Digital communication for field devices...

Digital Digital CommunicationCommunication

for Field Devices for Field Devices and Data Acquisition and Data Acquisition

NIDHIN MANOHARNIDHIN MANOHAR

Senior Research EngineerSenior Research Engineer

• Digital communication? Digital communication?

• What is serial communication?What is serial communication?

• Fieldbus?Fieldbus?

• What has Wireless got to do What has Wireless got to do

with field-devices?with field-devices?

We shall see…We shall see…

IEEE-488 (Parallel Data)IEEE-488 (Parallel Data)• Interface comprises 8 data lines, 8 control lines Interface comprises 8 data lines, 8 control lines

and 8 ground lines. and 8 ground lines. • Up to 15 devices can be interconnected on one Up to 15 devices can be interconnected on one

bus. bus. • Each device assigned a unique primary address, Each device assigned a unique primary address,

ranging from 4-30, by setting the address switches ranging from 4-30, by setting the address switches on the device. on the device.

• Devices are linked in either a daisy-chain or star Devices are linked in either a daisy-chain or star (or some combination) configuration (or some combination) configuration

• up to 20 m of shielded 24-conductor cable. up to 20 m of shielded 24-conductor cable. • A maximum separation of 4 m is specified A maximum separation of 4 m is specified

between any two devices, and an average of 2m between any two devices, and an average of 2m over the entire bus. over the entire bus.

• Data transfer at 1 MbpsData transfer at 1 Mbps

Digital serial communicationDigital serial communication• Introduced with earliest Digital ComputersIntroduced with earliest Digital Computers• Communicate between Central Computer Communicate between Central Computer

and peripheral devicesand peripheral devices• Mainframe and distributed terminalsMainframe and distributed terminals• Initially, specified for Distances 15-20 feet, Initially, specified for Distances 15-20 feet,

upto 50feetupto 50feet• interface between Data Terminal Equipment interface between Data Terminal Equipment

(DTE) and Data Communications Equipment (DTE) and Data Communications Equipment (DCE) (DCE)

RS-232CRS-232C• Electronic Industries Association (EIA) Electronic Industries Association (EIA) • send and receive bytes of information one send and receive bytes of information one

bit at a time.bit at a time.• Earlier, used to be slow (Compared to Earlier, used to be slow (Compared to

parallel data communication)parallel data communication)• But is simpler and good for longer distances But is simpler and good for longer distances • Distances > 1km (with repeaters OR Distances > 1km (with repeaters OR

Modems) Modems) • baud rate, data bits, stop bits, and paritybaud rate, data bits, stop bits, and parity

RS-232C: What Is Handshaking ?RS-232C: What Is Handshaking ?• for the data to be transmitted, both sides for the data to be transmitted, both sides

must be clocking the data at the same must be clocking the data at the same baud rate. baud rate.

• In many applications, problems due to In many applications, problems due to overloading receivers. overloading receivers.

• handshaking helps in asynchronous handshaking helps in asynchronous communication. communication.

• Three most popular handshaking are:Three most popular handshaking are:– software handshaking, software handshaking, – hardware handshaking, and hardware handshaking, and – Xmodem. Xmodem.

Serial RS-232CSerial RS-232C

RS-422, RS-423RS-422, RS-423

• serial connection used on Apple Macintosh serial connection used on Apple Macintosh computers. computers.

• uses differential signal, as opposed to the uses differential signal, as opposed to the unbalanced signals referenced to ground unbalanced signals referenced to ground with RS232. with RS232.

• two lines each for transmit and receive two lines each for transmit and receive signals, signals,

• results in greater noise immunity and results in greater noise immunity and longer distances as compared to RS232. longer distances as compared to RS232.

• advantageous for industrial applications advantageous for industrial applications

RS-485RS-485• improvement over RS422improvement over RS422• number of devices from 10 to 32number of devices from 10 to 32• defines electrical characteristics to ensure defines electrical characteristics to ensure

adequate signal voltages under maximum adequate signal voltages under maximum load. load.

• enhanced multidrop capability to network enhanced multidrop capability to network of devices to a single RS485 serial portof devices to a single RS485 serial port

• RS485 is a superset of RS422; RS485 is a superset of RS422; • all RS422 devices may be controlled by all RS422 devices may be controlled by

RS485 RS485

Firewire IEEE-1394Firewire IEEE-1394• Also popular as Firewire. Featured on Also popular as Firewire. Featured on

laptopslaptops• use of bus cable to power devices.use of bus cable to power devices.• Firewire transmits data in packets and Firewire transmits data in packets and

incurs some overhead as a result. incurs some overhead as a result. • Devices when used in Devices when used in isochronous modeisochronous mode, ,

guarantees a time slot for data transfer in guarantees a time slot for data transfer in every frame. every frame.

• Firewire frames are 125 msec longFirewire frames are 125 msec long• transfer speed of 400 Mbit/s but may result transfer speed of 400 Mbit/s but may result

in substantially slower responses to in substantially slower responses to instruments' service requests. instruments' service requests.

Firewire IEEE-1394Firewire IEEE-1394• uses a peer-peer protocol, similar to IEEE-488. uses a peer-peer protocol, similar to IEEE-488. • standard cable has maximum length bus standard cable has maximum length bus

comprises 16 hops of 4.5m each. comprises 16 hops of 4.5m each. • Each hop connects two devices, Each hop connects two devices, • but each physical device can contain four logical but each physical device can contain four logical

nodes. nodes. • A Firewire cable contains two twisted-pairs (signals A Firewire cable contains two twisted-pairs (signals

and clock) and two untwisted conductors (power and clock) and two untwisted conductors (power and ground). and ground).

• FireWire is 'hot pluggable' FireWire is 'hot pluggable' • devices operating at different communication devices operating at different communication

rates can co-exist on the same communications rates can co-exist on the same communications chain, though a slower device must not be placed chain, though a slower device must not be placed between two higher speed devices. between two higher speed devices.

USB (Universal Serial Bus)USB (Universal Serial Bus)• serial data transmission, serial data transmission, • device powering, device powering, • data sent in 1 ms packets. data sent in 1 ms packets. • USB 1.1 has 1.5 and 12-Mbit/s speedsUSB 1.1 has 1.5 and 12-Mbit/s speeds• USB 2.0 has signaling rate from 12 USB 2.0 has signaling rate from 12

MHz to 480 MHz MHz to 480 MHz • PCs, Laptops, devices, etc.PCs, Laptops, devices, etc.

Ethernet (TCP/IP)Ethernet (TCP/IP)• Instruments with Ethernet interfaces Instruments with Ethernet interfaces

have great advantage: can be have great advantage: can be interrogated and controlled from a interrogated and controlled from a desktop anywhere in the world. desktop anywhere in the world.

• A 'web enabled' device behaves like a A 'web enabled' device behaves like a website and can be operated with website and can be operated with standard browser. standard browser.

• Fast data transfer up to 1GB/s if the Fast data transfer up to 1GB/s if the network infrastructure is good. network infrastructure is good.

MODBUS with RS-485MODBUS with RS-485

• messaging structure developed by Modicon messaging structure developed by Modicon in 1979, in 1979,

• master-slave/client-server communication master-slave/client-server communication between intelligent devices.between intelligent devices.

• master initiates transactions (queries). master initiates transactions (queries). Slaves respond with requested data to the Slaves respond with requested data to the master, or takes action requested in the master, or takes action requested in the query. query.

MODBUS with RS-485MODBUS with RS-485• slave is any peripheral device (I/O slave is any peripheral device (I/O

transducer, valve, network drive, or other transducer, valve, network drive, or other measuring device),measuring device),

• Masters address individual slaves, or Masters address individual slaves, or initiate a broadcast message to all slaves. initiate a broadcast message to all slaves.

• Slaves return response to all queries Slaves return response to all queries addressed to them individually, but not to addressed to them individually, but not to broadcast queriesbroadcast queries

MODBUSMODBUS

• devices include Register Map. devices include Register Map.

• MODBUS functions operate on these MODBUS functions operate on these registers to registers to monitor, configure, and control monitor, configure, and control module I/Omodule I/O

• Standard MODBUS networks employ one Standard MODBUS networks employ one of two types of transmission modes:of two types of transmission modes:

– ASCII ModeASCII Mode

– RTU Mode.RTU Mode.

MODBUS: MODBUS: ASCII Transmission Mode

• American Standard Code for American Standard Code for Information InterchangeInformation Interchange

• each character byte in a message is each character byte in a message is sent as 2 ASCII characters. sent as 2 ASCII characters.

• allows time intervals of up to a second allows time intervals of up to a second between characters during between characters during transmission without generating errorstransmission without generating errors

MODBUS: MODBUS: RTU

• Remote Terminal Unit Mode, Remote Terminal Unit Mode, • each 8-bit message byte contains two 4-bit each 8-bit message byte contains two 4-bit

hexadecimal characters, hexadecimal characters, • message transmitted in a continuous message transmitted in a continuous

stream. stream. • Greater effective character density Greater effective character density

increases throughput over ASCII mode at increases throughput over ASCII mode at same baud rate.same baud rate.

MODBUS MESSAGE FRAMINGMODBUS MESSAGE FRAMING

• A message frame is used to mark the A message frame is used to mark the beginning and ending point of a message beginning and ending point of a message

• receiving device determines which device receiving device determines which device is being addressed is being addressed

• knows when message is completed.knows when message is completed.

• partial messages detected and errors partial messages detected and errors flaggedflagged

ASCII Mode Message frameASCII Mode Message frame

• messages start with colon ":" (ASCII 3AH) messages start with colon ":" (ASCII 3AH) end with a carriage return-line feed (CRLF, end with a carriage return-line feed (CRLF, ASCII 0DH & 0AH). ASCII 0DH & 0AH).

• The only allowable characters for all other The only allowable characters for all other fields are hexadecimal 0-9 & A-F.fields are hexadecimal 0-9 & A-F.

• each character requires 7 data bits. Thus, each character requires 7 data bits. Thus, each character is 10 bits with start bit, each character is 10 bits with start bit, parity bit, and stop bit of the data frame.parity bit, and stop bit of the data frame.

RTU Mode Message FramesRTU Mode Message Frames• messages start with silent interval of atleast 3.5 messages start with silent interval of atleast 3.5

characterscharacters

• when first field is received (address) after when first field is received (address) after silent interval of at least 3.5 char-times, device silent interval of at least 3.5 char-times, device decodes it to determine if it is being addressed. decodes it to determine if it is being addressed.

• Following the last character transmitted, a Following the last character transmitted, a similar silent interval of 3.5 char-times marks similar silent interval of 3.5 char-times marks end of messageend of message

• a new message can begin after this interval.a new message can begin after this interval.

• a silent interval of more than 1.5 char-times a silent interval of more than 1.5 char-times before completion of frame (not a continuous before completion of frame (not a continuous stream), causes receiving device to flush the stream), causes receiving device to flush the incomplete messageincomplete message

• the immediate next byte is assumed to be the immediate next byte is assumed to be address field of a new messageaddress field of a new message

• if new message begins earlier than 3.5 char-if new message begins earlier than 3.5 char-times following previous message, receiving times following previous message, receiving device assumes it is continuation of previous device assumes it is continuation of previous message.message.

• This will generate a CRC errorThis will generate a CRC error

• MODBUS ADDRESSESMODBUS ADDRESSES

• MODBUS FUNCTIONSMODBUS FUNCTIONS

• MODBUS DATA FIELDMODBUS DATA FIELD

• MODBUS ERROR CHECKINGMODBUS ERROR CHECKING

– Parity checking of the data character frame Parity checking of the data character frame (even, odd, or no parity)(even, odd, or no parity)

– Frame checking within the message frame Frame checking within the message frame (Cyclical Redundancy Check in RTU Mode, or(Cyclical Redundancy Check in RTU Mode, or

– Longitudinal Redundancy Check in ASCII Longitudinal Redundancy Check in ASCII Mode).Mode).

MODBUS with TCP/IPMODBUS with TCP/IP

Fieldbus?Fieldbus?• fully-digital, bidirectional fully-digital, bidirectional

communication networking schemes for communication networking schemes for automation and instrumentation automation and instrumentation applications. applications.

• term much misunderstood; referred to term much misunderstood; referred to by different cartels of manufacturers by different cartels of manufacturers and vendors to advance their own and vendors to advance their own versions of networking products and versions of networking products and brands at the expense of products from brands at the expense of products from other groups.other groups.

• concept of digital networking started concept of digital networking started in the early 1970sin the early 1970s

• large number of players in the large number of players in the FieldBus each with own proprietary FieldBus each with own proprietary digital communication protocolsdigital communication protocols

• exercise high degree of caution and exercise high degree of caution and reluctance to acceptance of reluctance to acceptance of FieldBuses.FieldBuses.

• fears of unavailability of support-fears of unavailability of support-after-sales, after-sales,

• fear of technology getting obsolete,fear of technology getting obsolete,• high cost of proprietary technologies high cost of proprietary technologies

due to lower competition, etc. due to lower competition, etc.

Fieldbus AdvantagesFieldbus Advantages

Fieldbus advantages over traditional Fieldbus advantages over traditional instrumentationinstrumentation

• Digital data eliminates analog conversions and Digital data eliminates analog conversions and related drifts and calibration issues.related drifts and calibration issues.

• Significant reduction in the amount of wiring, Significant reduction in the amount of wiring, cable trays etc.cable trays etc.

• extensive bi-directional data communication for extensive bi-directional data communication for diagnostic information, device parameters, etc.diagnostic information, device parameters, etc.

• Centralised commissioning of devices.Centralised commissioning of devices.• Asset management capabilities.Asset management capabilities.• Greater flexibility in system layout and design.Greater flexibility in system layout and design.• Ease of future expansion and modification.Ease of future expansion and modification.

Open, InterOperable systemsOpen, InterOperable systems• Open:Open: to denote easy availability of to denote easy availability of

(licensed) technology to developers (licensed) technology to developers and end-users for enhancements or and end-users for enhancements or product developmentsproduct developments

• InterOperable:InterOperable: easy availability of easy availability of products and services for any products and services for any particular technology from multiple particular technology from multiple sources; to mix and match from sources; to mix and match from different suppliersdifferent suppliers

Year Open and available Technology Developer 1976 Ethernet DEC, Intel, Xerox1977 ARCNet Datapoint1980 Remote I/O Allen-Bradley / Rockwell 1984 Interbus-S Phoenix Contact, Interbus-Club1988 WorldFIP WorldFIP orgn.1990 Seriplex Automated Process Control 1991 LonWorks Echelon1993 AS-I (Actuators AS-I consortium1994 DeviceNet Allen-Bradley/ Rockwell 1994 Profibus-DP Siemens, PNO1994 Smart Distributed Honeywell (MICROSWITCH)1995 Profibus-PA (process Siemens, PNO 1995 CAN Open CAN in Automation, Phillips1995 FOUNDATION FIELDBUS FOUNDATION1996 ControlNet Allen-Bradley/ Rockwell

- Modbus Plus Modicon- Modbus RTU/ASCII Modicon- Profibus-FMS PNO, Siemens

• FIP : Factory Instrumentation FIP : Factory Instrumentation ProtocolProtocol

• PROFIBUS : Process Field BusPROFIBUS : Process Field Bus • CAN : Controller Area NetworkCAN : Controller Area Network • LonWorksLonWorks• P-NetP-Net • AS-IAS-I • WorldFIPWorldFIP • ISP : InterOperable Systems Project ISP : InterOperable Systems Project • FF : Foundation FieldBus FF : Foundation FieldBus • Industrial Ethernet Industrial Ethernet

IEC 61158 IEC 61158 •FF version H1FF version H1•ControlNetControlNet•Profibus (CENELEC EN-50170, Profibus (CENELEC EN-50170, EN-50254)EN-50254)•P-NetP-Net•FF version HSEFF version HSE•SwiftNetSwiftNet•WorldFIP (Vol.3 of EN-50170)WorldFIP (Vol.3 of EN-50170)•Interbus-SInterbus-S

Fieldbuses Fieldbuses and the IEC61158 standard and the IEC61158 standard

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PROFIBUSPROFIBUS• DPDP : Decentralised Periphery; : Decentralised Periphery; high speed, simple field high speed, simple field

level communications; Low costlevel communications; Low cost..• FMSFMS : Fieldbus Message Specification; : Fieldbus Message Specification; Higher-end, Higher-end,

applications level communications.applications level communications.• PAPA : Process Automation; : Process Automation; Developed specifically for Developed specifically for

process industry, very cost effective two-wire connection process industry, very cost effective two-wire connection carrying both power and datacarrying both power and data. .

• PROFInetPROFInet : : Simlar to FF-HS version, is the Ethernet Simlar to FF-HS version, is the Ethernet stream of Profibusstream of Profibus..

• PROFIDrivePROFIDrive : Drive control; : Drive control; Deterministic operation (sub Deterministic operation (sub micro-second timing for servo and drive synchronisation).micro-second timing for servo and drive synchronisation).

• PROFISafePROFISafe : Safety related; : Safety related; High Integrity Emergency stop High Integrity Emergency stop systems, safety interlocks and safety-shutdown systems.systems, safety interlocks and safety-shutdown systems.

Profibus PAProfibus PA

10mA

0

-9mA

time Tbit = 32s (f = 31.25kbit/s)

…Etc.

Current/mA

+9mA

1 1 0 0

ProfibusProfibus

• GSD (Generic Station Description) GSD (Generic Station Description) files for PROFIBUS DP devices files for PROFIBUS DP devices

Profibus Nutzerorganisation e.V. Karlsruhe : PNOProfibus Nutzerorganisation e.V. Karlsruhe : PNO

Foundation FieldbusFoundation Fieldbus

• 31.25kbps Physical Layer31.25kbps Physical Layer• Signaling Method similar to smart transmittersSignaling Method similar to smart transmitters• Supply voltage through impedance conditioner, Supply voltage through impedance conditioner,

(typically inductors). (typically inductors). • DC current through impedance conditioner feeds DC current through impedance conditioner feeds

devices. devices. • Supply voltage between 9V and 32V at device Supply voltage between 9V and 32V at device

terminals. terminals. • The impedance conditioner ensures output The impedance conditioner ensures output

impedance of the power supply is higher than impedance of the power supply is higher than 400Ω in the signal frequency bandwidth.400Ω in the signal frequency bandwidth.

FFFF• Number of devices between 2 and 32.Number of devices between 2 and 32.• Trunk: twisted pair individual shielded 18- Trunk: twisted pair individual shielded 18-

AWG cable; cable length <1900m; else AWG cable; cable length <1900m; else <1200m<1200m

• Spurs: <120m total; depends on no. of devicesSpurs: <120m total; depends on no. of devices

FFFF

• only one device on a link is allowed to only one device on a link is allowed to communicate; Link Active Scheduler (LAS) communicate; Link Active Scheduler (LAS) is the mediator.is the mediator.

• Address:Address:

Virtual Field Devices Virtual Field Devices & Function Blocks& Function Blocks

• device may contain user applications device may contain user applications independent of each other; do not independent of each other; do not interact. interact.

• Management VFD Management VFD where network and where network and system management applications reside. system management applications reside. used to configure network parameters used to configure network parameters and manage devicesand manage devices

• Function Block VFD Function Block VFD ; possible for a field ; possible for a field device to have two or more Function Block device to have two or more Function Block VFDs.VFDs.

Function Block VFD

• three classes of blocks: – Resource Block; manufacturer’s name, device name,

DD, etc. – how much resource (memory and CPU time) is Available, – Status of hardware,– controls the overall device hardware and Function Blocks– Function Block; – Standard Block as specified by Fieldbus Foundation, – Enhanced Block with additional parameters and

algorithm, – Open Block (Vendor-specific) designed by individual

vendors.– Transducer Block.

Resources on the Web forResources on the Web forFieldBusesFieldBuses

• http://www.profibus.com/pall/meta/downloadshttp://www.profibus.com/pall/meta/downloads• www.interbus.comwww.interbus.com• www.interbusclub.comwww.interbusclub.com• www.phoenixcontact.comwww.phoenixcontact.com• www.fieldbus.com.au/techinfo.htmwww.fieldbus.com.au/techinfo.htm• www.ee.latrobe.edu.au/~stanley/infoseek.htm www.ee.latrobe.edu.au/~stanley/infoseek.htm • www.lonworks.comwww.lonworks.com• http://www.p-net.dk/fieldbus/fieldbus.htmlhttp://www.p-net.dk/fieldbus/fieldbus.html

• SCOPE Exists! SCOPE Exists! Discrepancies / Discrepancies / Errors, may be Errors, may be brought to the brought to the attention of attention of lecturer for lecturer for correction.correction.

Teachers No Most of the Teachers No Most of the times!times!

Thank you!Thank you!

For your patience & toleranceFor your patience & tolerance

[email protected]@fcriindia.com

HART Communication ProtocolHART Communication Protocol• Bell-202 standard Frequency-shift-Bell-202 standard Frequency-shift-

keying (FSK) keying (FSK) • bit ‘1’ : 1200 Hzbit ‘1’ : 1200 Hz• bit ‘0’ : 2200 Hzbit ‘0’ : 2200 Hz

• Baud rate :Baud rate :• 1200 bit/s1200 bit/s

• Signal structure:Signal structure:• 1 start bit1 start bit• 8 data bits8 data bits• 1 bit for odd parity1 bit for odd parity• 1 stop bit.1 stop bit.

HART Communication between HART Communication between master and slavemaster and slave

• The master sends messages with The master sends messages with requests for actual/specified values, requests for actual/specified values, and/or any other data/parameters and/or any other data/parameters available from the slave device. available from the slave device.

• The slave interprets these instructions The slave interprets these instructions as defined in the HART protocol. as defined in the HART protocol.

• The slave responds with status The slave responds with status information and data for the master.information and data for the master.

HART Commands HART Commands

• Universal commandsUniversal commands• Common practice commandsCommon practice commands• Device-specific commands Device-specific commands

• HARTHART follows the Open Systems follows the Open Systems Interconnections Interconnections (OSI) (OSI) model of the model of the International Organization for International Organization for Standardization Standardization (ISO)(ISO). .

• The HART protocol uses a reduced OSI The HART protocol uses a reduced OSI model, implementing only model, implementing only layers 1, 2 layers 1, 2 and 7and 7

• Layer 1, Physical LayerLayer 1, Physical Layer• Layer 2, Link LayerLayer 2, Link Layer• Layer 7, Application LayerLayer 7, Application Layer

QuestionQuestion TopicsTopics OSI OSI layerlayer

How do we make the How do we make the connection?connection? Plugs, sockets, cablePlugs, sockets, cable PhysicalPhysical

What signals can I What signals can I send?send? Voltage, current, frequencyVoltage, current, frequency PhysicalPhysical

How do I address a How do I address a message?message?

None (point-to-point), numerical address, None (point-to-point), numerical address, tagtag

Data Data LinkLink

When can I send a When can I send a message?message?

Access rules: master-slave, token-Access rules: master-slave, token-passing, collision-detectionpassing, collision-detection

Data Data LinkLink

What messages can I What messages can I send?send?

Coding: bits, characters, parityCoding: bits, characters, parity Data Data LinkLink

Data types: bits, integers, floating point, Data types: bits, integers, floating point, texttext App.App.

What does a message What does a message mean?mean?

Standard functionsStandard functions App.App.

Function blocks, Device DescriptionsFunction blocks, Device Descriptions "User"*"User"*