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HART Communication Problems

ESTABLISHING HART COMMUNICATIONS CAUSING YOU HEARTACHE?

Sometimes we have all experienced difficulty in establishing communications with a HART Smart devices in the field. Stepping back from the situation, and having a checklist of potential problems and how to check to pinpoint the source of your problem could possibly get you home in time for dinner some evening.

The term “HART modem” is used to describe a “converter” that when used with a laptop software like Siemens PDM, will allow programming of HART devices by turning your laptop into a handheld. These are available in both DB9 as well as a USB port versions, and have alligator clips for the loop side connection.

“Hart Modem” is also the name used for a chip that resides in the transmitter capable of reading the 1100Hz and 2200Hz frequency shift signal know as HART, and communicating back and forth between the transmitter processor and a handheld, laptop, and/or a control system running a HART capable software for diagnostic and configuration purposes. We formulated a list of potential problems as follows:

BAD CONNECTION TO LOOP – It is always a good practice to minimize the amount of bare wire other than what is essential to make contact with terminals. It is important that terminals are loosened and the loop wires pulled out far enough for the alligator clips to make good contact with the wire, especially on compression type terminals. Remember to readjust inserting the loop leads again when done.

LOOP RESISTANCE – HART connection can be made anywhere in the loop in parallel as long as there is enough resistance (250 to 1100 Ohms). The impedance is needed to get the amplitude of the frequency pulses high enough to be read. An indication that this may be the problem is when you start reading the transmitter, and it faults out, or you drop out in the middle of communications. This situation can also arise in the case of a PLC or DCS card being replaced in a loop where no resister was required before.

Now with newer higher resolution analog-to-digital converters, many replacement I/O cards no longer have 250 ohms input impedance, only 50 ohms. The voltage drop in the loop is lower. Given the wide range of acceptable resistance, one can always add a resister in series in one of the legs, and then connect across that resister.  Many people opt to make it standard practice of always adding a 250 ohm resister on the negative loop leg at the control system terminal.

LOW POWER ON HANDHELD – HART communicators have a diagnostic to indicate low battery. Communications problems can rear their ugly head prior to this diagnostic flag appearing. It is a good idea to have access to fresh batteries or full charged batteries at any point in time.

OLDER NON-ISOLATED HART MODEMS – Some of the early HART serial modems were not electrically isolated. This could cause a problem when using a laptop/desktop with the AC charger connected. If possible, try the laptop with the AC charger disconnected running on battery power. If this does not work, try using another computer. We have found that some notebook computers will not work as the voltage to the port pins is outside the acceptable range, leaving the modem with insufficient power.

NON-ISOLATED TRANSMITTERS OR FAULTY PRIMARIES – Some lower cost temperature transmitters are not electrically isolated. Should a grounded junction thermocouple be the primary, an elevated ground potential in the field could be introduced to the transmitter and introduced to the loop, inhibiting HART communications. The same could be said for a capacitance probe with cracked insulation. Remove the primary element, and reattempt communications while keeping the loop side of the transmitter powered.

TRANSMITTER RUNNING ON MARGINAL POWER – Each component added to a loop has a voltage drop. That drop becomes more significant as the measured variable drives upscale. If the transmitter runs on 12-45VDC, and starting with a 24VDC source, there remains only 12 volts after the transmitter is satisfied.

Loading a loop too heavily not only presents the risk of not having enough voltage to drive to full scale 20mA when required, but also causing HART communications difficulty by starving the transmitter processor and internal modem. Try to minimize the input by either disconnecting the field primary, or lowering the measured variable such as flow rate in the case of a magnetic flowmeter, to get the transmitter output below 16mA.

LOOP ISOLATORS – Loop isolators not only help eliminate the possibility of grounds in a loop dragging down other loops references to a common electrical point, such as on PLC or DCS non-isolated cards with a common negative loop connection, but they also filter out electrical noise.

That includes 1100 and 2200 Hz.  If a standard loop isolator is present, it will not allow passing of the HART signal, and therefore communications should be attempted with the isolator removed from the loop, or on the transmitter/field side of the isolator. Utilizing an isolator like the Moore Industries HIX (2-wire) or the HIT (4-wire) isolators will eliminate this possibility as they are designed to allow HART to pass.

INTRINSIC SAFETY BARRIERS – Consult with the barrier manufacture as to whether the specific barrier will pass HART signals or not. Some are designed to allow HART, some are not.

BAD OR LOW COST LOOP POWER SUPPLY – A power supply is basically a rectifier, taking in AC and having a resulting DC output. The amount of ripple refers to the amount of AC that is left after all filtering. Low cost power supplies can leave enough AC that it corrupts the HART data packet, and communications will be rejected. The filtering can be damaged or degraded by power surges. This can be checked by simply switching a voltmeter to read AC, and check the loop for presence of AC.

To verify, remove the instrument for the loop source, and power it with batteries by itself, either from a sourcing battery powered loop meter, or simply from a couple batteries making sure you have enough voltage. Connect and try to re-establish communications, keeping in mind that you still need the 250 ohms impedance even in a small transmitter/power supply loop. A power supply can also loose windings as either vibration or heat takes a toll on coil insulation. This can be detected by monitoring the voltage output for stability under load. An unloaded supply will rarely reveal its faulty condition.

Many newer devices such as panel meters have switched from full bridge rectifiers to half wave rectifier, then add in AC filtering to limit noise. Sometimes this filtering also trims off the HART frequency. Removing process displays and other auxillary devices, then adding them back into the loop one at a time could pinpoint the source of the problem.

INDUCED ELECTRICAL NOISE – Electrical noise can come in along the run of loop leads. Check that the 4-20mA loop is run in grounded conduit. This helps drive any induced electrical voltage to ground with a low impedance.

Make sure that cable shields are present on the loop wiring, and that they are tied to ground on only one end, not both. Once again, bringing out a battery source with short leads, and eliminating the rest of the loop can help confirm electrical noise as the culprit.

Bluetooth interference – Turn off the Bluetooth on the computer where the Hart modem is plugged into. For some unknown reason we have found this to interfere with the hart signal.

The chips for HART Smart transmitter must be obtained through a small number of approved manufacturers approved by HART Foundation.

Specifications for HART allow tolerances on the window of acceptable line noise, and therefore you may be on the fence with two transmitters of the same model in a noisy electrical environment, and find one unit allows communication, while the other does not.

The above list can help narrow down the source of your HART  communication problems.

Source : gilsoneng

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