HART Communicator does not connect with transmitter


Thread Starter


In a gas plant.

We use the HART COMMUNICATOR to interrogate with the transmitter while carrying out PM or CM. This is a process plant, the transmitter are connected in a vessel or tank to control or monitor level of temperature or a control valve. when you hook-up your HART COMMUNICATOR it will display "NO COMMUNICATION".

when you don't 24VDC to the transmitter, it will show "NO COMMUNICATION". On further troubleshooting the auxiliary room end to the field will have 24VDC (4-20MA).

Jonas Berge - W

Here are a few points you can check:

Is the device powered on? Verify local display
-Check polarity

-Is auxiliary power required?

-Connection in the system-end depends on the device being loop-powered or separately powered

Is sufficient power supply available (check the voltage at the device terminals, at 20 mA)?

-Measure DC with digital multimeter (DMM)

-Note that 4-20 mA can work at low voltage that causes HART to fail

-Some devices require only 10.5 V, others require 19 V or maybe more. Each device type is different. Check the device manual.

-A critical point especially when safety barriers are used or if multiple receivers are connected in series (indicators, signal conditioners, trip amplifiers, chart recorder, DCS/PLC, wireless adapters, etc.)

-Check for poor connections or corroded terminals at device and junction boxes etc.

-Ohm's law applies: the supply voltage must exceed the minimum power needed by the device plus, series receivers, plus sufficient voltage to overcome loop resistance

Is there ripple/noise on the power supply?
-The 4-20 mA negative is usually ground/earth, so it must be 'clean'-Note that 4-20 mA can work in spite of ripple/noise that causes HART to fail

-If noise is present, check the installation: twisted pair, shield, ground/earth, separation from AC power cables (particularly drives).Check inside housing of AC powered devices like magflow meters where signal and AC wires may be rolled-up together

-Measure AC with digital multimeter (DMM) and also check with oscilloscope

Is shunt resistor (>230 ohm) present (for transmitter)?
The handheld shall be connected on the device-side of the shunt resistor (not the power supply side)

Is HART filter required by the control system?
Is safety barrier used? If so it must be compatible with HART protocol

Compatible HART version? Host s (handheld field communicator, laptop software, control systems, intelligent device management software, calibrator, signal converter) designed for HART version 5 and 6 do not work for HART7 (no forward compatibility), but systems designed for HART version 7 also work for version 5 and 6

Conflict with other primary or secondary master (DCS, modem/interface, multiplexer, signal converter, wireless adaptor etc.)?
-Some adapters and converters etc. can be configured as either primary or secondary master

Correct device address (usually zero, but not in multi-drop mode)?
-For WirelessHART adapter the address may be higher than 15. Check the manual



Not trying to be stupid here but is the device Hart compatible?

I have had problems on startups with the manufacturer slipping in a modbus transmitter and no one catching it until I did the startup checks.

Also most of the time if the hart does not see anything the Error will be "NO DEVICE FOUND".
It's possible that the control system doesn't have enough impedance for the HART signal. some input cards I have seen are only 50 Ohms. A clue would be the Voltage you see across the transmitter terminals 23 Volts at 0% would suggest the impedance is 250 any more than 23 would indicate < 250 Ohms.

The HART signal wants to pull the lines up and down, without enough resistance in series it has difficulty. The resistance doesn't have to be 250 Ohms it will work with more or less. a 220 or 330 Ohm would be fine.
I have a Rosemount pressure transmitter (using for level) that is connected to a Allen Bradley Micrologix PLC analog card. To communicate with a 475 does the resistor need to be in series or parallel? I also cannot get the analog to read at the PLC, do I need to put a 250ohm resistor in series with the 24VDC+?
HART needs a minimum of 230 ohms in the loop. An analog input (AI) supplies some resistance, but the value varies from manufacturer to manufacturer. If that A-B AI uses, say 100 ohms, then you need more resistance in series in the loop, but not across the (+)/(-) of the AI.

There's a multitude of reasons why you might not be able to read an AI value: wiring, configuration, scaling, logic.

You need to separate reading HART with a 475 issue from reading the value on an analog input issue.

Putting 250 ohms in the loop will (probably) not help you read the analog value at the input, it might help the 475 to read the HART data.
Hi to every one.....

Thanks in advance for ur reply.....

I am facing the same problem with my 3051 rosemount PT. The tx. is powered from siemens PLC AI card. if i connect HART it shows NO DEVICE FOUND. When i connect a 24V DC source it communicates properly.

As per the earlier post, does it mean that the 24V source has sufficient loop impedance which is not present in field wiring (tx. to PLC's AI card)? Then, whats the proper way to measure the loop impedance in both cases (through source & through field wiring).
Four things are needed for HART communication:

1) a HART enabled transmitter
2) a DC power supply
3) a minimum of 230 ohms of resistance
4) HART communicator (with good batteries) or a HART modem/PC/software

If this is done on a bench, then the resistance needs to be a physical resistor.

If this is done in the field, then the resistance is usually supplied by the analog input. But as others, like Roy, have pointed out, sometimes analog input resistors are less than the required 230 ohms so a resistor needs to be added to the loop to increase the loop resistance. The old 275 had a resistor on banana plug adapter that would provide loop resistance.

When you say, 'if I connect HART it shows ____", do you have all the required components?

The DC power supply does NOT have sufficient impedance, in fact, it is a low pass filter that eats the HART signal.
Does the transmitter power "on"? Does it have an LCD which is lit? Is the
current in the loop > 4 mA? What is the voltage on the transmitter

Depending on the system, you must wire the transmitter differently depending on if it is loop powered (two-wire) or separately powered (four-wire). If card is wired incorrectly the transmitter will not power on.

For other systems you may need to configure the AI card to operate as two-wire or four-wire. If the card is configured incorrectly, it will not power the transmitter.

I had the same issue with a level transmitter of ours, but was eventually able to solve it like this:

On the Hart Application Menu (says Offline, Online, Utility, etc)
-Select 'Utility'
-Select 'Configure HART Application'
-Make sure 'Poll by Address' is selected under 'Polling Options'
-Change 'Polling Addresses' to 0-15

Most transmitters have polling addresses of 0, but some have higher addresses, so they won't be found unless the communicator is looking for higher addresses.

This did the trick for me!