FOUNDATION FIELDBUS
The real intention behind this article is to make the reader read another wonderful article given at the end. A few revision questions are given which can be attempted before and after reading the said article to gain insight into this technology.
Foundation Fieldbus is a great technology. It stands right at the top of the pyramid of evolution of process control devices achieving many check marks for features desired over the years for process control devices. Foundation fieldbus started as an organization of leading vendors of industry who wanted to take the control out of control room relying on the processing power built right into the field devices thus unburdening the DCS controllers in control room and saving their cycle time. In fact, with the right configuration, the DCS could be disconnected from the field devices and process control would go on as usual.
Devices subscribing to the standards of Fieldbus foundation were marked by a special F denoting interoperability with other fieldbus devices and controller cards of various other OEMs. The devices communicated to each other within the loop using a H1 network which used Manchester encoded signals. Ethernet was the network of choice for communication between the DCS controllers and the operator and engineering workstations.
Before FF devices, the field transmitters, be it Pressure, Temperature, Level and Flow would send a proportional current w.r.t to the process variable being measured. The lower end of this current was 4 ma denoting zero process variable whereas 20 ma was full or SPAN of the process variable. These transmitters were connected to electronic controllers or Analog input cards of a DCS system. The PID algorithms executed inside the electronic controllers or DCS.
The next major improvement came about by way of Profibus devices where the Analog input and Analog output block was confined to the field devices and a digital data stream was communicated to the Profibus controller situated in the control room. However, the control block like PID was still inside the controller and not in the field device.
Foundation Fieldbus took the next step in which the control blocks like PID could also be accommodated inside the field device thus unburdening the DCS controller from block processing. FF offered the following obvious advantages:-
(1) Instead of a dedicated cable for each transmitter and Final control element (like control valve) which needed to run between the device and control room in case of Analog devices, many FF devices could be connected to a single trunk control cable coming from the control room, this simplified the cabling layout.
(2) The process control algorithms could be executed in the field devices thus unburdening the DCS controller in control room.
(3) Rather than just the process value, the status of the value (whether good, bad etc) along with engineering units can be communicated.
Some disadvantages:-
(1) Commissioning requires attention to proper cabling practices and shielding from noise sources and if proper type (A,B,C,D) cable is not used or not properly terminated, lot of intermittent issues can be faced.
(2) Requires proper device descriptor files (DD) to be pre installed in the DCS for DCS to recognize and commission the device.
An extract from this book presents a very helpful description of FF concepts. It is written in simple and easy to understand language. The whole book is an open source project by its author and presents very useful insights.
Some questions to test understanding of FF technology, mostly pertaining to data link layer :-
1. what is FUN & NUN?
2. For macro cycle time < 1 second, how many devices are recommended within the segment and how many final control elements are recommended?
3. what is size of FF identifier or device address?
4. what does LAS stand for and its use?
5. what are scheduled and unscheduled transfers?
6. What is the function of CD & PT tokens?
7. What kind of values are necessarily transferred using the CD tokens?
8. what is backup LAS? how many LAS can be active in a segment at one time?
9. what are TD tokens?
10. What is a 'live list'? and who maintains it?
11. what is a PN token?
12. Who retains the transmission right in a segment? How is the transmission right transferred to a device?
13. Is publisher/subscriber or BNU another name for CD token?
14. Is Client/Server or QUB a type of PT token?
15. Is Source/Sink or QUU or 'Report Distribution' a type of PT token?
16. what is the recommended % time for scheduled communications vis a vis unscheduled communications in a H1 segment?
17. What happens to a device if it does not use or return PT tokens(unscheduled comms)? after repeated attempts does it remain on live list?
18. In such a case of Q17, how can you reduce the count of scheduled transfers (CD tokens) by moving function blocks between devices to reduce CD tokens?
19. What do you understand by a VCR?
20. Is flow control applied in client/server tokens?
21. Is flow control applied in Source/Sink tokens?
22. How does Queuing take place in server(FF device)? and when and how does server dole out all the responses to queued messages?
23. What kind of values are transferred using client/server PT tokens?
24. What kind of values are transferred using Source/Sink PT tokens?
25. What are three types of FF devices in increasing order of capability?
26. All FF devices contain one Resource block. What is specified in the resource block?
27. What are the function block modes in FF?
28.What is the basic difference between manual and OOS mode? Why OOS mode is preferred during calibration?
29. what is .sym and .ffo file? which file is human readable?
30. what is a .cff file?
31. What is the approx range of resistance of a FF cable disconnected at both ends between + and - , + & shield, - & shield, shield & ground? (This is useful for quick test on cable)
32. What is the voltage range in mV for a good FF signal?
All these questions can be answered by going through the extract of pdf file given above. I framed these questions on purpose as all of us have varying retention issues :-) .Anytime, one can come back to the questions to see how much we are retaining about the topic.