to 60mA of current per fault, deprives other instruments on the segment of power, overloads the segment power supply, and can cause catastrophic failures on the segment.
The new TG200 from Moore Industries uses a "fold-back" technique which disconnects the shorted spur from the segment, thus preventing loss of an entire segment.
Historically, users have been wary of relying on "current limiting" couplers, and limited each segment to only 16 devices to prevent large-scale segment failures.
With TG200 couplers, users no longer have to worry about such failures and can have more confidence about placing more devices on their Foundation fieldbus and Profibus segments.
Since the cost of H1 cards (US$2,500) can be cost prohibitive, allowing users to place more devices on a segment can save users a considerable amount.
Every fieldbus segment should be terminated at both ends for proper communication.
If a segment is not terminated properly, communications errors from signal reflections may occur.
A frequent commissioning problem during startup is determining that terminators are correctly located.
Most device couplers use manual on/off DIP switches to terminate couplers.
During installation of the fieldbus system, the DIP termination switches sometimes are set incorrectly, creating problems during startup.
The instruments can behave erratically, drop off the segment mysteriously, and generally raise havoc - all because the terminations are not set properly.
With the TG200 Automatic Segment Termination feature, these problems are eliminated, allowing commissioning and start-up timescales to be drastically reduced.
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