Practical DNP3, 60870.5 & Modern SCADA Communication System

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Fundamentals of SCADA
Communications

Topics
• SCADA Systems
• Remote Terminal Units
• Communication Architectures
• Communication Philosophies
• RS-232 and RS 485
• MODBUS Protocol

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

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

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

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

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

A/D Converters
• Integrating A/D
– low sample rate
– high accuracy
• Successive Approximation A/D
– high sample rate
– lower accuracy

Digital Inputs (Optically isolated)

Counter or Accumulator Inputs
Optical Isolation

Polled (or Master-Slave)
• Advantages:
– simple, predictable, no collisions
– quickly detect link failure
• Disadvantages:
– performance
– all slaves must be polled (cannot spontaneously
report alarms)

Prioritised Polling Technique
RTU 9
RTU 1
RTU 2
RTU 3
RTU 4
RTU 8
RTU 3
RTU 2
RTU 1
RTU 1
RTU 5
RTU 7
RTU 6
RTU 2
High Priority
RTU 1
RTU 2
RTU 3
Polling
Cycle
RTU 3

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

Connections between DTE & DCE

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

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

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

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

Installation of Bias Resistors
to Minimize Noise
B A
560Ω 560Ω120Ω
120Ω

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.

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)

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

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

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

Modbus Protocol Structure

Memory Structure

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

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

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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Practical DNP3, 60870.5 & Modern SCADA Communication System

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