Detcon 700 HRT Bridge - Manual (Page 7)

HRT Bridge
700 Sensor HRT Bridge IM Rev. 0.2 Page 3 of 18
3.0 Operation
When power is applied to the HRT Bridge PCA with the Model 700 sensor attached, the HRT Bridge will go
through a boot up sequence that will last for approximately 30 seconds. During this time, the 4-20mA line will
be held at 1mA. After the boot up sequence the HRT Bridge will enter normal operation, and communication
with the Host will begin. A red LED (D7) Labeled “HART” on the HRT Bridge PCA will illuminate when the
PCA is communicating with the HART Host.
The 4-20mA signal from the HRT Bridge must be connected to a load resistor for HART communication to
operate properly. If this signal is not terminated properly, the HRT Bridge, and the HART Interface will fail to
work. Normal termination for the 4-20mA signal is accomplished by connection to a Host device, which will
have the correct load to terminate the signal properly.
If the HRT Bridge senses a fault in the sensor, it will take the 4-20mA signal down to 1mA. This 1mA signal
will signify to the Host that a sensor fault has occurred, and the Host should, in turn, flag an error with the
associated sensor.
The HRT Bridge communicates with the Model 700 sensor through the Modbus™. The 4-20mA signal from
the 700 sensor should be connected to the HRT Bridge PCA even though it is not used. The HRT Bridge reads
the appropriate Modbus™ register and creates the 4-20mA signal from the register reading. This allows the
HRT Bridge complete control of the HART Communications. A red LED (D6) Labeled “MODBUS” will
blink when communication with the sensor occurs.
The HART interface has the ability to take the sensor into calibration. If the sensor is taken into calibration
via the HART interface, the HART Communication Protocol will inform the Host that the sensor is in
calibration mode, and will not set a fault. The 4-20mA signal will be set at 2mA. Starting a calibration using
the sensor interface and magnetic tool will also cause the 4-20mA to be set to 2mA.