PLC and SCADA communication

PLC and SCADA
Communication
P R E S E N TAT I O N B Y:
S U M A N T R E D D Y
J I T E S H S A N A P
A R P I T S A R A P
TA L H A S H A I K H
Department of Instrumentation and Control Engineering
Vishwakarma Institute of Technology, Pune.

Content
• Introduction
• What is PLC ?
• PLC-SCADA Communication
• PLC and SCADA communication protocols
• Modbus
• Profibus
• ControlNET
• DeviceNET
• HART
• Ethernet

Introduction
PLC also known as Programmable Logic Controller is a
combination of Hardware and software which is a kind
of programmable controller operated in 24v DC.
SCADA also known as Supervisory Control and Data
Acquisition systems are used to monitor and control a
plant or equipment in industries.
The PLC and SCADA communicate via the SCADA
reading the measured flow, level, or any other
parameter and sending the setpoint to the PLC.
When the measured flow is compared to the setpoint,
the PLC adjusts the needed parameter to match the
setpoint.

What is PLC ?
• PLC is an industrial computer that monitors inputs,
makes decisions based on its program and controls
outputs to automate a process or machine.
• A PLC has a built-in operating system(OS). This OS is
highly specialized to handle incoming events in real-
time, or at the time of their occurrence.
• PLC is a digital computer designed for multiple inputs
and output arrangements, extended temperature ranges,
immunity to electrical noise, and resistance to vibration
and impact. A PLC is an example of a real time system.
Fig 1. Siemens S7-1200 PLC

PLC Scan Cycle & Scan Time
• While the PLC is running, the scanning
process includes the four phases, which are
repeated continuously as individual cycles of
operation:
• Input Scan- Scan the state of the Inputs.
• Program Execution- Processes and
executes the program logic.
• Housekeeping- This step includes
communications, Internal Diagnostics,
etc.
• Output Scan- Energize/de-energize the
outputs
Fig 2. PLC Scan Cycle & Scan Time Flow Chart

Working of PLC and Architecture of PLC
Fig 3. Source Mode and Sink Mode Fig 4. PLC Architecture

SCADA
• Supervisory Control And Data Acquisition
• It is a software through which we can create
visualization of any industrial process.
• Earlier, PLCs were programmed, download
the program in it and it used to run for years
• The problem- no one knew what’s happening
inside the PLC.
• Solution = PLC+SCADA
Fig 5. SCADA Interface for Tank Level on CoDeSys Application

Tag’s
• It is the address of the memory location where
signals are being saved.
• We define a TAG in order to use it in the
SCADA software(Intouch wonderware).
• TYPES : 1) Analog 2) Digital 3) Strings
• These are further associated with either
“Memory” or “I/O” type tags.
• PC-SCADA – Memory analog/digital/string tag
• PLC-SCADA - I/O analog/digital/string tag
Fig 6. Tag’s from Siemens TIA Portal

PLC and SCADA
communication
Protocols
• SCADA needs a protocol for transmitting data
• DNP3 and T101
• Modbus RTU, RP-570, Profibus and Conitel
• Standard protocols are IEC 61850 , IEC 60870-5-101 or 104 and
DNP3
Fig 2. PLC-SCADA Standard Communication
Protocols

T101 Protocol
• Belongs to IEC 60870-5 Standards
• International standard prepared by TC57 for power
system monitoring, control & associated
communications
• Supports unbalanced (master initiated message) &
balanced (master/slave initiated message) modes of
data transfer
• Supports unbalanced (master initiated message) &
balanced (master/slave initiated message) modes of
data transfer
• Supports group interrogation
• Cyclic & Spontaneous data updating

DNP3 Protocol
Communications
between various types
of data acquisition
and control systems
Used by SCADA
Master Stations or
Control Centers,
Remote Terminal
Units, and Intelligent
Electronic Devices
Supports multiple-
slave, peer-to-peer
and multiple-master
communications
Provides
interoperability
between different
vendor’s equipment
Open protocol
Multiplexing, data
fragmentation, error
checking, link
control, prioritization,
and layer 2
addressing services
for user data
Use of Cyclic
redundancy check
codes to detect errors
Fig 9. Architecture of DNP3

Modbus
Open protocol
Common language for devices and equipment
to communicate with one and another
Peer-to-Peer message structure
Moves raw bits or words without placing many
restrictions
Allows communication between many devices
connected to the same network
[1] 2019 IEEE 19th International Conference on Communication Technology (ICCT) (pp. 420-423). IEEE.

Master-Slave
communication
Fig 01. Master-Slave communication
Source – By Real Pars

The master’s query:
– Slave address (broadcast address).
– Function code with a read or write
data command to the slave.
– The write command “Data” if a write command
was initiated by the master.
– Error checking field.
A slave’s response:
– Fields confirming it received the request.
– The data to be returned.
– Error checking data.
Fig 02. Master Query
Fig 03. Slave's Response
[1] 2019 IEEE 19th International Conference on Communication Technology (ICCT) (pp. 420-423). IEEE.

Profibus
PROcess FIeld
BUS
Supports single
cable
communication
for multiple
inputs
Convenient for
large assembly
Three types:
• Profibus DP
• Profibus PA
• Profibus FMS

Profibus DP1
• DP is Decentralized Processing
• Single RS-485 cable from i/o to plc
• 9.6 Kbps to 12 Mbps data-rate
• Cable length of 100 to 1200m
1Kyung Chang Lee, Suk Lee and Man Hyung Lee, "Remote fuzzy logic control of networked control system via Profibus-DP," in IEEE Transactions on Industrial Electronics

Profibus PA
• PA is short for Process Automation
• remove this I/O unit and cables and replace them with a segment coupler
• 31.25 kbps data rate with 1900 m long cable
• Allows 31 devices, can be expanded up to 126 nodes

Communication In Profibus1
1A. Willig, "An architecture for wireless extension of PROFIBUS," IECON'03. 29th Annual Conference of the IEEE Industrial Electronics Societ
• Token passing is used
for communication in
masters
• Polling method is used
for communication
between master and
slaves
• Master with token pulls
the information from
slaves

ControlNET
Used in applications that work best with
scheduled communications
5 Mbps data transfer rate
RG-6 coaxial cable types
1000 m cables and repeaters could use
afterwards
Trunkline, star or tree topology networks 3 .
[3] Lian et al, Performance evaluation of control networks: Ethernet,
ControlNet, and DeviceNet. IEEE , 21(1), 66-83.

Physical layers of ControlNET
Tap
Terminating
Resistor :
75 Ohm
Repeater
Segment
Bridge
[3] Lian et al, Performance evaluation of control networks: Ethernet, ControlNet, and DeviceNet. IEEE , 21(1), 66-83.
Fig 08. ControNet Physical Media 3 .

Token passing
Bus Control
Network
• MAC id for each node
• Node knows id of
predecessor and successor
node.
• Each node has a scheduled
time.
• One node can transmit at a
time 3 .
Fig 09. ControlNet Token Ring

Advantages
High speed
Strictly scheduled
Have deterministic data
transfer
Stable and Reliable
Disadvantages
Higher Cost
Difficult troublesooting
Advantages and
Disadvantages

DeviceNET
Open Protocol
64 possible
connections
Four pin connector
with both power
and signal cables
Trunkline and
dropline network
topology
Uses DeviceNet
scanner instead of
i/o module
Fig. Total Dropline Cable Length per DeviceNet Data Rates 3 .

DeviceNET
physical layer
• Terminating resistor of 121 Ohms, 1 percent, 0.25
Watts
• Taps for connecting
• Direct, branching or daisy chaining connection 3 .
Fig 10. DeviceNet Physical Layer 3 Fig 11. Trunkline Cabel Length per DeviceNet Data Rates 3

Advantages
low cost
widespread
acceptance
high
reliability
efficient use
of network
bandwidth
power
available on
the network
maximum
cable length
limited
message size
limited
bandwidth
Disadvantages
Advantages and Disadvantages

HART
• Highway Addressable Remote Transducer
• Analog sensors based on current loop
technology
• 4-20 mA current range for LRL and HRL value
• Single pair of wire connection
• Signal can be transmitted up to 1000m
• Tag name, or calibration settings, or sensor
diagnostics can be transmitted with sensor
values
• Provides remote monitoring
• Up to 63 sensor nodes can be established 4
[4] Muller, I., Netto, J. C., & Pereira, C. E. (2011). WirelessHART field devices. IEEE
Instrumentation & Measurement Magazine, 14(6), 20-25.

Frequency-Shift
Keying (FSK)
• 1200 and 2200 HZ Ac signals are used to transmit digital data as 1 and
0 respectively
Fig. HART
communication

HART Communication
• HART sensor must be requested by the
master node
• Requests like measurement value, node
key, other values of sensor.
• HART allows for multiple master nodes
• HART communicator is used to request all
sensor data from different location
[4] Muller, I., Netto, J. C., & Pereira, C. E. (2011). WirelessHART field devices. IEEE Instrumentation
& Measurement Magazine, 14(6), 20-25.

Ethernet
Multiple devices can create, store and share
information
Different type of cables: Coaxial cable, Twisted
cable, Optic cable
Rj-45 connectors are used in twisted cables for data
transmission in half duplex or full duplex mode
Optic cables uses light pulses to transmit the data
SFP and SC are most common connectors for optic
cables
Ethernet to Fiber Converter is used to transfer data
from twisted to optical cable
[3] Lian, F. L., Moyne, J. R., & Tilbury, D. M. (2001). Performance evaluation of control networks: Ethernet, ControlNet,
and DeviceNet. IEEE control systems magazine, 21(1), 66-83.

Ethernet Devices
• Switches and Routers connect multiple
computers or even networks together
• Gateway connects two dissimilar
networks together
• Bridge connects two similar networks
together
Fig. Physical layer device

Ethernet Data Link
Layer
• Logical Link Control” establishes
paths for data on the Ethernet to
transmit between devices.
• Media Access Control” uses hardware
addresses to identify devices
• Use of CSMA/CD (Carrier Sense
Multiple Access with Collision
Detection)
• Reduce data collisions and increase
successful data transmission Fig. Data link layer

Profinet
Ethernet-based industrial communications protocol
Provides both cyclic and acyclic communication
Fast and deterministic communication with 100
mbps data transfer rate
Uses standard RJ-45 Ethernet jack
Cable length can go up to 100 m
Different network topologies can be achieved by
using switches

Conclusion
In industrial automation, PLC and SCADA can be communicated by
various type of adaptive communication methods
According to specifications of sensors, PLCs and the physical
media, a proper communication type can be selected

References
1. You, W., & Ge, H. (2019, October). Design and implementation of Modbus
protocol for intelligent building Security. In 2019 IEEE 19th International
Conference on Communication Technology (ICCT) (pp. 420-423). IEEE.
2. Xu, J., & Fang, Y. J. (2004, June). Profibus automation technology and its
application in DP slave development. In International Conference on
Information Acquisition, 2004. Proceedings. (pp. 155-159). IEEE.
3. Lian, F. L., Moyne, J. R., & Tilbury, D. M. (2001). Performance evaluation of
control networks: Ethernet, ControlNet, and DeviceNet. IEEE control systems
magazine, 21(1), 66-83.
4. Muller, I., Netto, J. C., & Pereira, C. E. (2011). WirelessHART field
devices. IEEE Instrumentation & Measurement Magazine, 14(6), 20-25.
5. Li, H., Zhang, H., & Peng, D. (2011, December). Research and design of
Industrial Ethernet communication gateway on power station. In Proceedings
2011 International Conference on Transportation, Mechanical, and Electrical
Engineering (TMEE) (pp. 986-989). IEEE.
6. Yang, M., & Li, G. (2014, September). Analysis of PROFINET IO
communication protocol. In 2014 Fourth International Conference on
Instrumentation and Measurement, Computer, Communication and Control (pp.
945-949). IEEE.

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