Foundation Fieldbus Total Cable Length, Cable Type, Length of Branch Cables

This article is about Foundation Fieldbus Total Cable Length requirement, Cable Type used, Length of Branch Cables and number of branch cable can be used.

Foundation Fieldbus Total Cable Length, Cable Type, Length of Branch Cables

Fieldbus Cable Type and Total Cable Length

Another factor that affects the decision of the number of Fieldbus devices in a segment is the type of Fieldbus cable and the total cable length. The total cable length is the sum of the length of cable from the ALF111 module to the terminators of the terminal blocks, and the length of branch cable from the terminal blocks to the Fieldbus devices.

The total cable length vary depending on the connected Fieldbus devices and the cable type.

Types of Cable and Total Cable Length

• Type A cable, which has the lowest signal attenuation rate, is recommended.
• If type B cables are to be used, the following guidance should be observed.

In order to ensure sufficient signal amplitude, the total number of connected Fieldbus devices should be kept at 20 units or less and the total cable length to 600 m or less.
Alternatively, the total number of Fieldbus devices should be kept at 10 units or less and the total cable length to 1200 m or less.
• If type D cables are to be used, the following guideline should be observed.

Use up to 2 pairs of cables for the Fieldbus and prevent mutual interference by using remote pairs. If multi-conductor cables are used, use only Fieldbus signals or analog signals (including hybrid communication) in the same cable.

Number of Branch Cables and Length of Branch Cable in Foundation Fieldbus

In case of branch wiring, the total length of the branch cables is restricted according to the number of branch cables (the number of connected fieldbus devices) illustrated as below. The number of connected fieldbus devices includes the ALF111 module; however, if the ALF111 module is directly connected to the trunk cable instead of a branch cable, the ALF111 will not be included in the number of connected fieldbus devices.

Example of Multi-Drop Connection

Maximum Total Length of Type A Branch Cables (Reference Value Based on IEC and ISA standards)

The number of connected units on bus may be limited by conditions such as power supply limitations and communication
performance.

In general, the total length of branch cables should be limited within the range of the reference length regulated in the IEC and ISA standards. However, in the actual practice of wiring, the total length of branch cables may exceed the regulated range without hampering the communication performance. Thus, referenced with the IEC and ISA standards, Yokogawa provides the following recommendations that allow the wiring of CENTUM system to have longer total length of branch cables. The details are as follows:

• Use Type A cables for fieldbus wiring
• Number of Connected Devices : 12 (Actual number of field devices per segment)
• Maximum Length of Each Branch Cable: 120 m
• Maximum Total Length of All Branch Cables: 1440 m
• Maximum Length of Trunk Cable: 1900 m – (Total Length of All Branch Cables)

No restrictions on the number of terminal boards used for the wiring of the branch cables.