During the long lasting lifetime of a process plant many parts of the plant are exchanged, updated or replaced including field instrumentation and control systems to keep the plant operational and competitive. However, both control rooms and central cabling often re-main unchanged and at their primary locations. Consequently, in case of further plant expansions, plant operators will finally face the situation, where the trunk cable capacity (which has been planned years before) will reach its limit with only a few free wires left. The only way to overcome this situation is to use fieldbus technology such as PROFIBUS. But conventional multi-pole trunk cables are not suited for fieldbus communication, which requires special shielded and twisted cables and trained stuff for its installation. This however causes high installation costs and a longer production stop.
A modem/coupler based solution for using conventional trunk cable wires for fieldbus communication has been presented recently: The fieldbus signals are converted into robust SHDSL (Single Pair High Speed Digital Subscriber Line) communication signals at one end of a free trunk cable wire line and reconverted back to fieldbus signals at the other end of the trunk cable line by using special modems and couplers. This communication over the trunk cable wire is fully transparent and invisible for the PROFIBUS master. Therefore, the integration of the field devices into the master through GSD or DTM files is not required.
Two alternatives exist to apply this concept in reality: (a) to install a remote I/O system in the field and a PROFIBUS master in the control room or (b) to collect the signals in the field using a multiplexer and to switch them to free I/O cards in the control room.
The solution as described has already proved its suitability over years in a number of process plants of the chemical industry in Germany.
The solution offers advantages and benefits to all plant owners and operators who plan to expand their conventional plant with fieldbus systems without investing much money in the installation of a fieldbus cabling system.
Major user benefits are
• Low installation costs by using a free pair of wires of the existing trunk cable
• A minimal production stop due to short installation and commissioning times
• Broad application range due to optimal transmission quality up to 20 km
• Future-proof due to expandability of the integrated fieldbus solution