FOUNDATION FIELDBUS (3)

KEY DESIGN POINTS

1 - Total Length of Cable

Total length of cable including branch cable

is limited. Length is depended on type of cable as follows:

• Twisted pair and individual/overall shielded #18 AWG – 1900 m

• Single or multi-twisted pair with an overall shield #22 AWG - 1200m

• Twisted pair without shield # 26 AWG - 400m

• Multi-core, without twisted pairs, without shield # 16 AWG – 200m

• Multi-core, without twisted pairs, without shield # 16 AWG – 200m

2 - Control Cycle

- Conventional system: Instrument signal is totally parallel. So, number of instrument does

not affect on transmission speed.

• Transmitting of Analog signal is individual one after another.

• →A parallel transmission

- Fieldbus System: Instrument signal is in a series. So, number of instrument affects on

transmission speed.

• In case LOOP#1 and #2 connect up one segment, the control cycle is twice.

• →A series transmission

Transmission speed should be taken into the design, especially for control loop where speed is critical.Transmitting of Analog signal is transmission individual one after another.

3 - Power Consumption

- Number of instruments connected to one main cable are limited because power supply to

all instruments is provided through one main cable.

- Proper communication requires that field device have enough voltage. Each device should

have minimum 9 volts.

4 - Cost

Total cost introducing Fieldbus compared with Conventional System should be checked.

The factors to be checked are:

Cost of cable ---- Reduced

• Cost of main cable way ---- Reduced

• Cost of main cable installation ---- Reduced

• Cost of instrument system and instrument ---Increased

5 - Conclusion

Foundation Fieldbus System requires close attention and more study especially on wiring topology, cable length, control cycle time, power requirements and limitations. All of these areas have significant impact on project implementation, reliability and cost.