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Industrial Automation
Automation Industrielle
Industrielle Automation
4
Application Layer Protocols
4.1
Device Management Protocols
protocolos de gestión de dispositivos
Protocoles de gestion des appareils
Gerätezugangsprotokolle
SNMP and friends
Contents
This part treats device remote access in order of increasing complexity
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
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Device access protocols 4.1 - 2
4.1.1 Current Loop
The classical solution for analogue values
Example differential pressure transducer
4..20 mA current loop
Reminder:
fluid
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Device access protocols 4.1 - 3
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.
Reminder:
Reminder:
more info: http://www.hartcomm.org/
http://www.thehartbook.com/default.htm
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Device access protocols 4.1 - 4
The Round card
http://www.fint.no/ha-i4012.pdf
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.
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Device access protocols 4.1 - 5
HART commands summary
Master
Request
Slave
Indication
time-out
Response
Confirmation
Universal Commands
• 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
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Common Practice
Commands
Device-Specific Commands
(example)
• 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
Device access protocols 4.1 - 6
Device access
device
volumetric flow rate
type
FlowPro
manufacturer
ABB
volumetric flow rate
3 cm2
cross sectional area:
field device
pipe inside diameter
network
adapter
handheld
device
SCADA
2 cm
velocity
13.32 m2/s
diff. pressure
9.8
Pa
density
0.8
kg/l
network
adapter
network
modem
adapter
4-20 mA loop for HART
13.32
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9.8
0.8
Device access protocols 4.1 - 7
Installation
point-to-point
multidrop
universal hand-help terminal
taken from: www.hartcomm.org
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Device access protocols 4.1 - 8
4.1.3 SNMP
Simple Network Management Protocol for the Internet
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Device access protocols 4.1 - 9
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?
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Device access protocols 4.1 - 10
SNMP - MIB objects
managed
information
base
MIB
Agent
TCP/UDP/IP
NT network
DHCP
WINS
Appletalk
Nowell
IPX
DecNet
…..
CISCO
(171 objects)
(90 objects)
(14 objects)
(70 objects)
(proprietary)
Mostly parameters, statistics and error counters used exclusively for communication
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SNMP – ASN.1 Object example
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 }
http://net-snmp.sourceforge.net/docs/mibs/TCP-MIB.txt
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SNMP - Access to Managed Objects
User
manager
interface
agent
interface
object interface
managed
information
base
Manager
call
(request)
reply
Agent
reply
(confirm)
UDP
IP
ISO 8802-2 Type 1
ISO 8802-3 Token
ATM
(Ethernet)
Ring
MIB
(response)
management
messages
call
(indication)
UDP
IP
ISO 8802-2 Type 1
ISO 8802-3 Token
ATM
(Ethernet)
Ring
network
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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.
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Device access protocols 4.1 - 14
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
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Device access protocols 4.1 - 15
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
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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
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Device access protocols 4.1 - 17
4.1.4 MMS
Program
Invocation
Named
Variable
Named
Variable List
Domain
Types
Operator
Station
Transaction
File
Journal
Semaphore
Event
Condition
Event
Action
Event
Enrolment
Manufacturing Messaging Specification (MMS)
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Device access protocols 4.1 - 18
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)
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MMS - Application field
schedule
robot
configuration
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MMS - Concept
MMS (Manufacturing Message Specifications) defines:
• A set of standard objects which must exist in every conformant device, on which
operations can be executed (example: local variables, read and write) or which
can start a transmission spontaneously
• A set of standard messages exchanged between a manager and an agent station
for 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)
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)
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MMS - Manufacturing Message Specification
device
(e.g. SCADA)
MMS specifies a set
of messages which
allow an MMS client
to control an MMS
server
remote
procedure
call interface
MMS specifies a set
of objects which
an MMS server is
expected to contain
MMS
client
request
device
(e.g. PLC)
MMS
server
response
(command)
(reply)
communication
stack
linking
device
communication
stack
network
(example)
router
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Device access protocols 4.1 - 22
MMS - Communication Stack
Association Control Service Element, ACSE, ISO 8649/8650, N2526,N2327
“Application”
Abstract syntax notation,ISO 8822/8823, 8824/8825
Presentation
ISO 8326/8327
Session
ISO 8073 Class 4
Transport
ISO 8473 connectionless
ISO 8802-2 Type 1
ISO 8802-3
(Ethernet)
Network
Link
ISO 8802-4
(token bus)
MAC
Physical
quite heavy… Boeing decided to drop ISO for TCP/IP ("TOP“).
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MMS - Basic Communication Principles
MMS assumes that the communication stack offers two services:
MMS Requester
(client)
Request
network
MMS Responder
(server)
Indication
1) Remote Procedure Call
(Call paired with Reply,
synchronous, unicast)
processing
Confirmation
2) Event Reporting
(spontaneous messages sent
by server)
Response
event
Request
Indication
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.
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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
enables/disables
event conditions
MMS client
(MMS server)
Event
Enrolment
Event
Condition
Event
Action
Who?
When?
What?
event notification
and confirmation
Events are the most complicated part of MMS
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MMS - Event triggering
MMS client
MMS client
MMS Server
NETWORKTRIGGERED
Event
Enrolment
Event
Condition
boolean
variable
Event
Action
MONITORED
cyclic monitoring
plant
events are triggered by a change in a boolean variable in the server (monitored event) or
by an MMS client (trigger event) as an invitation procedure
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VMD: Virtual Manufacturing Device
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
robot
flow meter
cell
Virtual
Device
Virtual
Device
Virtual
Device
Application Programming Interface
(MMSI = MMS interface)
communication
presentation
stack
session
Program
Invocation
Named
Variable
Named
Variable List
Domain
Types
Operator
Station
Transaction
Journal
File
Semaphore
Event
Condition
Event
Action
Event
Enrolment
transport
network
link
physical
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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 ?
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