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Industrial AutomationAutomation IndustrielleIndustrielle Automation

4 Application Layer Protocols

4.1 Device Management Protocolsprotocolos de gestión de dispositivosProtocoles de gestion des appareilsGerätezugangsprotokolle

SNMP and friends

Device access protocols 4.1 - 2Industrial Automation 2013

Contents

4.1.1 current loop 4..20 mA

4.1.2 HART

4.1.3 SNMP: Simple Network Management Protocol

4.1.4 MMS: Manufacturing Messaging Specification

This part treats device remote access in order of increasing complexity


4.1.1 Current Loop

The classical solution for analogue values

Example differential pressure transducer

4..20 mA current loop

fluid

Reminder:


4.1.2 HART

Data over 4..20 mA loops

Practically all 4..20mA devices come equipped with HART today

About 15 Mio devices are installed worldwide.

more info:

http://www.thehartbook.com/default.htm

http://www.hartcomm.org/

Reminder:

Reminder:


The Round card

The round card is a standardized printed circuit board that can be mounted in an instrument, containing the modem, a processor, RAM, EPROM and all the logic

and software necessary to execute the HART protocol.

It is round because most hydraulic instruments have a round case.

http://www.fint.no/ha-i4012.pdf


HART commands summary

Universal Commands Common Practice Commands

Device-Specific Commands(example)

• Read manufacturer and device type

• Read variable and units• Read current output and

percent of range• Read up to four predefined

dynamic variables• Read or write 8-character tag,

16-character descriptor, date• Read or write 32-character

message• Read device range values,

units, and damping time constant

• Read or write final assembly number

• Write polling address

• Read selection of up to four dynamic variables

• Write damping time constant• Write device range values• Calibrate (set zero, set span)• Set fixed output current• Perform self-test• Perform master reset• Trim variable zero• Write variable unit• Trim DAC zero and gain• Write transfer function (square

root/linear)• Write sensor serial number• Read or write dynamic

variable assignments

• Read or write low-flow cut-off• Start, stop, or clear totalizer• Read or write density

calibration factor• Choose variable (mass, flow,

or density)• Read or write materials or

construction information• Trim sensor calibration• enable PID, write PID setpoint• Valve characterization• Valve setpoint• Travel limits• User units• Local display information

Master

Indication

Slave

Request

Confirmation

Response

time-out

command

response


Device access

type

manufacturer

modemnetworkadapter

device volumetric flow rate

FlowPro

ABB

velocity 13.32 m2/s

diff. pressure 9.8 Pa

density 0.8 kg/l

volumetric flow rate

cross sectional area:

pipe inside diameter

3 cm2

2 cm

13.32 9.8 0.8

field device

hand-held device

networkadapter

SCADA

4-20 mA loop for HART

networkadapter


Installation

taken from: www.hartcomm.org

point-to-pointmultidrop

universal hand-help terminal


4.1.3 SNMP

Simple Network Management Protocol for the Internet


Simple Network Management Protocol

• Configuration Management

Keeping track of device settings

• Fault management

Dealing with problems and emergencies (router stops forwarding, server loses power, etc)

• Performance Management

How smoothly is network running?

Can it handle the current workload?


SNMP - MIB objects

TCP/UDP/IP (171 objects)NT network (90 objects)DHCP (14 objects)WINS (70 objects)AppletalkNowellIPXDecNet…..CISCO (proprietary)

Mostly parameters, statistics and error counters used exclusively for communication

AgentMIBmanaged

informationbase


tcpMaxConn OBJECT-TYP SYNTAX Integer32 (-1 | 0..2147483647) MAX-ACCESS read-only STATUS current DESCRIPTION "The limit on the total number of TCP connections the entity can support. In entities where the maximum number of connections is dynamic, this object should contain the value -1." ::= { tcp 4 }

tcpActiveOpens OBJECT-TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The number of times that TCP connections have made a direct transition to the SYN-SENT state from the CLOSED state. Discontinuities in the value of this counter are indicated via discontinuities in the value of sysUpTime." ::= { tcp 5 }

SNMP – ASN.1 Object example

http://net-snmp.sourceforge.net/docs/mibs/TCP-MIB.txt


SNMP - Access to Managed Objects

call(indication)

call(request)

management messages

object interface manager interface

User

managed informationbase

reply(confirm)

reply(response)

agent interface

MIBManager Agent

network

UDPIP

ISO 8802-2 Type 1ISO 8802-3(Ethernet)

TokenRing

ATM

UDPIP

ISO 8802-2 Type 1ISO 8802-3(Ethernet)

TokenRing

ATM


SNMP - Operations on objects

Operations (PDU type):Get (read)Set (write)GetNext (transversal reading)GetBulk (optimized GetNext, v2 and v3)Response (variable bindings and acknowledgement)Trap (asynchronous agent notification, priorities)

Since SNMPv1/SNMPv2 do not provide authentication, “Set” commands are normally disabled. Traps are rarely used.


SNMP - How are objects identified ?

ISO defined a world-wide addressing scheme on a hierarchical basis:

MIB objects are identified by a concatenation of numerical identifiers

quite wasteful, but bearable in LANs


SNMP example of identification

.1.3.111.3.37.238.9999.1.1.2 ==.iso.org.ieee.standards-association-c-series-standards.std-c37.part238.

ieeeC37238TSMib.ieeeC37238Objects.ieeeC37238DefaultDS.ieeeC37238DefaultDSClkIdentity


SNMP - Assumptions about the underlying communication network

- the network is connectionless (datagrams): only UDP is used (no TCP).

- manager and agent can send messages to each other spontaneously

- all entities must be able to receive and send packets of at least 484 octets

- the network supports broadcast

Further reading: www.wtcs.org/snmp4tpc/files/reference/francois/snmp.ppt


4.1.4 MMS

Manufacturing Messaging Specification (MMS)

Action

ProgramInvocation

NamedVariable Named

Variable List

File

Types

Semaphore

EventEnrolment

Transaction

Domain

OperatorStation

Journal

EventCondition

Event


MMS - Manufacturing Message Specification

Developed 1980 for the MAP project (General Motor’s flexible manufacturing initiative)

Reputed for being heavy, complicated and costly (due to poor implementations)

But:• Boeing adopted MMS as TOPs (MMS on Ethernet)• Adopted by the automobile industry and power distribution

Standardized as:

[1] ISO/IEC 9506-1: Industrial Automation systems - Manufacturing Message Specification -Part 1: Service Definition (IS 1990)

[2] ISO/IEC 9506-2: Industrial Automation systems - Manufacturing Message Specification -

Part 2: Protocol Specification (IS 1990)


MMS - Application field

schedule

robot configuration


MMS - Concept

• A set of standard messages exchanged between a manager and an agent stationfor the purpose of controlling these objects

• A set of encoding rules for these messages

• A set of rules for exchanging messages between devices (basic protocol)

• A set of standard objects which must exist in every conformant device, on whichoperations can be executed (example: local variables, read and write) or whichcan start a transmission spontaneously

MMS (Manufacturing Message Specifications) defines:

MMS does not specify application-specific operations (e.g. change motor speed).This is covered by application-specific, “companion standards”(e.g. flexible manufacturing, drives, remote meter reading)


device(e.g. PLC)

MMSserver

communicationstack

MMS - Manufacturing Message Specification

network(example)

request

(command)

response

(reply)

linkingdevice

router

procedure call interface

device(e.g. SCADA)

remote

MMS specifies a set of objects which

MMSclient

an MMS server isexpected to contain

MMS specifies a set of messages which

to control an MMS server

allow an MMS client

communicationstack


MMS - Communication Stack

Association Control Service Element, ACSE, ISO 8649/8650, N2526,N2327

ISO 8473 connectionless

ISO 8802-3(Ethernet) Physical

Link

Network

Transport

ISO 8802-4

ISO 8073 Class 4

ISO 8326/8327

Abstract syntax notation,ISO 8822/8823, 8824/8825 Presentation

ISO 8802-2 Type 1

MAC

“Application”

Session

quite heavy… Boeing decided to drop ISO for TCP/IP ("TOP“).

(token bus)


MMS - Basic Communication Principles

Remote Procedure Call(Call paired with Reply,synchronous, unicast)

Event Reporting(spontaneous messages sentby server)

1)

2)

MMS does not specify how to address clients and servers. Messages contain only a communication reference (number which identifies the connection) obtained by unspecified means.

Indication

MMS Responder(server)

Request

Confirmation Response

network

Indication

Request

processing

event

MMS Requester(client)

MMS assumes that the communication stack offers two services:


MMS - Event services

MMS provides services to:- Event Condition (define the boolean condition that triggers an event and its priority)- Event Enrolment (define the MMS client(s) to notify when an event is triggered)- Event Action (define the MMS confirmed service to be executed when the event occurs)

MMS client MMS client

(MMS server)enables/disables event conditions

event notification and confirmation

AlterEventCondition EventN

otifica

tion

AckEve

ntNotifi

catio

n

EventAction

EventEnrolment

EventCondition

What?Who? When?

DefineEventCondition

Events are the most complicated part of MMS


MMS - Event triggering

MMS client MMS client

MMS Server

NETWORK-TRIGGERED

plant

EventAction

EventEnrolment

EventCondition

TriggerEvent

EventN

otifica

tion

events are triggered by a change in a boolean variable in the server (monitored event) orby an MMS client (trigger event) as an invitation procedure

cyclic monitoring

AckEve

ntNotifi

catio

n

booleanvariable

MONITORED


VMD: Virtual Manufacturing Device

ProgramInvocation

NamedVariable Named

Variable List

File

Types

Semaphore

EventEnrolment

Transaction

Domain

OperatorStation

Journal

EventCondition Event

Definition of objects, services, and behavior •Only specifies the network-visible aspects (device / application communication)•Internal implementation details (programming language, operating system, CPU type, input/output (I/O) systems) not specified by MMS

interoperability

Action

flow meterrobot

Application Programming Interface(MMSI = MMS interface)

Virtual Device

Virtual Device

Virtual Device

communication

stack

physicallink

networktransportsession

presentation

cell


Assessment

What is the purpose of the HART protocol ?

Which communication is used between a hand-help and a field device ?

Which categories of commands do exist ?

What distinguishes Hart from SNMP ?

What distinguishes SNMP from MMS ?

What are the (dis)advantages of MMS ?

industrial automation automation industrielle industrielle automation 4application layer protocols...

Documents

device access protocols

device type

treats device remote

device network adapter

field device hand

hart network adapter

damping time constant

hart protocol