FCU 2165/2175 Reference Manual - Emerson Process Management

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Rosemount Tank Radar REXReference Manual308018EN, Edition 3BSeptember 20073.2.2 ElectricalInterface3.2.3 LoopArchitecture3.2.4 CommunicationParametersThe electrical interface to the Whessoe gauges is based upon a digitalCurrent loop. In normal circumstances, the standing current is set to anominal 20mA. The current is set by adjusting a voltage source in theMASTER device. When transmission takes place, the current is modulated atthe configured baud rate between 0mA and the standing current. Although thestanding current is nominally quoted as 20mA it is quite common to find thatreliable communication is only possible at some other setting. Normally arange of 13mA to 30mA is used to get a reliable communication.The FCU2165/2175 has been designed to interface with the electricalinterface of the Whessoe gauge using an integrated Current Loop (C/L)interface.The integrated C/L interface design for the FCU2165/2175 has a limitation onthe maximum output voltage (48 VDC) available to drive the Current loop. Ifthe Data Loop contains a lot of transmitters and/or the Data Loop is very longthe available output voltage may be insufficient to drive the Loop. In this casethe external DLIC interface should be used (U max : 70 VDC).Where the FCU2165/2175 and/or DLIC are replacing an existing system, thestanding loop voltages of the system being replaced e.g the Whessoe DACS,Whessoe Fig. 1097 (mini receiver) or Fig. 1098 should be measured whilstdoing the site survey. This information can subsequently be used to select thecorrect device.The Whessoe Loop is a 2 wire serial loop and is based upon a singleMASTER multiple SLAVE configuration.The FCU2165/2175 is the MASTER that initiates all communication.The SLAVE device are the tank transmitters.The FCU2165/2175 device also provides the Current Loop power supply.The WM550 protocol allows for up to 32 devices.Each data loop can therefore accommodate up to 32 devices. Each device isconfigured with a unique address in the range 0 to 31.The data bits, parity setting and stop bits are fixed at:• 8 Data Bits.• EVEN Parity• 1 Stop BitThe available baud rates are:• 150• 300• 600• 1200• 1800• 2400Earlier models of Whessoe gauges have the baud rate set by links on thecommunications board in the Outstation or Transmitter. The FCU2165/2175must be set to use the same baud rate as the Outstations / Transmitters onthe Loop.3-2 Section 3. Interfacing to Whessoe Gauges
Reference Manual308018EN, Edition 3BSeptember 2007Rosemount Tank Radar REXLater Fig. 1315 Transmitters and ITG are provided with an automatic baudrate detection mechanism, which will attempt to “lock on” to the baud rateused on the Loop. The FCU2165/2175 should be set to use the desired baudrate, suitable for the length of the Loop and the number of Transmitters on it.Where possible, the baud rate of the existing system should be determinedbefore decommissioning the existing Loop Master.3.3 COMMISSIONINGTHE FCU2165/2175This chapter describes how to configure the FCU2165/2175 forcommunication to the Data Loop.3.3.1 Preparation Prior to undertaking this work the following data will be required:1. The Baud Rate of the Whessoe Data Loops.2. The number of transmitter on each Loop.3. The Gauges Type and Address of each gauge on each Loop.4. The output voltage at standing current from existing MASTER.This information needs to be entered through the FCU configuration tools, see“Configuration Tools Software” on page 2-10.Setting the Whessoe loop currentThere are two general techniques to set the Loop current:• General procedure: increase the Loop Voltage in steps until you getreliable communication.• Alternative procedure: use a meter to set the loop current to a precisevalue.General Procedure• Ensure the Field Loop is disconnected.• The Rx LED should be on, the Tx LED should be flashing periodically.• Adjust the Loop output voltage to a minimum.• Connect the Field Loop.• The Rx LED should still be on, the Tx LED should be flashingperiodically.• Increase the Loop Voltage until the Rx LED goes off and/or starts toflash periodically.• Then slowly increase the Loop Voltage until the Rx and Tx LED´s flashconsistently with no gaps.• Check that data is being displayed in TankMaster.If any tanks continue to give communication problems and the field transmitteris working properly, increase the Loop current in small steps until theproblems disappear.If by increasing the Loop Voltage the Rx LED does not go off or starts to flash,there may be insufficient loop voltage to drive the loop or possibly a problemwith the interface and/or data loop.Section 3. Interfacing to Whessoe Gauges3-3
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