How is power and data extracted from the Foundation Fieldbus?

[BEAMer]

Hello,

I am an Electronics & Instrumentation Engineering student and I am trying to understand the basics of Foundation Fieldbus and its implementation from an electronics stand-point. I understand that the bus carries data in the form of time-varying signals and also the bus is capable of powering field instruments. How is this achieved? I mean, I would like to know how the data is extracted from the bus and how is it filtered to power the device.

Thank you for your time. Any form of response will be helpful to me in getting a better understanding of the technology.

BEAMer

[Mike ONeill]

Hi BEAMer,

Normally we aren't so reticent about responding to questions like this. I'll start the reply going so that the usual crowd can come in with corrections and clarifications.

The 2-wire bus has DC power injected into it via a power supply, typically 27-28VDC (FF spec. max 32V). Up to 32 devices sit between those wires and each takes the current that it needs (typically 10-25mA) to drive its internal circuits, processors, etc. Cable resistance causes volt-drop along the bus cable due to the total current flowing (10 devices might need 200mA), and part of the design process is to make sure that at least 9V (FF min. operating spec) is left at the device connection.

On top of the DC component, the devices can transmit and receive a series of pulses at 31.25kHz. At any one time, only one device is allowed to transmit and the rest can listen. Transmit permission comes in the form of a software 'token' which is received and then passed on to the next active address based on a 'list' held by a 'master' (Link Active Scheduler). Receiving is basically a high-impedance state which can be used to generate the digital signals within the device, transmission is similar except the impedance is varied to add/subtract 10mA or so to/from the steady state current.

The DC power supply is prevented from interfering with the communications signal by an inline filter ('conditioner').

The bus is prevented from ringing/suffering undue reflections by a 'terminator' at each end. The terminator is a load resistor & capacitor, where the resistor is chosen to be 100R to suit the bus communication dynamics and the capacitor stops the resistor shorting out the DC power.

At the DCS end, there is a special device(s) called a H1 card/linking module which converts/stores/manages the FF signals into other formats which might be proprietary or open depending upon that DCS design.

[agupta]

The digital communication signal is above 1 kHz. The power is DC and can have some ripple at 50 - 120 Hz. This frequency separation is used to separate power from the communication signal. A high pass filter is used to extract the communication signal. There is additional low pass filter at approx. 40 kHz to filter the noise.
The field device uses DC current sink to extract the power from the Bus. The current source is necessary, so that the device has high input impedance at the digital communication frequencies.

[marisg]

For an explanation about power, signalling and other Fieldbus lore, see www.relcominc.com (http://www.relcominc.com technical)under technical information/fieldbus wiring guide.