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Bernd Schuessler

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How to Calculate Fieldbus Cable Voltage Drop

Posted by Bernd Schuessler on Fri, Mar 08, 2013

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.

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Topics: Cables/ Cordsets, Software, Fieldbus

Are FOUNDATION fieldbus and PROFIBUS PA the same thing?

Posted by Bernd Schuessler on Tue, Jan 29, 2013

The short answer is: although both “buses” are very similar, they are not the same!
Here is what’s different: PROFIBUS is the overall name for a process fieldbus solution that includes the application and industry-specific PROFIBUS DP, PROFIBUS PA, PROFINET, and PROFIsafe. In the process industry, we see mostly PROFIBUS DP and PROFIBUS PA.
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PROFIBUS DP (decentralized peripheral) is the RS485-based high-speed (up to 12 Mbit/s) solution for connecting remote I/O systems, MCCs (motor control centers), or similar externally powered devices to a PLC or DCS control system.

PROFIBUS PA (process automation) is defined in the IEC 61158-2 standard and used predominantly in process plants. It is suitable for general purpose and hazardous locations and delivers power (up to 32 V) and communications (31.25 kBit/s) on the same cable (bus powered) to PROFIBUS PA instruments such as flow meters and temperature or pressure transmitters.

Typically, PROFIBUS PA applications are connected to a PLC/DCS control system via a PROFIBUS DP network, which requires a DP/PA Segment Coupler. These coupler devices are necessary to connect the two different physical layers of the high-speed, non-bus-powered DP to the lower-speed, bus-powered PA. The Segment Coupler includes the power supply modules needed to condition 24 VDC bulk power for use on PROFIBUS PA. PROFIBUS PA does not support CIF (control in the field), where instruments can talk to each other even if the connection to the host is lost.

Just like PROFIBUS PA, FOUNDATION fieldbus is based on the IEC 61158-2 standard. This means that both networks are based on the same physical layer and used in similar industries and for the same types of application. The physical layer defines the cable type (Type A fieldbus cable with specific R, L, and C values, twisted shielded pair) as well as the min/max power levels (9 V … 32 V) and the transmission rate (31.25 kBit/s). Since PROFIBUS PA and FOUNDATION fieldbus are based on the same physical layer, the same type of field distribution devices like Segment Protectors and FieldBarriers can be used for both networks.

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Topics: Ask an Expert, Fieldbus

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