Down times included in the price … On the test bench: a PROFIBUS cable clone from the Far East

Users of PROFIBUS cables manufactured in the Far East may be taking an expensive risk.

LEONI Special Cables GmbH (LSC) is in possession of a sample from a Chinese manufacturer. At first glance, the product makes a good impression. However, although visually perfect, the cable reveals striking performance deficits which could lead not only to interruptions in production, but also to the complete failure of entire plants. The clone from the Far East fails to meet the requirements set out by PNO, the PROFIBUS user organization, in both its transmission and its thermal and mechanical properties.

Wave attenuation for example exceeds the specified limits by a clear margin, and its behaviour becomes completely undefined at increasing lay lengths (see diagram). Reflections are the cause of the attenuation peaks in the insertion loss characteristic. Reflections are attributable to the fact that at 135 Ohm, the impedance of the sample lies substantially below the desired value of 150 Ohm, to which the measurement system is adapted. The values for attenuation proper (without reflections) would however also be substantially higher than the limits stated by the PNO.

In practice, transmission rates of 1.5 MBit/s are generally required at lay lengths of up to 400 m. These values can be achieved with high-quality cables. As the lay length increases, however, so too do the requirements upon the quality of the cable. Were the sample tested to be used, errors would arise in data transmission and consequently interruptions in production, since the useful lay length is severely limited.

The extent to which numerous parameters influence the quality of a manufactured cable, and also influence each other, can be seen from the shield impedance, which is too high by 30 %. This impacts negatively upon the coupling impedance, and increases the cable's sensitivity to electromagnetic interference. The sensitivity to electromagnetic interference is further increased by the wholly unchecked coupling capacitance, which at >9000 pF/km exceeds the permissible maximum value of  1500 pF/km by over 500 %. In addition, a whole range of further parameters measured on the Chinese sample lie substantially outside the PNO limits: the loop resistance, which exceeds the limit by 15 %, is a sign of inadequate wire thickness and/or poor copper quality.

To sum up: the price per metre is only one criterion for product selection. The “inner qualities”, which are a function of experience and production expertise, are much more important.

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