This is a comprehensive workshop covering the essentials of SCADA communication systems focusing on DNP3 and the other new developments in this area. The workshop commences with a brief review of the fundamentals of SCADA systems hardware, software and the communications systems (such as RS-232 and RS-485 Ethernet and TCP/IP) that connect the SCADA operator stations together.
A solid review is then done on the DNP3 protocol where its features, message structure, practical benefits and applications are discussed. The workshop is intended to be product independent but examples will be taken from existing products to ensure that all aspects of the DNP3 protocol are covered.
It provides you with the tools to design your next SCADA system more effectively using DNP3 and draw on the latest technologies.
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Practical DNP3, 60870.5 & Modern SCADA Communication System
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Fundamentals of SCADA
Communications
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Topics
• SCADA Systems
• Remote Terminal Units
• Communication Architectures
• Communication Philosophies
• RS-232 and RS 485
• MODBUS Protocol
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SCADA System
• Supervisory Control and Data Acquisition System
• Comprises a number of Remote Terminal Units
(RTU’s) collecting field data connected back to a
master station via a communications system
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Typical SCADA System
S C A D A
S e r v e r
R S - 2 3 2
I n I nO u t
A n a lo g D ig ita l
O u t
I n s t r u m e n t a t io n
& C o n t r o l
H M I 1 H M I 2 P r in t e r
R a d i o
M o d e m
R a d io
M o d e m
P C P C
P C
I / O
D a t a b a s e
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Remote Terminal Unit
• RTU is a standalone data acquisition and control unit, generally
microprocessor based which monitors and controls equipment at some
remote location from the central station.
O p t io n a l
S p a r e R S - 2 3 2 P o r t
O p e r a t o r S t a t io n /
P r o g r a m m in g Te r m in a l
O p t io n a l
N o n
V o la t ile
M e m o r y
P o w e r
S u p p ly
a n d
B a c k u p
B a t t e r y
M o d e m
A n t e n n a
A n a lo g
In p u t
M o d u le
A n a lo g
O u t p u t
M o d u le
D ig it a l
In p u t
M o d u le
A C M a in s
P o w e r
V o la t ile
M e m o r y
( R A M )
C e n t r a l
P r o c e s s in g
U n it
D ig it a l
O u t p u t
M o d u le
C e n t r a l B u s
M a in R TU
S e r ia l C o m m s P o r t s
( R S - 2 3 2 / R S - 4 2 2 / R S - 4 8 5 )
P r o g r a m m a b le L o g ic C o n t r o lle r
E t h e r n e t
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Typical RTU Hardware
• Control Processor
• Analog I/O
• Digital I/O
• Counter Inputs
• Communications Interface
• Power Supply
• RTU Rack and enclosure
• Watchdog Timer
• Real Time Clock
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Analog Input Module
Gain
A/D Co nverter
B uffer/FIFO
S/H
Clock
S/H
S/H
S/H
S/H
B us Interface Tim in g S yste m
Ch annel-G ain
Arra y Me m ory
Pro gra m m a ble
G ain
A m pliferS a m ple &
H old for
A/ D
S a m ple
E nd of
co nversion
Start
Co nversion
Step/N ext
C hann el
E xt. Clock
Ext. Trigger
Sa m ple
Inp uts
Re ad
C onversion
P C
D ATA
B U S
D ata
Rea dy
Re ad
D ata
P G A
+
-
Input
C ha nnel
S a m ple &
Hold
Status &
Control
Lin es
Channel
Input
Multiplexer
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A/D Converters
• Integrating A/D
– low sample rate
– high accuracy
• Successive Approximation A/D
– high sample rate
– lower accuracy
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Digital Inputs (Optically isolated)
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Counter or Accumulator Inputs
Optical Isolation
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Polled (or Master-Slave)
• Advantages:
– simple, predictable, no collisions
– quickly detect link failure
• Disadvantages:
– performance
– all slaves must be polled (cannot spontaneously
report alarms)
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Peer to Peer (CSMA/CD)
• Advantages:
– Slaves can report alarms spontaneously
– Can have many stations
• Disadvantages:
– Need to control collisions (priorities)
– Can lead to network collapse
– Link failure detection slower
• Use Exception Reporting to cut down
unnecessary traffic
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Connections between DTE & DCE
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Voltage Levels for EIA-232
D a t a L i n e D r i v e r
C o n t r o l L i n e D ri v e r
0 ( S p a c e )
1 ( O n )
1 ( M a r k )
0 ( O ff )
D a t a L i n e s
D r i v e r R e c e i v e r
C o n t r o l L i n e s
D a t a R e c e i v e r
C o n t r o l R e c e i v e r
+ 2 5 V
+ 2 5 V
+ 2 5 V
+ 2 5 V
- 2 5 V
- 2 5 V
- 2 5 V
- 2 5 V
+ 5 V
+ 5 V
+ 3 V
+ 3 V
- 5 V
- 5 V
- 3 V
- 3 V
0 V
0 V
0 V
0 V
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EIA-232 Transmitters and Receivers
DCE DeviceDTE Device
TTL TTL
Data Transmit
Data Receive
Signal Common
Logic 1 : V = -V
Logic 0 : V = +V
t
t
Line DriverLine Receiver
+V+V
+V+V
22
33
77
OV
OV
-V-V
-V
Line ReceiverLine Driver
Vt
Note: TTL 0V = 0 +5V= 1
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RS 485
• RS-485 is the most common voltage standard in
use today for multi-drop communication systems.
• Because:
• It is very resistant to noise
• It can send data at high speeds
• And can go long distances
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EIA-485 Interface Standard
• Distance: up to 1200m (4000 ft)
• Speed: up to 10 Mbps
• Up to 32 line drivers - balanced, tri-state
• Up to 32 balanced receivers
NOTE can only address 32 devices
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Installation of Bias Resistors
to Minimize Noise
B A
560Ω 560Ω120Ω
120Ω
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Modbus Protocol
• Developed by Modicon for use with Process
Control Systems.
• Estimated by Control Engineering Magazine
that over 40 % of industrial interfacing was
done using this Protocol.
• Even the new Fieldbus state of the art
standards appearing today have a Modbus
Gateway.
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Open Protocol
• While Modbus is not an actual system it is a
protocol used on many industrial systems
• Although it is an old standard (written in Fortran in
the 1970s) it has become a highly successful Open
protocol (defacto standard)
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MODBUS Protocol
• Originally intended to link Modicon PLC’s with
intelligent terminals
• Half-duplex, Master/slave operation
• Connects 1 Master with up to 247 slaves
• Typically used with a multidrop bus
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A De-facto Standard
• Relatively Slow compared to other buses.
• Widespread acceptance.
• PLC jargon used in describing the standard....but
used in instruments/gas chromatographs....etc
• Two versions:
– Modbus RTU (Modbus-B sometimes)
– Modbus ASCII
– Very similar structure
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Physical Layer
• No physical layer specified so Modbus can be used
with a variety of interface standards:
– RS-232/RS-422 (point-to-point)
– RS-485 (multidrop bus)
– Bell 202 modem (switched and leased lines)
– “Modbus modem” (baseband line driver on twisted pair
up to 5000m)
– Asynchronous, byte-oriented frames
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Modbus Protocol Structure
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Memory Structure
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Application Layer
• Two types of messages:
– Query/response - specific slave addressed
– Broadcast - address 00, no response required
• Function codes specify:
– what the slave is to do
– what section of the memory map to find or deposit
the data
• e.g. Coil Status, Input Status, Input Register, Holding
Register
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Exception responses
• Are returned if error conditions occurred during
communications
• the most significant bit of the function code is
set in the returning message
• the error code is returned in the data field
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Validating Changes
• Can you trust the response that comes back
when you are changing a critical parameter on
the device?
• Methods to check:
– Do a subsequent read request of that parameter.
– Check some other associated parameter..e.g. start a
pump, check flow rate...
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DO YOU WANT TO KNOW MORE?
If you are interested in further training or information,
please visit:
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