Foundation Fieldbus Communication Protocols

The structure of the fieldbus communication entity is consistent with the concept specified in the ISO's OSI reference model, but there is an important difference in that the structure defined here includes only three layers instead of the seven layers in the ISO's OSI reference model. This architecture is described and illustrated below.

The basic goal of each layer protocol is to provide Virtual Communication Relationships (VCRs, Virtual Communication Relationships). FMS provides VCR application messaging services such as read and write variables. Some device APs do not use FMS, but access FAS directly. SMK, in addition to using FMS services, accesses the data link layer directly when communicating with its SMKP. fas provides messaging services for FMS and APs.

The protocols of the communicating entities are managed by the Network Management Agent NMA. Conceptually, the NMA manages the objects within the protocol through the layer management entity associated with each layer. From an interoperability perspective, how the NMA accesses information is invisible from the network.

1. Network Management Agent

The NMA provides access to the configuration and statistical information of the protocol stack. This information is represented in the Network Management Information Base (NMIB, Network Management Information Base). Some information is defined as writable by the Network Management Specification document and some is defined as read-only. Network-visible, read-only information is read-only from the network. The NMA provides access to three types of information.

Information about the communication stack entity as a whole         

Information about the VCR

Information about each protocol layer

2. Bus Message Specification Layer FMS

The application layer services in FF are defined by the FMS. this layer describes the communication services, message formats, behavior states, etc. required by the application. the FMS provides a set of services and a standard message format. User applications can use this standard format to pass messages to each other on the bus and access AP objects and their object descriptions through the FMS service. the FMS service specifies the services and message formats for accessing AP objects and OD descriptions of these objects. The network-visible entities of an AP and their corresponding OD descriptions are represented in FMS as Virtual Field Devices (VFDs).FMS services are provided to APs at the VCR endpoints.

3. Bus Access Sublayer FAS

There are three types of VCRs: Client/Server VCR, Report Distribution VCR, and Publisher/Subscriber VCR. client/server VCR is a queued, non-scheduled communication, user-triggered, one-to-one communication between devices, This type of VCR is often used to set parameters or implement certain operations, such as changing given values, accessing and adjusting regulator parameters, acknowledging alarms, and uploading and downloading devices; scheduling report subtype VCR is a queuing, non-scheduling communication, user-triggered, one-to-many communication between devices, and this type of VCR is often used to notify the operator console of alarm status, trend data, etc.; publisher/subscriber type VCR is a buffer type. The publisher/subscriber VCR is a buffered, one-to-many communication method. This type of VCR is often used to refresh data for the input and output of user application modules, such as refreshing process variables, operation outputs, etc. in a cyclic scheduling manner.

4. Data link layer DLL

The fieldbus data link layer is a subset of the IEC definition. This subset is selected to support the defined VCR types.

To transfer messages between applications, the DLL provides access to the bus media for SMKP and FAS. The access is controlled by a specific data link layer entity called LAS, which provides two types of access to the bus: scheduled and unscheduled communication.

The DLL is divided into two levels of operation, one to provide access to the bus and one to control the transfer of data between data link users. These two levels of operations are integrated for performance improvement and therefore cannot be considered as separate sub-layers.

a) DLL media access function

Each bus has a centralized media access control point, the LAS, and one or more token-holding devices, the base devices. Each bus on the network is a link. When the network includes more than one link, it is called an Extended Link. The links in an extended link network are connected by a bridge. Therefore, extended link networks are sometimes referred to as bridging networks.

Basic devices are those that can receive tokens and can respond to them. All devices, including LAS and bridges, have the capabilities of basic devices, i.e., they all have the authority to receive tokens (initiate data transfers).

The token grants the device the authority to send on the bus. Only one device holds a token at a given time. the LAS can authorize two types of tokens to the device. The device must return data with address immediately when it receives the first type of token, which is issued by the CD (Compel Data). The second type of token authorizes the device to access the bus at one of the three priority levels for a specified period of time. This kind of token is issued by PT (Pass Token).

CD DLPDUs are used in FF to support publisher/subscriber VCRs. each publisher VCR endpoint publishes its data through the DL-BUFFER. The contents of the buffer are issued as a response to the CD DLPDU. Usually the CD is sent by the LAS to the Data Link Layer address of the publisher's buffer. In this case, the CD is issued periodically by the LAS according to its scheduling.The CD can also be issued by the subscriber VCR endpoint, in which case the CD can be sent only when the device receives a PT DLPDU.

PT DLPDUs are used to send all other data in the device. When a device receives a PT DLPDU, it is authorized to send data to the link for a period of time. This time period is specified in the PT DLPDU.

The device browses the transmission requests sent by the FAS and SMK to its DL Service Access Points (DL-SAPs, DL Service Access Points) and its DL Connection End Points (DLCEPs) to select the type of DLPDU to be sent. DL-SAPs are used to send all SMKP messages and all message release VCR messages using the connectionless DLL service. DL-SAPs are also used to DL-SAPs are also used to send connection establishment messages for the publisher/subscriber VCRs and client/server VCRs services. DLCEPs are used to send all other publisher/subscriber VCRs and client/server VCRs messages.

The selected request to be sent is the earliest, highest priority (greater than or equal to the priority specified in the PT DLPDU) request that can be sent within the remaining holding time of the token.

When the device has no data to send, it explicitly returns the token to the LAS; if it still has requests to send, it will request more token holding time from the LAS.

The link master device is the device that can become the LAS. There are more than one link master device per link.

Bridge devices connect the links into a tree network. The bridge device is always the link master device and must be the LAS, which includes the system management time publisher for each of its downstream links. When the bridge device is not one of these roles, data link time and application clock time cannot be republished.

b) DLL Advanced Functions (Data Transfer Functions)

FF provides three DLL mechanisms for transmitting data: connectionless data transfer and two connection-oriented data transfers. Connectionless data transfer supports message publishing VCRs, one type of connection-oriented data transfer supports publisher/subscriber VCRs, and one type of connection-oriented data transfer supports client/server VCRs.

Connectionless data transfer is a queued transfer of mutually independent DLPDUs between DL-SAP addresses. No context is reserved in the DLL to control the data transfer. This type of data transfer is used for message distribution because the destination address can be configured as a group (multiple) DLSAP addresses, allowing multiple message sources to be sent to the same group of addresses. This also allows the recipient of the message to be insensitive to the identification and location of the message source.

Because all connectionless data transfers are independent, the priority of each DLPDU is specified separately, all for each VCR, only one priority is specified.

Connection-oriented publish data transfer is a buffered data transfer from the publisher. The publisher does not need to know the subscriber's identity and the DT DLPDU to be sent contains only one address, that is, the publisher's address. The subscriber knows the publisher's address and listens to the data sent by the publisher.

When a publisher starts, it broadcasts an Establish Connection EC (Establish Connection) DLPDU to all subscribers on the local link. this EC DLPDU does not require a response. Subscribers perform a similar operation by sending an EC DLPDU with an address to the publisher to request a response. When the publisher receives an EC DLPDU from a subscriber, it issues another EC DLPDU to the local link. the subscriber uses the first EC DLPDU it receives after it issues a request to complete opening its connection. the EC DLPDU it uses may have been issued by the publisher in the first place, or it may have been issued by the publisher as a response to the subscriber's EC DLPDU. After the connection is established, the subscriber starts listening to the data sent by the publisher and ignores the EC DLPDUs sent later by the publisher.

Publish data transfer is started when a CD DLPDU with the publisher's DLCEP address is received by the publisher and a DT DLPDU is sent as its response. If this DLCEP is not yet open, or if the associated buffer has not yet been updated, no DT DLPDU will be sent at this time.

Posting data transfers may be scheduled (triggered by the LAS sending CD DLPDUs) or non-scheduled (triggered by the subscriber sending CD DLPDUs). For a particular connection, all DT DLPDUs are transmitted with the same priority. This transmission may be sequential, allowing data duplication to be monitored. Data duplication occurs when a publisher is unable to update its buffers during a data transfer.

Scheduled publisher data transfers are sent only on this link. When publishers and subscribers are located on different links, separate connections are established between the publisher and the bridge and between the bridge and the subscriber. Therefore, EC DLPDUs sent by the subscriber must be sent to the publisher DLCEP when they are on the same link, or EC DLPDUs sent by the subscriber must be sent to the bridge's republisher DLCEP when they are not on the same link.

Non-scheduled releases are different. In this case, the bridge is configured to forward EC, CD, and DT DLPDUs between the publisher and the subscriber. therefore, there is no republishing at this point.

Connection-oriented request/response data transfer is a queued data transfer between the client and the server. The client VCR endpoint or peer VCR endpoint acts as the initiator and sends an EC DLPDU requesting a connection to the server, and the server response instructs the server AP whether to accept this connection request. Once data transfer is started on this connection, all DT DLPDUs will be transferred with the same priority.

5. Physical Layer

The physical layer of the fieldbus consists of the physical media and the signaling protocol used to transmit the data. All devices include at least one physical layer entity.

The physical layer protocol provides the data link layer with the ability to send and receive data independently of the physical layer media type. The type of physical layer media supported by a device can be any of the types defined in the IEC physical layer specification document.

In FF networks, three transmission rates are used: 31.25kbps, 1Mbps, and 2.5Mbps.