Main Differences with 4..20 mA technology:

Energy available

Connection topologies

Functionality

Configuration

Energy available

1. The 4-20 mA analog signal is a "communication" system  standard for industrial instrumentation
2.  The 4-20 mA current loop represents the transmitter measurement signal
3. The current loop itself also powers the transmitter,
4. The measurement signal is valid between 3.8 and 20.5 mA while, in the event of an anomaly, the signal goes to the selected alarm threshold of  3.6 mA or more than 22.0 mA
5. The most critical condition is when the instrument goes into alarm at 3.6 mA because the instrument must continue to operate    with less than this current.
6. This reduced availability of energy does not allow 4 ... 20 mA instruments to perform complex calculations and functionalities that would be feasible if there was more energy available.

note:

Communication and modulation with a current di 9ma like the photo below:

Connection topologies

See photo below :

Distinctive features of FF

Function Blocks

1. Distributed control in the field LAS function (Link Active Scheduler)
2. The types of communications used by FF
3. Macrocycle and Determinism The DD and CFF files

"Blocks" structure given in FF instruments

FF instruments contain many variables and information distributed / mapped in different "Blocks"

The FF instruments  have at least 3 "Blocks"

Resource Block

Contains data describing the general characteristics of the device:

1. Revisions
2. Serial number

Transducer Block

Connection between the physical system (process) and the computer world (conversions)

1. Acquisition and conversion of the process measurement in engineering format
2. Limits and Calibration of the sensor

Function Blocks

Input (Analog Input) for transmitters or Output (Analog Output) for Actuators or control valve which are directly connected to the relative Transducer block from which they receive the measurements in engineering format, and apply the appropriate scales required for the specific process control.

Note: In addition there may be many other Function Blocks

Function Blocks

What are they and what do they do?

The function Blocks are software "modules" that:

1. receive input measures or variables ...
2. they elaborate them through formulas / calculation algorithms
3. produce the output results

See photo below:

Function Blocks .... Which and how many they are?

The function Blocks ... .. How to use them?

The Function Blocks can reside in the (traditional) Controller and / or in field instruments such as Transmitters or Positioners

1. The control Function Blocks contained in an instrument can also be used by other instruments connected on the same segment / network
2. By linking several Function Blocks to each other (link), a control strategy is designed
3. If the Function Blocks are resident in the instruments, a distributed control in the field

Typical Applications

Function Blocks ... ..in traditional use

Function Blocks ... in the FF solution

Function Blocks….Controllo di CascataConvenzionale

Function Blocks .... Cascade Control with FF

Function Blocks..What does it change for the instruments?

1. The FF tools are not evaluated only for their traditional measurement function but also for their own calculation capacity.
2. If you want to achieve distributed control, you mustselect tools that contain number & typesFunction Blocks necessary for this purpose.
3. It is also necessary to check their 'Execution Time'to understand if they are adequate to control performancerequests

LAS (Link Active Scheduler) What is it?

The LAS function is the coordinator of an FF network

1. Its function is also called "BUS Arbitrator" because one of its tasks is to manage the traffic of communications that must exist between instruments and between instruments and DCS
2. It can be imagined as that of the "conductor" that coordinates the "orchestral" (field instruments) that they must follow a "score" (Macrocycle)

LAS (Link Active Scheduler) ... .. What is it for?

1. Self-detection of instruments connected on the bus (live List)
2. Automatic assignment of the network address and search with TAG
3. Scheduled and Unscheduled communications management
4. Time Distribution (Synchronization between instruments)

 Note: The LAS function in limited mode can be developed also in field instruments (Backup LAS)

LAS (Link Active Scheduler) ... .. The types of Communications

Unscheduled (asynchronous)

• Client Server (one to one)

1. Used for Configuration / Maintenance / Monitoring tool messages
2. Transmitted only upon intervention / request by the operator

•  Report Distribution (one to many)

1. Used for Event / Alarm Notification and for Trend transmission.
2. Transmitted from instrument with active alarms only when the LAS passes the Token

Scheduled (Sincrone)

• Publisher Subscriber (one to many)

1. Used for the publication / exchange of variables between Function Blocks connected "link" to each other
2. Exclusively dedicated to the realization of Process Control

The Macrocycle

Deterministic System

H1 - redundancy of the "LAS"

1. FF distinguishes 2 types of instruments: "Link Master" and "Basic"
2. Only "Link Master" tools support the Backup-LAS function

H1 - redundancy of the "LAS"

Multimaster system

1. The FF distinguishes 2 types of instruments: "Link Master" and "Basic"
2. Only "Link Master" tools support the Backup-LAS function

Device Description (DD)

The Device Description is an extended description of each object / parameter within the instrument, to be made available through the bus for readings / writes

The DD allows each Host FF to recognize and use, automatically, the corresponding Field Device

for example: It should be considered as the driver of a printer

Capability File (CFF)

The Capability File describes the resources of the tool type:

1. Number and types of Function Blocks
2. Function Blocks execution time (Execution Time)
3. Default setting of the parameters / objects of the Function blocks ....

Advantages of Fieldbus

Analog / Digital communication

What are the differences between a communicationanalogue and digital in process automation?

Summary of the comparison

• The 4 ... 20 mA output signal, including the  D to A converter will disappear
• The function of the I / O Card that converts the 4 ... 20 mA signal in digital format into the transmitter itself ...... The measurement is transmitted in engineering format (ex: 27.5 ° C or 300.45 mbar)
• It is not essential to set a measurement or output range in when the fieldbus instrument measures and transmits in digital format everything that is within the physical limits of the sensor ... the setting of the output field are:

1. If you want to read the measurement as a percentage of the measuring range
2. If the measurement is derived, for example a flow measured with a dP
3. As simple information for the user or the DCS but has no effect on the calculations
4. Multidrop topology, a twisted pair for multiple instruments

• More energy for instruments if we compare with the 4-20 mA current loops.

1. More information available for the Monitoring and  Diagnostics – control

• Measurement available as a real value expressed in the selected unit of measurement and in addition a validation of the value itself (Status Byte)

1. The measurements are composed of 5 Bytes (Input / Output)
2. The 5 Bytes are composed of 4 Bytes floating point and 1 Byte of Quality Status

BAD + Detail

UNCERTAIN + Detail

GOOD + Detail

• Integration between Local Devices / Control System
• Better accuracy due to the absence of A / D and D / A conversions
• Less update / propagation delay of information due to fewer elements between the instrument and the DCS
• More Flexibility

Summary of advantages Reduction of:

1. Connection cables and cabinets
2. S. Barriers
3. Input / Output Converters
4. Power Supplies and protection
5. Space for control rooms of the plant

Comparative control tests in the field and traditional

Control in the Field Provides Superior Reaction to Deterministic Disturbance in the Process

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