Foundation Fieldbus Cables and Wiring
Foundation Fieldbus is a Local Area Network widely used for process control. It replaces traditional "point-to-point" wiring with shared wiring for powering devices and carrying signals between devices. Because each device on the network uses the same physical wiring, less wire needs to be used, and upgrading and adding to the system can be easily done.
The Foundation Fieldbus specification covers two physical network types: High Speed Ethernet for communicating large amounts of
maintenance and diagnostic data and H1 for communicating control information. There are a few rules associated with H1 systems:
- a maximum of 32 field devices may be installed in a single segment,
- the maximum cable length of the entire system can not exceed 1,900 meters (without incorporating repeaters),
- the maximum drop (spur) length is 120 meters.
The Foundation Fieldbus specification defines and recommends a particular cable type to be used for an H1 network line. This is referred to as Type A cable. Three alternate cable types are also defined, with the intent of allowing the use of existing cable runs when retrofitting or upgrading existing installations to Foundation Fieldbus H1.
The four cable types are discussed below.
Type A Fieldbus Cable
Fieldbus Type A cable uses 18 AWG twisted-pair wires. A twisted-pair is used, rather than a pair of parallel wires, to reduce external noise from getting onto the wires. A shield over the twisted-pair further reduces noise.
Type A cable has the following characteristics:
- Wire size (minimum) 18 AWG (0.8 mm),
- Shield 90% coverage,
- Attenuation 3 dB/km at 39 kHz,
- Characteristic Impedance 100 Ohms +/-20% at 31.25 kHz,
- Maximum network length 1,900m.
Type B Fieldbus Cable
Type B cable is multiple shielded pairs of 22 AWG wire. This cable type also has a characteristic impedance of 100 Ω at 31.25 kbps, but a higher electrical resistance due to the smaller wire size. The maximum network length is 1,200 meters when using Type B cable.
Although the cable has multiple pairs, fieldbus only uses one of them to transmit its power and receive communication.
Type C Fieldbus Cable
Type C cable is multiple unshielded pairs of 26 AWG wire. The maximum network length is further reduced when using this cable to 400 meters. This cable is generally not preferred for fieldbus use because it is not shielded. Again, only one of the pairs is needed for the fieldbus network.
Type D Fieldbus Cable
Type D cable is multicore unshielded and untwisted 16 AWG wires. The maximum network length is drastically reduced to 200 meters when using this cable.
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