Detcon 700 Series Remote Sensor/Alarm Relay Module With HART Interface - Manual (Page 15)

700 HART-RAM I.M.
ALM1 ALM2 FLT
NO NC COM NO NC COM NO NC COM
J7
J2
J1
J4
IN
SENSOR
OUT
+
+ mA A B
mA
A
B +
A
B
K1
K3
K2
Input Power and
mA/HART Output
Modbus ™
Interface to Host
U1
Figure 9 Hart Interface Connection
5. HART Operation
When power is applied to the HART-RAM with the Model 700 sensor attached, the HART interface 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 HART interface will enter normal operation, and
communication with the Host will begin. A red LED (D7) labeled “HART” on the HART Bridge PCA will
illuminate when the PCA is communicating with the HART Host.
The 4-20mA signal from the HART-RAM must be connected to a load resistor for HART communication to
operate properly. If this signal is not terminated properly, the HART-RAM 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 HART-RAM 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 HART-RAM communicates with the Model 700 sensor through the Modbus™. The HART-RAM reads
the appropriate Modbus™ register and creates the 4-20mA signal from the register reading. This allows the
HART-RAM complete control of the HART Communications. A red LED (D6) labeled “MODBUS” on the
HART Bridge PCA 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.
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