Emerson MSM400 manual Appendix A3 Hart and Psion Operating Instructions, Hart Considerations

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.

1.1Multi-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).

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