The Profibus guideline (http://www.profibus.com/download/installation-guide/) recommends the shield be grounded at cabinet entry points which is rather unusual since the shield is typically grounded at one and only one end of the cable to prevent ground loops. To reduce the effects of ground loops, a large gauge equalizing cable is recommended to be installed between the cabinets to ensure the system is at the same earth ground potential.
Profibus uses a variation of the RS-485 standard and the RS-485 standard requires that a third wire be used to provide a return path if the earth return path is not available. Isolated Profibus circuits such as the ISO1176 provides full galvanic isolation so the earth return path is not available unless the bus ground is tied directly to earth which defeats the purpose of the galvanic isolation. Additionally the typical Profibus cable (Belden 3109A) does not provide a third conductor for the common reference so the shield must be used if it is indeed required. And typical Profibus connectors, such as the Phoenix Contact SUBCON-PLUS-PROFIB/PG/SC2 - 2708245, does not provide a terminal to terminate a third wire for a common reference. Only the A and B conductors are terminated inside the D-SUB and the shield is tied to the casing via compression.
Does Profibus require a third conductor? Is the third conductor supposed to be via the shield since only two conductors are provided in typical Profibus cables? Doesn't grounding the shield to earth at multiple locations defeat the purpose of the galvanic isolation? If possible, please elaborate and provide references to all questions. Thanks in advance.
References:
http://www.profibus.com/download/installation-guide/
www.robustdc.com/.../index.php
From the RS-485 Standard
"Proper operation of the generator and receiver circuits requires the presence of a signal return path between the circuit grounds of the equipment at each end of the interconnection. The circuit reference may be estab-lished by a third conductor connecting the common leads of devices, or it may be established by connections in each using equipment to an earth reference."