When designing FOUNDATION™ fieldbus or PROFIBUS PA networks, it is extremely important to make sure that the devices on those networks have enough power to start up properly and operate normally.
The fieldbus physical layer specification IEC 61158-1 defines the min/max voltage levels for FOUNDATION fieldbus and PROFIBUS PA. The minimum voltage required for any fieldbus device is 9 V and the maximum voltage allowed on a fieldbus network is 32 V.
To calculate the voltage drop along a fieldbus cable, you must know the number of connected instruments and their current consumption, as well as the resistance of the cable, cable length, and power supply voltage. Typical fieldbus devices consume 15 mA to 20 mA. The exact number can be found on instrument data sheets. A standard “type A” fieldbus cable has a typical resistance of 44 Ohm/Km and a typical fieldbus power supply supplies 30 V.
Knowing the total current draw, supply voltage, cable resistance, and cable distance of a segment, and applying Ohm’s law, allows you to deliver maximum distance and/or voltage drop at a certain distance.
To simplify these calculations, companies like Pepperl+Fuchs have created free-of-charge segment design tools like Segment Checker.
These tools eliminate the need to manually perform these calculations. Instead, you can build the network layout by dragging and dropping components from the left into the design window.
After you enter trunk distances, spur distances, and instrument current consumption, the tool automatically checks to see if the segment can function at the specified parameters. If there are errors, the tool indicates exactly what they are so that you can correct them.