Romilly's HART® and Fieldbus Message Board

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Re: Fieldbus cable Specification

Message posted by Romilly ( : on 17 July 2006 at 20:34:29.

in reply to: Re: Fieldbus cable Specification posted by N.Ramkumar on 15 July 2006 at 11:25:13.

Unfortunately, the parameters you have listed are not the most important. A good fieldbus cable (type A or B) should have a specified characteristic impedance of 100 ohms at 31.25kHz. The tolerance is 20% (type A) or 30% (type B). Also, the capacitance unbalance must be no more than 4nF/km (type A) or 6nF/km (type B), and the attenuation at 39kHz must be no more than 3db/km (A) or 5db/km (B).

The less-well-specified types C and D are unscreened, and do not specify these parameters.

BS5308 does not specify these parameters, so it cannot be regarded as type A or B. It is perhaps not as bad as type C. But unless you can get these critical parameters measured for the particular cable, I think you have to assume a performance more like that of type C.

The result of using lower grade cable is a limitation of cable length (due to signal loss and distortion). Including trunk and all spurs added, the expected lengths are 1900, 1200, 400 and 200m for types A, B, C and D.

I doubt if you would be satisfied with type C or D limitations for a new project. Those cables should really only be considered for retrofits, when already installed. I recommend you insist on a better-specified cable for your project.

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