Lowest cost Home Run – Fieldbus or wireless?

Wireless is being proposed as the easiest and lowest cost way to install additional infrastructure between a field device and the control system when the home run cable is ‘full.’ Unfortunately, this is not true since as the title implies another alternative exists and that alternative can be used in any area classification. The solution is Fieldbus. Because fieldbus protocols are wired in parallel and support multiple connections on a single home run (trunk) cable, provided the cable is capable of carrying a fieldbus signal it can be converted from analog to digital communications service while providing ALL the benefits of a wired connection while using open standard technologies proven in use for over a decade. Now to describe how this can be done.

The first step is to confirm that the existing cable will support a fieldbus message by using a paired signal generator and network analyser to put a signal on the network. Relcom Inc. (http://www.relcominc.com/fieldbus/fbtesters.htm) makes a pair of such devices. The FBT-5 is a signal generator while the FBT-6 is a network analysis tool. Because the Fieldbus specification was designed to be able to use existing cable, I have yet to run a test with these tools that did not pass. Once the cable has been confirmed as acceptable, the next requirement is to confirm that your control system supports an H1 I/O card as you will need to install a card in your Host to be able to communicate with the new Fieldbus devices. If your Host does not support H1 directly, there are other alternatives but then things get more complicated with OPC and such starting to get involved but the basic concept described here will still work. Besides if the Host does not support native H1, I imagine it is unlikely to directly support Wireless either.

Fieldbus multiplexing
With a secure way to provide signal transmission and power to the field, the next step is to select one of the existing transmitters that can be converted to a Fieldbus device as it will be replaced as part of the conversion. The new devices will also be Fieldbus, though if you do not have confidence in the technology and wish you could replace additional existing monitor only devices and then use analog signals for the new control loop.

The only remaining steps are to insert your Fieldbus Power Conditioner in the interface room between your host and the field cable, land the field cable on one side and the H1 card on the other. A Field Device Coupler (preconfigured terminal block that is wired in parallel) is then added to the terminal strip in the field Junction box and the new fieldbus devices are then connected to this new set of terminals.

Because Fieldbus is able to support a minimum of 3 devices in the IS environment, and up to 16 devices in other installation factors you can expect to be able to increase the capacity of your existing WIRED infrastructure 5 – 10 times using this simple process. The end result is as shown  in the figure.

End result, fully integrated digital transmitters complete with diagnostics, power and compliant with a widely supported international standard.

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