field comm

Course 9050 - October 1996 Principles of Instrumentation and Control

Field Communications 3 - 1

FIELD COMMUNICATIONS

Think with the wise buttalk with the vulgar

- Greek Proverb

Principles of Instrumentation and Control Course 9050 - October 1996

3 - 2 Field Communications

SynopsisA definition of "signal" leads into an analysis and comparison of pneumatic, analogelectrical and digital protocols.

In analog electrical signals, the importance of power supply and the relative merits ofa current/voltage standard are considered.

Digital communications' standards are viewed from the perspective of pure and hybrid(smart) systems. Comparisons are made between competing network topologies for afuture Field Standard.

v v v v v v vv v v v v v vv v v v v v vv v v v v v vv v v v v v v

SignalInformation in the form of a pneumatic pressure, an electric current, or mechanical position thatcarries from one control loop component to another.

Pneumatic Signals

The basic pneumatic mechanism converts a small motion or force into an equivalent (proportional)pneumatic signal. Most systems use 3-15 psig/20-100 kPa span which corresponds to 0-100% ofscale.

Using the traditional flapper nozzle control element, it may be seen that the scale movement is notlinear (see Fig. 3.1). Electronics can correct for this non-linearity.

Fig. 3.1



Pneumatic SignallingAdvantages

1. Sufficient power levels for positive valveactuation.

2. Safe in Hazardous areas.

Disadvantages

1. The longer the run of tubing the moresignificant the errors due to lag andtemperature.

2. Dirty air can clog orifices in those sensorsusing traditional technologies.

In common practice, a 0-100% input signal to a current to pressure transducer can be calibrated toa 1-17 psig (7-115 kPa) output to ensure firm closing and complete opening of valves.Greater non-standard pressures (500 kPa say) are used to reduce Actuator sizing and save money.

Electrical Signals - AnalogIn the past, competition for the Analog field standard was between 0-20 mA, 10-50 mA and 4-20mA.

The latter emerged as the preferred option. In practice it is better to have a 'live zero' input to adevice. Otherwise a zero milliamps input could be ambiguously related to either zero output or acomplete loss of signal due to power supply failure. In the standard, 4 mA corresponds to zeroinput and 0 mA to complete loss.

Using the Analog standard gives rise to considerations best appreciated in the case of 2 wiretransmitters in series with a power supply and various control room indicators.

Fig. 3.2 Wiring Connection



The transmitter acts as a variable load. To enable the meter to register the full 4-20 mA deflection,the combined effects of the transmitter and control room instrumentation's load can't exceed a valuedetermined from the table below.

In the example shown, the combined load cannot exceed about 800 ohm for a 30 VDC supply.

In reverse, a given load demands a minimum driving voltage. If the power supply should fall belowthis minimum, the effects on output are shown below.

It can be seen to be in someways preferable to have a VOLTAGE field standard. In this way indicatorswould be in parallel with the instrument. However, current based standards are less noise susceptibleand are not subject to voltage drop on long cable runs.

Fig. 3.3 Load Limitations

Fig. 3.4



Electrical Signals - DigitalThe latest move in increased communications effectiveness is to employ digital communications inpure or hybrid systems.

In general, no universally accepted standards exist and many groups are in existence to complicatedeliberations.

MAP - Manufacturing Automation Protocol. A specification being developed by GeneralMotors for standardizing communications on the factory floor.

MPII - MAP Process Industry Initiate. Coordinates process industry activities andrequirements within the MAP specification.

TOP - Technical and Office Protocol. A specification being developed by Boeing ComputerServices for standardizing communications between information systems.

Mini-MAP - A specification that is a subset of the full MAP specification for standardizing realtimecommunication networks.

ISO - International Standards Organization. Responsible for developing the model thatthe communication specifications are based upon as well as standards for each layerof the MAP and TOP communication specification.

IEEE - Institute of Electrical and Electronics Engineers. Formed the IEEE 802 Project fordefining standards for network media and access methods.

SP72 - Instrument Society of America, Standards & Practice committee Number 72.Developing EIA1393 companion standards for process control messaging.

SP50 - Instrument Society of America, Standards & Practice Committee Number 50.Developing standards for digital communication between field devices (i.e.transmitters and control equipment).

MAP/TOP Users Group -

An independent Organization made up of users and vendors of MAP based equipmentthat are recommending changes to the MAP and TOP specifications to make themore acceptable industry wide.

F.I.P - Factory Information Protocol. Approved French National Standard.

Profibus - Process Fieldbus. Approved German National Standard.

Fieldbus consortium of manufacturers and end users developing an international digitalFoundation - communication system.

*World FIP - A U.S./French conglomerate of instrument manufacturers.

*ISP - Interoperable Systems Project. a multinational group of some 80 manufacturersspanning U.S./Europe and Japan.

(* now merged to form the Fieldbus Foundation.)



Pure Digital SystemsThe OSI (Open Systems Interconnections) Model is used to delineate the various parts of the digitalstandard.

LAYERS

7 Application Provides Services to User Applications

6 Presentation Restructures data to/from standard format

5 Session Synchronizes & Manages data

4 Transport Provides reliable data transfer

3 Network Performs routing functions between nodes

2 Data Link Manages access control

1 Physical Physically transfers bits

This model modularises the components of the networking hardware and software based onfunctionality. Each module takes the form of a layer in the model and is responsible for providingservices to the layer above. A service is an abstract capability provided at the boundary betweenany two layers of the model. Services are provided by either hardware, software, or through theservices available from the layer below.

FieldbusWith the merger of World FIP and ISP into Fieldbus Foundation, progress towards an internationalagreed standard of digital communication took a leap forward and Beta testing of software is nowcomplete. Full plant trials are now under analysis.

Hybrid SystemIn these systems, 2-wire transmitters are employed. The Analogue signal is so modulated that itcarries digital control communication simultaneously with analog process measurement.

Such a system is called "SMART".

A 'SMART' instrument employs simultaneous analogue and digital communications

The standard of SMART instrument is the HART (Highway Addressable Remote Transducer)Protocol, and is supported by more than 75 manufacturers.



Features:

1. The Analogue signal can drive local devices, such as remote indicators, whilst re-ranging(say) is accomplished from the Control Room.

2. These devices may accommodate 'multiple masters' for example a control system, hand-heldcommunicator, process computer, or in some cases, a combination of the three.

Fig. 3.6 Smart transmitter linked to control.

Fig. 3.5 Two basic signallingmechanisms.



However no common standard yet exists for apurely digital system. The more instrumentson the loop, the slower it is and the singlecommunications link leaves the systemvulnerable to disruption if it is severed.

4. A vast amount of information can flow between the device and the Control System eg.Process Variable, Transmitter Status, Configuration Data, Device physical layout, maintenance,construction information, temperature compensation data, alarm units, diagnostics and loopchecks.

Communication can be managed remotely.

Fig 3.7a Typical point-to-point link.

Fig 3.7b Multi-drop or bus network.

3. Multi-dropping" is possible with digital communications

Fig 3.8a Leased telephonecommunications.

Fig. 3.8b Radiocommunications.

No reranging requiredCommunicate transmitter statusRemote diagnostics(Digital communications only)

No RTU requiredNo reranging requiredCommunicate transmitter statusRemote diagnosticsTranmitters powered locally

Analog or digital process valueautomatic reranging based onanalog output.Commicate transmitter statusRemote diagnostics.



Fig. 3.9 Bus topology

Fig. 3.10 Ring topology

TopologiesThere are several versions of topologies that may be considered for a fieldbus network.

1. The Bus Topology shown lends itself to a wide separation of devices because each drop onthe network is independent of the rest. Devices are not required to complete communicationsto other devices.

2. The ring topology shown provides two potential communication paths, increasing the reliabilityof the network. Each device acts as a repeater, so a fiber optic implementation is easilyaccomplished. This version is some what more expensive because of the more complexnature of the electronics involved.

Design atrributes: Efficient wiring for wide separation of devices. Potentialfor simpler protocols. Single device failure cannotcause a catastrophic network failure.

Design liabilities: Greater installed cost due to the number of taps.Spacing and length of drop are critical. Maintenancecomplexity. Up-front engineering to plan for deviceconnection.

Design attributes: Potential for simple point-to-point connections. Lends itself tofiber optics. Enhanced reliability.

Design liabilities: Increased planningfor wiring. Increased hardware for complexity.Each device is active and/or a repeater of information.



The tree topology shown is an alternate version of the bus configuration. The junction box can beeither active or passive and is consistent with current wiring practices. It also allows field wiring tobe done independently of the "home-run" cabling from the junction box back to the controller.

Integrated SystemsWith the advent of more powerful computers (PCs) and software systems (Windows NT),manufacturers are starting to combine process control with maintenance, engineering andmanagement systems to give an integrated approach to instrumentation.

Such a system enables an operator or maintenance engineer to plug a portable notebook computeranywhere in the system loop and obtain information on the whole plant system, both currentinformation and plant history; check maintenance programs and obtain a variety of reports.

Such a system is very flexible and easily expandable and when working under Windows environment,easy for all to understand and communicate with.

Fig. 3.11 Tree topology

Design attributes: Uses current wiring practices. Power is easily appliedfrom the network. A single failure cannot cause acatastrophic network failure.

Design liabilities: Communications are sensitive to the number of drops.Loss of trunk cable causes loss of network.



Case Study

IntroductionThe HART (Highway Addressable Remote Transducer) Communicator is the hand-held interfacethat provides a common communication link to all HART-compatible, microprocessor-based in-struments. This case study discusses the HART Communicator display, keypad, connections, bat-tery pack, and maintenance.

The HART Communicator interfaces with any HART instrument from any wiring terminationpoint in a 4-20 mA loop provided a minimum load resistance of 250 ohms is present between thecommunicator and power supply. The HART Communicator uses the Bell 202 frequency shiftkeying (FSK) technique. This technique uses high frequency digital communication signals super-imposed on the standard 4-20 mA transmitter current loop. Because the net energy added to theloop is zero, communication does not disturb the 4-20 mA signal.

Liquid Crystal DisplayThe LCD is an eight-line by twenty-one-character display that provides communication betweenyou and the connected device. When connected to a HART-compatible device, the top line of eachon-line menu displays the model name of the device and its tag. In addition, the bottom line of eachmenu is reserved for a dynamic label for each software-defined Function key, F1-F4 (found di-rectly below the display).

Fig. 3.12 The HART communicator

LCD

Function Keys

Action Keys

AlphanumericKeys

Shift Keys



The dynamic labels reflect available functions. For example, the label HELP appears above the Flkey when access to on-line help is available.

The Action KeysAs shown in Figure 3.12, the Action keys are the six keys located above the Alphanumeric keys.The function of each key is described as follows:

On/Off Key

Use this key to turn the HART Communicator on and off. When the communicator is turnedon, it searches for a HART-compatible device on the 4-20 mA loop. If a device is not found,then the communicator displays the message, No Device Found. Press OK. Press OK

(F4) to display the Main menu.If a HART-compatible device is found, the communicator displays the Online menu. See yourdevice manual for more information on the options available in this menu

Disabled Off Key

When performing certain operations, the message OFF KEY DISABLED indicates that the HARTCommunicator is unable to be powered off. This feature helps to avoid situations where the HARTCommunicator could be unintentionally powered off while a devices output is fixed or when con-figuration data has not been sent to a device.

Up Arrow Key

Use this key to move the cursor up through a menu and to scroll through lists of available charactersand options when editing a field.

Down Arrow Key

Use this key to move the cursor down through a menu and to scroll through lists of availablecharacters and options when editing a field.

Left Arrow and Previous Menu Key

Use this dual-function key to move the cursor to the left or to back out of a menu.

Right Arrow and Select Key

Use this dual-function key to move the cursor to the right or to select menu options.

Hot Key

Use this key to quickly access important, user-defined options when connected to a HART-compat-ible device. Pressing the Hot Key turns the HART Communicator on and displays the Hot keymenu.

Software-defined Function KeysUse the four software-defined Function keys, marked Fl through F4, located below the LCD to

F1 F2 F3 F4



perform software functions as indicated by the dynamic labels. On any given menu, the labelappearing above a Function key indicates that keys function for the current menu. As you movebetween menus, different function key labels appear over the four keys. For example, in menusproviding access to on-line help, the HELP label may appear above the Fl key. In menus providingaccess to the Home menu, the HOME label may appear above the F3 key. Table 1 describes thelabels that appear above each function key throughout the menus.

Alphanumeric and Shift KeyThe Alphanumeric keys perform two functions: the fast selection of menu options and data entry.

Selecting Options Using the Fast Select Feature

From within any menu, you can select avail-able options in two ways: You can use the Upand Down Arrow keys and the Select key toselect available options, or use the Fast Selectfeature. Just press the corresponding numberon the alphanumeric keypad to select the de-sired option. Figure 2, shows how to quicklyaccess the Utility menu from within the Mainmenu by pressing the indicated key.

Table 3.1 Function Key Labels.

Fig. 3.13 Quickly Accessing Menus.



Data Entry

Some menus require data entry. Use the Alphanumeric and Shift keys to enter all alphanumericinformation into the HART Communicator. If you press an Alphanumeric key alone from withinan edit menu, the bold character in the center of the key appears. These large characters include thenumbers zero through nine, the decimal point (.) and the dash symbol (-).Example: To enter one of the letters or other symbols appearing above a large bold character on anAlphanumeric key, first press and release the corresponding Shift key then press the alphanumerickey. Do not press these keys simultaneously, but one after the other. For example, to enter the letterR press the following key sequence:

Pressing the right shift key activates shift and causes the right shift arrow icon to appear in theupper right corner of the display. Pressing the Shift key again deactivates shift. With shift acti-vated, pressing the 6 key causes an R to appear in the editable field.

Hart Communicator Connections

Explosions can result in death or serious injury. Do not make connectionsto the serial port or NiCad recharger jack in an explosive atmosphere.

The HART Communicator can interface with a transmitter from the control room, the instrumentsite, or any wiring termination point in the loop via the rear connection panel as shown in Fig. 3.14.To communicate, connect the HART Communicator in parallel with the instrument or load resistor.The connections are non-polarized.

Battery Charger Jack(R) option only)

Serial PortLeadset Jacks

Fig. 3.14 HART Communicator rearconnection panel with optional battery



Fig. 3.15 illustrates the wiring connections for the communicator and any HART compatible in-strument. Fig. 3.16 illustrates the same connection as Fig. 3.15 using the optional 250 ohm loadresistor.

Explosions can result in death or serious injury. Before connecting theHART Communicator in an explosive atmosphere, make sure the instru-ments in the loop are installed in accordance with intrinsically safe ornonincendive field wiring practices.

NOTE

For the HART Communicator to function properly, a minimum of 250 ohms resistance must bepresent in the loop. The HART Communicator does not measure loop current directly.

Fig. 3.16 Connecting the HART Communicatorwith the Optional Load Resistor.

Fig. 3.15 Connecting the HARTCommunicator to a Transmitter Loop.



Servicing the HART CommunicatorAs shown in Fig. 3.17, the modular construction of the HART Communicator allows easy disas-sembly of the battery pack and the memory module. This section discusses how to change alkalinebatteries, recharge the NiCad battery pack, and upgrade the communicator software.

Battery Pack

The HART Communicator is available with a battery pack that holds five AA alkaline cells, or witha rechargeable nickel-cadmium power pack. The alkaline cells last approximately 150 hours, andthe NiCad cells last approximately 60 hours with normal usage. When approximately one hour ofbattery life remains a low-battery icon appears in the upper right corner of the display.

Changing Alkaline Batteries

Explosions can result in death or serious injury. Do not remove or replacebattery pack in an explosive atmosphere

Refer to Fig. 3.18 and use the following steps to change alkaline batteries:

Fig. 3.18 Battery Pack Removal.

Figure 3.17 HART Communicator Exploded View.



1. Completely loosen the three captive screws holding the communicaton battery pack.

2. Grasp the battery pack and pull it away from the communicator. Make sure not to bend thepins connecting the battery pack to the communicator.

3. Remove and properly dispose of batteries.

4. Referring to the battery orientation diagram on the inside of the battery pack, insert fiveAA alkaline batteries.

5. Carefully align the pins with the communicator base and replace the battery pack.

6. Tighten the three screws.

7. Turn on the communicator to ensure that the batteries are installed properly.

Replacing the Memory Module

Explosions can result in death or serious injury. Do not remove or replacebattery pack in an explosive atmosphere.

Refer to Fig. 3.19 and use the following steps to replace the Memory Module:

1. Completely loosen the three captive screws holding the communicator battery pack,

2. Grasp the battery pack and pull it away from the communicator. Make sure not to bend thepins connecting the battery pack to the communicator.

3. Loosen the two captive screws holding the Memory Module.

4. Grasp the Memory Module and pull it away from the communicator.

5. Align the new Memory Module with the communicator and tighten the two screws.

6. Replace the battery pack.

Fig. 3.19 Memory Module Removal.



Functional SpecificationsMEMORYNonvolatile memory retains contents when the HART Communicator is not Powered.Program (and Device Descriptions): 1.25 MB.Transmitter data: 2 KB.

POWER SUPPLYFive AA 1.5 V batteries (rechargeable Nickel-Cadmium battery pack optional).BATTERY CHARGER OPTIONS110/120 V ac, 60 Hz, US plug.220/230 V ac, 50 Hz, European plug. 220/230 V ac, 50 Hz, UK plug.

MICROPROCESSORS32-bit Motorola type 68331.8-bit Motorola type 68HC05.

CONNECTIONSLeadset: Two 4 mm banana plugs.Battery charger: 2.5 mm jack.Serial port: PC connection through optional adapter.Memory Module: 26 pin, 0 1 -in. Berg connector.

Performance SpecificationsTEMPERATURE LIMITSOperating Limits32 to 1220F (0 to 500C).Storage Limits-4 to 1580F (-20 to 700C)Humidity LimitsOperates in 0-95% relative humidity under noncondensing conditions below 1040F (400C) withouterror.

HAZARDOUS LOCATIONS CERTIFICATIONSBaseefa/CenelecFactory MutualCanadian Standards Association

Physical SpecificationsDISPLAY8-line liquid crystal display with 21-character line width (128 by 64 pixels).KEYPAD25 large keys including six Action keys, a complete alphanumeric keypad, four software definedfunction keys, ON/OFF, and cursor control keys; membrane design with tactile feedback.



WEIGHT Approximately 3 lb (1.4 kg) including batteries.

Ordering InformationTable 3.2 lists possible options as either standard or optional selections. The symbol indicatesselections that make up the model number 275D9E.

STANDARD SELECTIONS

Table 3.2 Ordering Information.



Fig. 3.20 HART Communicator Spare Parts.

Table 3.3 Spare Parts.

Spare PartsRefer to Table 3.3 and Fig. 20 to choose spare parts for the HART Communicator.



DiscussionWhat are the pros and cons of moving to plant-wide Smart instrumentation?

What practical implications arise in the maintenance of a Plant-wide, purely digital network?

TestQ. 1 What is the pneumatic signal field standard?

Ans. ___________________________________________________________________

Q. 2 What is the analog electrical signal field standard?Ans. ___________________________________________________________________

Q. 3 Explain "live zero"Ans. ___________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

Q. 4 Explain "Fieldbus"Ans. ___________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

Q. 5 Define a "Smart" InstrumentAns. ___________________________________________________________________

_______________________________________________________________________

Q. 6 Explain "OSI Seven Layer" ModelAns. ___________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

Q. 7 Describe a "Bus" NetworkAns. ___________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________



Q. 8 Describe a "Ring" NetworkAns. ___________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

Q. 9 Describe "HART""Ans. ___________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

Third Printing: October 1996Second printing: July 1993First printed: October 1991

SynopsisSignalPneumatic SignallingElectrical Signals - AnalogElectrical Signals - DigitalPure Digital SystemsFieldbusHybrid SystemTopologiesIntegrated Systems

Case StudyIntroductionLiquid Crystal DisplayThe Action KeysAlphanumeric and Shift KeyHart Communicator ConnectionsServicing the HART CommunicatorChanging Alkaline BatteriesReplacing the Memory ModuleFunctional SpecificationsPerformance SpecificationsOrdering InformationSpare PartsDiscussionTest

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