Emerson Rosemount 644h, Reference Manual

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Reference Manual
00809-0100-4728, Rev HA
December 2002
3-13
Models 644H and 644R
Transmitter Behavior with Intermittent Sensor Detect OFF
When the Intermittent Sensor Detect feature is switched
OFF
, the transmitter
tracks all process temperature changes, even if they are the consequence of
an intermittent sensor. (The transmitter in effect behaves as though the
threshold value had been set at 100%.) The output delay due to the
intermittent sensor algorithm will be eliminated.
Open Sensor Holdoff
The Open Sensor Holdoff option, at the normal setting, enables the Model
644 to be more robust under heavy EMI conditions. This is accomplished
through the software by having the transmitter perform additional verification
of the open sensor status prior to activating the transmitter alarm. If the
additional verification shows that the open sensor condition is not valid, the
transmitter will not go into alarm.
For users of the Model 644 that desire a more vigorous open sensor
detection, the Open Sensor Holdoff option can be changed to a fast setting.
With this setting, the transmitter will report an open sensor condition without
additional verification of the open condition.
MULTIDROP
COMMUNICATION
Multidropping refers to the connection of several transmitters to a single
communications transmission line. Communication between the host and the
transmitters takes place digitally with the analog output of the transmitters
deactivated.
Many Rosemount transmitters can be multidropped. With the HART
communications protocol, up to 15 transmitters can be connected on a single
twisted pair of wires or over leased phone lines.
A Model 275 HART Communicator can test, configure, and format a
multidropped Model 644 transmitter in the same way as in a standard
point-to-point installation.
The application of a multidrop installation requires consideration of the update
rate necessary from each transmitter, the combination of transmitter models,
and the length of the transmission line. Each transmitter is identified by a
unique address (1–15) and responds to the commands defined in the HART
protocol.
Figure 3-4. Typical Multidropped
Network
Fast Key Sequence 1, 3, 5, 3
Power Supply
Power Supply
Impedance
Handheld Terminal
Computer or DCS
Model 644 HART
transmitter
4–20 mA
HART Interface
250
Ω