Reference Manual 00809-0100-4728, Rev HA December 2002
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 Fast Key Sequence
1, 3, 5, 3
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
Power Supply Impedance Power Supply
4–20 mA
250 â„Ś
Handheld Terminal Computer or DCS
HART Interface
Model 644 HART transmitter
3-13