Eaton MTL

MTL Instruments Group

FAQs in HART Interfacing

Questions

Answers

Does removing or repowering a MTL4841 affect the analogue signals?

The MTL4841 and MTL4842 can both be removed or the MTL4841 re-powered without interfering with the analogue 4-20mA signal.

Why does an AMS reset cause the MTL4841 to reset and lock up?

You are probably using a very early MTL4841 unit. This condition was corrected at software version 3.4. You should contact your local MTL representative for details on upgrading.

What are the maximum interconnection cable lengths?

The maximum recommended length of 20 way HART ribbon cable between the BPHM64 backplane and the HART connection unit or IS backplane is 6 metres or 20 feet. Flat ribbon cables should not be taken outside the cabinet as they are not robust and susceptible to noise. Ground loops may also be an issue. If possible, locate the HART mux units close to the take-off points. For larger distances use more than one BPHM64 with MTL4841 modules fitted and link via the RS485 bus.

The maximum length of 10 way interface bus cable between a single MTL4841 communications module and up to 16 MTL4842 HART interface modules / backplanes is 4 metres or 13 feet, (between modules 1 and 16).

What value terminating resistor should I use for RS485?

The termination will normally be a simple resistive terminator with an impedance that matches the characteristic of the cable; this will normally be in the range 100-220 ohms.
RS485 interfaces should ideally be provided with a “matched” termination to prevent reflections and “ringing” of the signal on the bus cabling.

Why does the MTL4841 scan slowly after a reset?

The search mode on the MTL4841 has been selected incorrectly. There are three selections, Single Analog, Single Unknown and Multidrop.
The normal selection is “Single Analog”, this is selected if all of the HART devices have their addresses set to 0.
“Single Unknown” is used if the poll addresses of the devices are unknown. It will search each loop until it finds a HART device on any poll address, 0-15, as soon as it finds a device it will move to the next loop.
“Multidrop” is used where there are multiple devices on loops, this will search every poll address, i.e. 0-15, on every loop.

Why doesn’t the IMS show the new HART device I’ve added?

When a new device is added, either a “loop rebuild” or a “reset” must be carried out to enable the new device to be seen in the IMS.

Why can I see only 49 of the connected HART devices?

There is a software bug in version 6.0 of the AMS Software that limits visibility to 49 devices. This bug has been fixed in versions above 6.0. Contact your local Emerson office or download the patch from the Emerson website.

Why does resetting the IMS confuse the listing of devices?

This is often because the converter is either not a recommended type or the settings are not correct. MTL recommends using the Westermo MA-45 RS485 converter with the MTL4841.
Lack of termination and biasing can be another reason for seeing this effect. The RS485 serial line should be terminated and biased and the Westermo converter has internal switches that enable you to do this.

Why are the attached devices not found when using AMS?

This usually means that the MTL4841 is in the wrong mode. The MTL4841 is available in three different pre-set modes, MTL, AMS and PRM and should be ordered in the required mode to suit your Instrument Manager Software (IMS). If you are unsure which mode your IMS requires, please contact your local MTL representative