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The ties that bind: Why cable is one of the most critical components of any control system


Cable is a part of the control system we often take for granted. Nevertheless, it is one of the most critical components of any system. Not only is it normally the medium by which we transmit the signals and information we use for control, as the Physical Layer of the Open System Interconnect (OSI) model, it is also the one on which all the other layers build. Just a like it is impossible to build a plant without pipes, a facility without a Physical Layer cannot exist. Therefore, cable (or its equivalent for those fiber and wireless enthusiasts in the audience) is truly the component that binds all our control systems together.

The Fieldbus Foundation has recently released a specification describing the requirements for cable to be used to connect the various components in a system. What is most interesting about the specification is what is not tested. The parameters not tested include: signal propagation velocity, wire insulation colours (though they do mention as an option that it should be brown for positive and blue for negative), cable jacket colours and characteristics such as temperature pull and cable wire size/diameter.

Of interest to all of us here in the northern climes is that the specification calls for a temperature range of -30 to +90 degrees Celsius, not great for Northern Alberta, where the specification is normally for -50 degrees Celsius. All of you need to be aware of that as you specify your cable. Besides specifying the key cable properties themselves, the specification also standardizes how connectors are to be mated to the cable for M12 and 7/8″-16 UN-2A Thd connectors:

Other key parameters covered in the specification include:
• Characteristic impedance (Zo) – 100 +/- 20 Ohms
• Attenuation – 3db/km at 39 kHz
• Wire – both 18 AWG for trunks and 22 AWG for spurs gauge wire is included.
• Shield construction – Each pair must be individually shielded using metalized polyester tape as the preferred choice though other equivalent options are allowed.
• Wire-to-wire capacitance – a minimum of 12 picoFarads/meter based on a minimum 30 m cable length.
• Wire twists per meter – a fairly typical 18 to 22 twists per meter.
• Jacket resistance – 1 MegaOhm/330 meters (1000 feet) minimum resistance between the cable shield and the metal structure the cable may be in or on.

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As an end user you should note that the characteristic impedance can vary by +/–20 per cent from what is considered the norm of 100 Ohms – and, as we all know, resistance can change considerably with temperature, so be sure you know how Zo changes as a function of temperature and at what temperature the cable’s characteristic impedance is being given. Otherwise if you purchase a cable with an impedance of 80 Ohms, still within the specification, you may have an “out of specification” cable at operating temperatures. If you, as an end user, think the above may be a bit too “wishy-washy,” I encourage you to become a member of the Fieldbus Foundation and as such you will be able to cast a ballot to confirm, deny or, as a minimum, influence and change this or any FF standard.

The end result of this specification is that customers will now be able to specify that the cable to be used for their FF project be compliant with FF-844 and have the associated FF “check mark.” Of course an offshoot of this is that adventurous end users now have a clear picture of what characteristics are needed to use any other type of cable for their design – provided it meets all the identified criteria. I am sure that some people will go this route, but the question remains – are you willing to risk your entire project on infrastructure that “might” be okay?

Ian Verhappen is an ISA Fellow, ISA certified automation professional, adjunct professor at Tri-State University and director of industrial networks at MTL Instruments, a global firm specializing in fieldbus and industrial networking technologies. E-mail him at Ian.Verhappen@ICE-Pros.com or visit his website at www.ICE-Pros.com.