Calibration and Maintenance UMSITRPSD3-1
November 2005
7-8
7.3.2 Digital Output (Communication)
A malfunctioning digital output can indicate a defective communication circuit. More commonly,
however, these problems are caused by an incorrect or poor installation. It is possible to install a
transmitter such that the 4-20 mA signal is correct, yet the digital HART signal is not.
The most common symptom of a communication problem is the inability to locate a transmitter on the
loop using a HART Master Device, such as the HART Communicator. Typical messages from the HART
Communicator include: device disconnected, no device found, or communication error.
If communication problems occur, check the following. Refer to the specifications in Section 9 and to
Section 4 Installation as necessary.
• Check that loop resistance is between 250Ω and 1100Ω.
• Check that electrical noise on the loop is not excessive: power supply ripple should not exceed 12
mVp-p.
• Check that there are no high inductance devices in the loop (I/P for example). Install a HART
communication filter across such a device.
• Check that the power supply voltage is high enough for the installed total loop resistance. Refer to
Section 4 Installation.
• Refer to Section 4 Installation and confirm that loop cable length is not excessive.
• Check that the HART Master is connected across a load.
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