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APPENDIX A3 : HART and PSION Operating Instructions
1.0. HART CONSIDERATIONS
Special Considerations for HART
If the HART communications facility built into the control unit will be used at the time of installation or
during its future working life, then it is essential that a resistive load of at least 250 Ohms is connected in the
supply cable. This may be provided by other devices in the loop (Chart recorder, meter, etc.) or more usually
by installing a standard 270 Ohm 0.25W resistor in series with control unit. In this way the master device will
be able to signal to the control unit without the power supply short-circuiting the data.
In addition, excessive cable capacitance will attenuate the HART signalling and so the capacitance must
anyway be limited if HART communications is to be used. The RC time constant for the network must not
exceed 65µs, e.g. if the network resistance is, say, 650Ω then the maximum network capacitance is 0.1µF.
HART transmitters are often given a Cn number where n is the multiple of 5000pF which the device presents
to the network. In the case of MSM400 the value of n is 1 since its capacitance is below 5000pF.
Total loop resistance and capacitance
Additional equipment such as indicators or recorders may be inserted in the loop subject to the resistance and
capacitance limits discussed above.
Cable trays
It is recommended that HART signal cables are not run alongside power cables.
1.1 Multi-drop installations (MSM400 in HART digital mode)
Up to 15 transmitters may be connected in parallel with each other, and each must be set to a different
HART address between 1 and 15. When HART transmitters are connected in multi-drop mode, each control
unit draws a fixed current of 4mA.
Compatibility with other HART instruments.
Any HART instrument, regardless of the manufacturer, may be connected in parallel with another to create a
multi-drop network. They may be combined with separately powered, current sourcing or sinking devices but
care must be taken to ensure that the HART signal current passes through the 250W minimum impedance to
establish communications. For full details of HART instrument availability, refer to the HART Communication
Foundation publications or individual manufacturers' literature.
Handheld Communicators
A handheld communicator (HHC) may be connected across the network (downstream of the minimum 250
Ohm loop resistance) to programme or interrogate a HART control unit. Some HHC’s support only a subset of
a transmitter’s functions, and will thus only access the control units Universal and Common Practice
commands. However, the MSM400 is fully supported by the MOBREY-CK1 HHC and by the UNIVERSAL 275
HHC (provided the 275 is loaded with the transmitter’s Device
Description – contact Mobrey Measurement or the HART Communications Foundation for details).