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WorldFIP Protocol
Physical Connections
Producer-Consumer Model
Coding the Data Frame
WorldFIP Interoperability Guides
A Single Protocol
In plain words this means that, no matter what you intend to use your fieldbus for, there is only one protocol to learn. Some other bus systems require two or even three different protocols to cover the range of tasks, which makes them harder to learn as well as requiring extra interfaces and conversions within the system.
A Simple Protocol
WorldFIP protocol is also simple to learn. There are no lengthy vocabularies of specialised terms, and maximum use is made of existing International Standard formats such as the Manufacturing Message Specification (MMS). You can confidently expect that any work you carry out with WorldFIP will align very easily to full International Fieldbus standards when they ultimately emerge.
In this module, we'll work through the basic terms that you will meet when configuring a WorldFIP network, and learn the ground-rules for building and linking networks.
Overview of Key Parameters
Physical Layer
- Bus topology to IEC 1158-2.
- Data rates of 31.25kbit/sec, 1 Mbit/sec or 2.5 Mbit/sec via twisted pair copper cable, overall screen.
- Data rate of 5 Mbit/sec via optical fibre.
- 64 nodes per cable segment, up to 4 segments through repeaters.
- Segment length: 1 Km or more depending on data rate, cable and number of nodes.
- Redundant cable as an option.
Data Link Layer
- Uses "producer-consumer" model (described later in this module) with centralised bus-scheduler (redundant bus-schedulers can be used if required).
- Supports time-critical "variables" and "messages" (described later in this module).
- Time-critical "variables" can be "cyclic" or "event" variables.
- Up to 64000 message identifiers. Up to 64000 variable identifiers. (A few identifiers are reserved).
- Size of variables from 2 bytes to 127 bytes. Message size up to 256 bytes.
Application Layer
- Variables accessible through MPS interface.
- Messages accessible via sub-MMS interface. Maximum size of message 64kBytes using sub-MMS.
Physical Connections
The WorldFIP cable system consists of a Trunk Cable with Drop Cables connecting to each Node. The drop cables are connected to the trunk by Tap Boxes.A node may be connected to its drop cable in either of two ways:
- Sub-miniature 9-pin D-type connector, for use in benign environments.
- Circular connector, for harsh environments.
The trunk cable requires a terminating impedance at each end; this is physically small, and for convenience is normally included in the tap boxes of the nodes located at the ends of the trunk cable.
The network may be extended by a combination of Junction Boxes and Repeaters, to achieve the desired cable layout.
Basic Connection Rules
Up to 64 devices can be linked on a single 1Mb/sec WorldFIP fieldbus cable segment, which runs in "trunk + spurs" form, with spurs of up to 0.5m and maximum recommended total segment length of 750m. Up to 4 repeaters may be used to extend the trunk, giving a maximum end-to-end length of 3.75km for the network. A repeater counts as one device on each of the segments which it links.Recommended maximum lengths for the other operating speeds (31.25kb/sec and 2.5Mb/sec) are shown in the diagram on right. The overall configuration rules for these speeds are exactly the same as described above for a 1Mb/sec network.
Cables and Tap Boxes
Minimum cable requirement is a single twisted pair plus screen, with characteristic impedance 150 ohms. Cable with 2 separately-screened pairs may also be useful (the second pair may be used either as a duplicate bus or to distribute power). The main trunk requires termination, to match cable impedance and prevent unwanted signal reflections. The simplest terminator is a 150 ohm resistor connected between the conductors; more sophisticated termination may be used, for example, to balance the bus to ground or provide DC blocking.Connecting Devices
Each device is connected in parallel between the two conductors. The wires connecting the device to the trunk form a Spur -- usually connected to the trunk cable via a Tap Box. The 0.5m spur length is sufficient for local connection within an equipment enclosure; the main trunk is arranged to visit each enclosure.For field-mounted equipment it is more appropriate to loop out the main trunk to each device, from a tap box on the main trunk. For loop connections of this type the recommended cable type has two twisted pairs and a single overall screen. These loop connections must be included in the calculation of overall trunk length.
Tap boxes are available which support looping of the trunk and switched isolation of the loop during maintenance or fault conditions. Connector options include bayonet style and screened 9-pin "D" type.
Producer-Consumer Model
WorldFIP's producer/consumer model is central to the effectiveness of WorldFIP's protocol.Why Producer/Consumer?
Many networks are based upon protocols developed for long-distance communications. They were developed to transmit messages from point to point, with built-in re-try mechanisms to cope with the high risk of data corruption on public message routes.Though well-suited to their original application, such protocols are quite unsuitable for time-critical data-transfer between devices on a fieldbus, since:
- The possibility of re-try prevents a deterministic response.
- Data must be re-sent to each node which requires it.
The "producer/consumer" model gives deterministic high-speed response, by allowing the data produced by one node to be consumed at the same time by any or all other nodes on the network.
How it Works
The "producer" node does not need to know which nodes are consuming the data. The Bus Arbiter function controls the "producers" rather like the conductor of an orchestra, so transmission of data at known time intervals (equivalent to the conductor's "beat") is guaranteed. Data integrity is guaranteed by status flags, which show the "timeliness" and the "freshness" of data. There is no re-transmission in the event of corrupted data, the next value simply overwrites the previous value."The "producer/consumer" mechanism is very effective for time-critical data and event data. It allows easy construction of a distributed real-time database in a WorldFIP-connected system.
As a simple example, imagine a PLC using WorldFIP cyclic data to communicate with its distributed I/O system.
You may add a PC into that network, set up to consume all the data that is already passing; the PC is effectively invisible to the PLC. The PC might be supporting SCADA or other packages, and thus be able to provide links into the information networks onthat site, all without requiring any change to the program in the PLC and with no effect on the data-traffic across the fieldbus.
Coding the Data Frame
Coding a WorldFIP Frame
All WorldFIP frames (frame_question, response, message, etc.) are composed of three parts:- Frame Start Sequence (FSS)
- Control and Data field(CAD)
- Frame End Sequence (FES)
Frame Start Sequence (FSS)
This contains two fields:- Preamble (PRE)
This series of 8 "1" bits is used by receivers to synchronize themselves with the transmitter's clock. - Frame Start Delimiter (FSD)
This series of bits indicates to the data link layer the beginning of useful information (CAD).
Control and Data Field
This field contains only the logical information ("0" and "1") from the data link layer.Frame End Sequence (FES)
This series of bits is used by the data link layer as a delimiter, to locate the end of the CAD field.The User May Define the Data Field
The above description explains the way in which each frame is encoded for secure transmission. This is interesting for a product developer -- but it is largely "invisible" to a user, who is far more interested in the information which he can send and receive via the frames.The Control & Data Field (CAD) has three parts:
- Control Field: Contains control and address information used by the protocol
- Data Field: Contains the user data
- Check Field: Contains a 16-bit CRC computed and checked by the protocol
All the "real-life" data is transmitted within the Data field defined above, and the user (or his system builder) can define exactly how the Data field is built up, according to the needs of his specific system. The definition of the contents of the Data field is part of the system-builder's task when he builds the system.
The Application Layer of WorldFIP provides both time-critical and non-critical (or messaging) services.
Time-critical services are managed via WorldFIP's distributed database (functionally identical with those in the draft IEC Standard). Non-critical services are provided by a sub-set of the Manufacturing Message Specification (MMS).
Time-Critical Data
Time-critical data (eg control-loop variables) are passed around the network using "MPS" real-time periodic/aperiodic services.
Cyclic variables are sent to the network using the periodic services. The repetition rate may be fixed (down to 2msec) or free-running. Such variables always have absolute priority, and are typically used to transmit data used in fast speed-control loops etc.
Event variables are sent only if requested, using the aperiodic service -- for instance, to respond to a change of state (eg alarm) or a demand for an operator display. Use of event variables is very appropriate in systems where change is normally infrequent but there are occasional cascades of alarms, such as gas/water/electricity industries.
Non-Critical Data
Less time-dependent data (such as diagnostic reports), is sent via the message service. This may be typically used for:
- Installing & setting up an application
- Network supervision and diagnosis
- Integration with higher-level systems.
How It All Works
Network time is split into "macro-cycles" -- usually based on the update time of the slowest periodic variable in the system. These are further divided into "elementary cycles" each of the same length (min. 2 msec).Data is allocated to the cycles in the following priority order:
- Cyclic variables
- Event variables
- Messages (non critical)
Unlike token-passing systems, WorldFIP leaves no doubts about transmission time and regularity for cyclic data variables.
For more information, download the WorldFIP Protocol document from our download page
WorldFIP Interoperability Guides
In order to have interoperable products from different vendors, it is necessary to define in generic terms how the WorldFIP Protocol is used by each different main class of products (eg Drives, I/O, Simple Sensors, etc). This is achieved by creating "Companion Standards".
Guides Available WorldFIP Interoperability Guides include:
- Profiles:
- General Rules
- Profile 1
- Profile 2
- Profile 3
- Companions Standards:
- Temperature transmitters
- Differential pressure sensors (see Newsletter Iss: 9 for an example)
- Absolute and Gauge pressure sensors
- Variable speed drives (available shortly) (see Newsletter Iss: 10 for an example)
- Motor control centres
- I/O multiplexers (see Newsletter Iss: 11 for an example)
- On/Off actuators (available shortly)
What is a Companion Standard?
It is a specification which allows vendors to produce equipment that will work together with equipment from other vendors (interoperability). It has a communications part, defining the communications profile and data fields used, and an applications part, defining the structure and meaning of the application information.The WorldFIP Approach
WorldFIP's Interoperability Guides are the basis of the WorldFIP approach. They define the general structure of a companion standard, the key common elements and the permitted communication profiles. The Companion Standards for each class of product then detail the remaining information specific to each product category.Application Function
The application function is expressed using function blocks. These blocks produce application variables and connect to the process via process interface variables. The application variables are made available to the network through exchange blocks. Exchange blocks can be used to group application variables together into larger communications variables for efficient data transmission.Communication Profiles
The Interoperability Guides define four permitted communication profiles, of increasing complexity:
| Profile 1: Plug & Play |
Little data and no configuration.
(examples: simple sensor, bar code reader.) |
| Profile 2: Simple Equipment type A |
The equipment has a few simple parameters and data is normally exchanged cyclically, with the possibility of aperiodic exchange.
(examples: simple I/O rack, simple MCC.) |
| Profile 3: Simple Equipment type B |
The equipment is configurable. The data exchanged is both periodic and aperiodic, with the possibility of many aperiodic variables at any time (alarm avalanche).
(examples: I/O racks, variable speed drive, actuator, complex sensor.) |
| Profile 4: Complex Equipment |
The equipment can be configured and downloaded. There is a large amount of data of all types.
(examples: Complex I/O racks, large variable speed drives, PLCs...) |