Troubleshooting Fieldbus Devices



Suggestions obtained from industrial customers of FF reveal a standard sample: Fieldbus is a robust and dependable expertise, however solely whether it is correctly put in. Poor installations, often pushed by a need to attenuate capital bills, will trigger quite a few issues throughout commissioning and operation.

One comparatively simple strategy to keep away from issues attributable to short-circuits in FF wiring is to make use of coupling units with built-in short-circuit safety. This function doesn’t add significant value to the coupling gadget, and it’ll stop the whole phase from failing as a result of a short-circuit on a single spur cable or inside a tool. Use coupling units with indicator LEDs as nicely, since these give straightforward visible verification of community energy which can tremendously speed up FF phase troubleshooting when the necessity arises.

Cable resistance

A easy verify of an H1 phase’s cabling consists of a collection of resistance measurements carried out with the section unpowered (as is commonplace with any electrical resistance test), with all FF units disconnected, and with the cable completely disconnected (all three conductors) on the host finish. The next desk exhibits tips revealed by the Fieldbus Basis for H1 section cable resistance measurements:

Fieldbus Foundation cable resistance measurement

The final resistance examine proven within the desk checks for the presence of floor connections within the protect conductor apart from the one floor connection on the host finish (which has been disconnected for the needs of the take a look at). For the reason that defend ought to solely be grounded at one level (to keep away from floor loops), and this one level has been disconnected, the protect conductor ought to register no continuity with earth floor in the course of the check.

The need of disconnecting all FF gadgets and host system interfaces is crucial in order that the resistance measurements reflect the well being of the cable and nothing else. The presence of any FF gadgets on the phase would considerably affect the resistance measurements, notably resistance between the sign (+ and −) conductors.

Observe : An alternate methodology of protect grounding is to instantly join it to earth floor at one finish, after which capacitively couple it to floor at different factors alongside the phase size. The capacitor(s) present an AC path to floor for “bleeding off” any induced AC noise with out offering a DC path which might trigger a floor loop.

Sign energy

The Fieldbus Basis specifies a sign voltage (peak-to-peak) vary of 350 mV to 700 mV for a wholesome FF section. Extreme sign voltage ranges level to an absence of terminator resistor(s), whereas insufficient voltage ranges level to an over-abundance of terminators (or even perhaps a tool quick):

Fieldbus Signal strength

Electrical Noise

FF, like all digital networks, are unaffected by noise voltage beneath a sure threshold. If noise voltage is current in extreme amount, although, it could trigger bits to be misinterpreted, inflicting knowledge errors. The Fieldbus Basis provides the next suggestions for noise voltage ranges on a FF section:

Fieldbus Electrical Noise

Fieldbus diagnostic instruments measure noise on the community section throughout occasions between message frames, when there ought to be purely DC voltage between the 2 conductors.

Be aware :  Keep in mind the tolerable degree for noise will differ with sign voltage stage as nicely. All different components being equal, a robust sign is much less affected by the presence of noise than a weak sign (i.e. the signal-to-noise ratio, or SNR, is essential).

Utilizing an oscilloscope on H1 Fieldbus segments

A instrument obtainable in most instrument retailers is a digital-storage oscilloscope, which can be used to measure and show FF H1 sign waveforms for evaluation of issues. Analog oscilloscopes are additionally helpful for community troubleshooting, however to a lesser diploma .

When utilizing an oscilloscope to measure FF H1 indicators, it is rather necessary to not join both of the FF section conductors to earth floor by means of the oscilloscope. Introducing such a “floor fault” to the community section will virtually definitely trigger communication issues, along with no matter issues exist already that compel you to diagnose with an oscilloscope. If a single channel of the oscilloscope is linked throughout the phase wires, the “floor” clip of the probe will power a type of conductors to earth floor potential through the metallic chassis of the oscilloscope which is grounded by the third prong of the facility plug for security. An exception to this rule is that if the oscilloscope itself is battery-powered and has an insulated case the place no floor connection is made by means of the floor it sits on or the human hand that holds it. In any other case, utilizing a single channel on a line-powered oscilloscope to measure community indicators is inviting bother.

If a line-powered oscilloscope should be used, the correct option to configure it’s for differential channel measurement. On this mode, the oscilloscope will register the voltage between two probe suggestions, slightly than register the voltage between a single probe tip and earth floor.

Troubleshooting Fieldbus Devices

Configuring a dual-trace oscilloscope for differential mode is kind of easy. On the entrance panel of the oscilloscope, you could set the multi-trace controls to the Add mode, the place one hint on the display represents the instantaneous sum of the 2 inputs (channels “A” and “B”). The volts per division “sensitivity” of each channels must be set to precisely the identical worth.

Additionally, the Invert management should be engaged for the second enter channel, forcing that channel’s sign to be inverted (register upside-down on the display screen). The summation of channel “A” and an inverted channel “B” is equal to the mathematical difference (subtraction) between “A” and “B,” which implies the one hint on the display now represents the difference of potential between the 2 probe suggestions. The oscilloscope now behaves as an ungrounded voltmeter, the place neither of the check leads is referenced to earth floor.

Message re-transmissions

Except for voltage parameters (sign power, noise amplitude), one other good indicator of FF section well being is the variety of message re-transmissions over time. Sure varieties of communication on an H1 phase require verification of a acquired sign (notably shopper/server VCRs equivalent to these used to speak operator setpoint adjustments and diagnostic messages). If the sign acquired by the shopper FF system seems corrupted, the system will request a re-transmission of the message from the server system. Re-transmission occasions, subsequently, are a sign of how typically messages are getting corrupted, which is a direct perform of sign integrity in a Fieldbus section.

Most host techniques present re-transmission statistics in a lot the identical manner that computer systems speaking by way of TCP/IP protocol have the power to show the variety of “misplaced” information packets over time. Since almost all FF segments operate with a number system linked, this turns into a built-in diagnostic software for technicians to troubleshoot FF community segments.

Hand-held diagnostic instruments are additionally manufactured to detect sign voltage ranges, noise voltage ranges, and message re-transmissions.


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