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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

SCADAServer

RS-232

In InO ut

Analog D ig ita l

O utInstrum enta tion

& Contro l

HM I 1 HM I 2 Printer

RadioM odem

RadioM odem

PC PC

PC

I/OD atabase


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 tio na l

Sp a re RS-232 Po rt

O p e ra to r Sta tio n/Pro g ra m m ing Te rm ina l

O p tio na l

No nVo la tile

M e m o ry

Po we rSup p ly

a ndBa c kupBa tte ry

M o d e m

Ante nna

Ana lo gInp ut

M o d ule

Ana lo gO utp utM o d ule

Dig ita lInp ut

M o d ule AC M a ins

Po we r

Vo la tileM e m o ry

(RAM )

C e ntra lPro c e ssing

Unit

Dig ita lO utp utM o d ule

C e ntra l Bus

M a in RTU

Se ria l C o m m s Po rts(RS-232 / RS-422 / RS-485)

Pro g ra m m a b le Lo g ic C o ntro lle r

Ethe rne 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 C o nv erte r

B uffe r/F IF O

S/H

Clo ck

S/H

S/ H

S/H

S/H

B u s Inte rfa ce Tim in g S yste m

C h a nnel-G ainA rra y Me m ory

Pro gra m m a bleG ain

A m pliferS a m ple &H old for

A/ D

S a m ple

E nd ofco n version

StartC o nv ersion

Step/N e xtC han n el

E xt. C lo ck

E xt. Trig ger

S a m pleIn p uts

R e a dC on versio n

P CD ATAB U S

D ataR e a dy

R e adD ata

P G A

+

-

In putC ha n nel

S a m ple &H old

Status &C o ntro lL in es

Channel

Inp

ut

Mu

ltip

lexer


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-232D 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 r i v e r

0 ( S p a c e )

1 ( O n )

1 ( M a r k )

0 ( O f f )

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 = +Vt

t

Line DriverLine Receiver+V+V

+V+V

22

33

77OV

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 A560 560120

120


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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