A FF is a control system digital, two-way, multi drop communication link among intelligent devices and control devices. It uses voltage level for communication; timing and connections for each function define by protocol.
A segment can have 4 to 16 devices.
For any communication protocol explanation we have to consider OSI (Open System Interconnection) model of ISO, this gives us a general architecture of network specification,
OSI model has 7 layers
1) PHYSICAL LAYER- SAYS ABOUT ELECTRICAL CONNECTIONS
2) DATA LAYER SAYS ABOUT DATA TRANSFER FOR ADJACENT NETWORK
3) NETWORK LAYER SAYS ABOUT ENCAPSULATION OF DATA INTO PACKETS FOR TRANSMISSIONS AND RECEPTION
4) TRANSPORT LAYER SAYS ABT DATA HANDLING OVER ENTIRE NETWORK
5) SESSION LAYER SAYS DATA ORGANISING START AND END OF SPECIFIC TRANSMISSION
6) PRESENTATION LAYER DEFINES CHARACTER SETS AND CONTROL AND GRAPHIC CONTROLS
7) APPLICATION/SOFTWARE LAYER SAYS ABOUT ACTUAL SOFTWARE PROGRAMS
Foundation field bus uses the layer 1 physical layer, 2 data layer and 7 software or application layer.
So let us explain one by one
A} Layer 1: (Physical layer)
The Physical Layer receives messages from the communication stack and converts the messages
into physical signals on the field bus transmission medium and vice-versa.
In FF a number of transmitters are combined along a two wire twisted pair cable to form a group referred to as segment or network. A FF is made up of segments which are the smallest unit of a FF having all the characteristics of a FF network.
A simple network of segment of FF is as shown below
HOST which has DCS/PLC
This first component of a segment is the control system interface module here H1/ HSE host whose function is to control the segment and ensure communication.
TRUNK is the main cable which runs along the entire segment. Is similar to a tree which supports all branches.
FF Power supply The FF offers power (with redundancy) and communication on the same twisted pair cable. The communication rectangular signal is superimposed on this power supply. Typical output ranges are13 to 30 V and 110mA to 500mA.Higher voltage allows longer cable and higher current allows more devices.
JUNCTION BOX: There must be some JB between Trunk and filed devices Some of the JB types are a)device couplers, b)field barriers and c)segment protectors
This is the basic terminal block device coupler having connections for trunk in, trunk out, and one field spur. This has no short circuit current limitation.
b) Segment Protector. This option have short circuit current protection module and have various spurs ranging from 4 to 12 spurs.SC protection ensures that a short on one spur don’t affect other spur
c). Field Barrier: It is used for high power trunk concept having galvanic isolators. The purpose of field barrier is to keep the intrinsic safety barrier as far as possible into the field and still provide the most power to the network
5) SPUR: A spur is a wire connecting field devices to the main trunk line.
6) DEVICES: The field device the end measuring instrument used in the process. The process variable collected by the field device are transmitted on the bus for use in the controlling the overall process.
Every segment requires two terminators to operate properly. The terminators are equivalent to 1micro F and 100 ohm resistance in series. The terminators are placed at the segment as shown below
The terminators serve as shunt for field bus current and protect against reflections.
The individual bus devices on a segment can communication is called topology and the topology available for FF are
a)Star b)Line/bus c) Tree or trunk and spur technology
a)Star topology connects all devices directly to control room.
This structure uses most cabling and is rarely used in FF
In line topology the network wiring is symbolized as single line between two devices. A twisted pair of wire connects all nodes with each other using a line. This structure is frequently used in FF
c)Tree or Trunk and spur technology
The tree structure topology is made upon line topology .The trunk line is run from control room to JB in the field. Each device is connected through spur connection to the device coupler. In a distributed architecture a trunk line goes out to a single JB and all spurs branch out from this enclosure. The Single JB may contain one or more device coupler to accommodate the desired number of spurs.
In a highly distributed architecture, the trunk is linked to multiple junction boxes. The boxes are small and placed near to field devices .Each JB contains one device coupler or segment protector
The technology used to communication between the devices and host is Manchester Encoding.
A rectangular alternating voltage is used for communication and is modulated onto DC signal for the power supply of the device. The DC power supply is of 31 V and each device works on a minimum of 9V Power must be available to all network stations. Message transfer is performed by an alternating current by with a current intensity of 9mA at a frequency of 31.25 kHz. In Manchester encoding the transition across the bit times zero line – that is the DC supply voltage level-are analyzed to find a 1 or 0.A positive transition is considered as 0 and a negative transition is assumed as 1
B} DATA LAYER
The data link layer controls the data onto the field bus. The data link layer manages the access to the field bus through a centralized scheduler. The communication of FF is based on publisher/subscriber or otherwise called peer to peer communication model. In this type only one node has the authorization to transmit a signal on the bus at any given time. After the transmission is complete, the next node receives the token transmit device communication. The token is passed on a circular pattern where each node is prompted one after another to send its data. This active device broadcasts the relevant process variable information to the entire network. This network publication allows all other interested devices to receive the process data simultaneously. These interested listening devices are the subscribers on the network.
The process of determining the which network device is the publisher is accomplished with a defined scheduled of communication in the DCS/PLC.The H1 host which contains the segment schedule ensures that each device has a scheduled amount of time to transmit its data. The device is prompted to transmit necessary data when the host sends a specific signal to the device. This signal indicates that receiving device is now the active publisher on the network.
Three type of devices are defined in data link layer namely,
a) Basic devices
b) Link Master Devices
c) Linking bridge devices
a) Basic devices is the device class that incorporates field instrument for the segment.
b) Linking master devices class contains Link Active Scheduler (LAS) capabilities The LAS is the traffic officer for the H1 segment. The LAS is responsible for traffic monitoring on the segment and for keeping the communication on schedule. Basic device don’t have the capability to become a LAS.
The LAS is responsible for probing every available address for new devices and for ensuring known devices are still active. It compiles the execution schedule for instruments process data transmission during scheduled communication. A LAS device sends scheduled “Compel Data” CD to query devices on network for time critical information. Once a device has completed transmission, it returns the token to LAS
The Link Active Scheduler (LAS) has a list of transmit times for all data buffers in all devices that need to be cyclically transmitted.
When it is time for a device to send a buffer, the LAS issues a Compel Data (CD) message to the device.
Upon receipt of the CD, the device broadcasts or “publishes” the data in the buffer to all devices on the field bus. Any device that is configured to receive the data is called a “subscriber”.
All of the devices on the field bus are given a chance to send “unscheduled” messages between transmissions of scheduled messages.
The LAS grants permission to a device to use the field bus by issuing a pass token (PT) message to the device. When the device receives the PT, it is allowed to send messages until it has finished or until the “maximum token hold time” has expired, whichever is the shorter time
Typically LAS is a part of DCS H1 CARD
c)Linking / Bridge devices: This allows the connection of the H1 devices to other high level networks such as HSE (High Speed Ethernet)
C} APPLICATION LAYER OR SOFTWARE LAYER (LAYER 7):
The Field bus Foundation has defined a standard User Application based on “Blocks.” Blocks are representations of different types of application functions
The types of blocks used in a User Application are
The Resource Block describes characteristics of the field bus device such as the device name, manufacturer, and serial number. There is only one resource block in a device.
Function Blocks (FB) provide the control system behaviour. The input and output parameters of
Function Blocks can be linked over the field bus. The execution of each Function Block is precisely scheduled.
There can be many function blocks in a single User
The Field bus Foundation has defined sets of standard Function Blocks.
Function blocks can be built into field bus devices as needed to achieve the desired device functionality. For example, a simple temperature transmitter may contain an Al function block. A control valve might contain a PID function block as well as the expected AO block.
The following additional objects are defined in the User Application:
Link Objects define the links between Function Block inputs and outputs internal to the device and across the field bus network.
Trend Objects allow local trending of function block parameters for access by hosts or other devices.
Alert Objects allow reporting of alarms and events on the field bus. View Objects are predefined groupings of block parameter sets that can be used by the human/machine interface. The function block specification defines four views for each type of block.
A detailed explanation of application layer is beyond the scope of this article.