What are the basic differences, advantages and disadvantages, between 4ma-20ma system and Foundation fieldbus?
The difference in the technologies is that 4-20mA uses current loops to communicate to / from field devices whereby each instrument or valve positioner requires a pair of wires connected to the control system. This allows only the measured variable or the control signal to be transmitted (unless HART devices are used) which represented by a milliamp value.
Fieldbus consists of a series of networked field devices (usually up to 12 devices per segment) which communicate serially over a 31.25kHz bus, this allows devices to communicate data between each other and to / from the host control system using a single pair of wires. Data is not limited to the measure variable but also includes diagnostics data, status, alarms, and manufacturers data. Also the devices themselves can perform control functions such as PID control.
4-20mA has the following advantages: simple to fault find, reliable, proven technology, can be used for safety related applications, simple intrinsic safety application, wide range of instruments available, doesn’t require further training of maintenance or installation personnel.
The disadvantages of 4-20mA are: large homerun cables to control buildings, limited data can be transmitted, larger cabinet footprint, control must always be in the control system
Fieldbus has the following advantages: many instruments connected to single trunk cable (much less cabling), smaller cabinet footprint, capable of transmitting diagnostic data, control (e.g. PID) can be performed in the field devices which lessens the effort of the control system, measured variable is more accurate as it isn’t converted to a current loop…the disadvantages are: requires further training, limitations in intrinsic safety applications, not really accepted for safety systems, devices require interoperabilty testing with host system, extra design effort required (segment calculations, additional configuration data required for control system)
The main difference I have noticed is that homerun cables are much smaller and easier to install and take up less space in the control buildings. The additional design effort required is minimal and all instrument and control system vendors offer a wide range of compatible equipment. Many large end users have now accepted fieldbus as their standard requirement.
Author - Jim Hughes