Embedded_system_design_assignment 2

HCA3110 Embedded Distributed Systems – Distributed Processing in a box

University of Technology – Mauritius

SCHOOL OF INNOVATIVE TECHNOLOGIES & ENGINEERING

BEng (Hons.) Electronic Engineering & Computer Science – Part-Time (Upgrade)

Module Title: Embedded Systems and Design

Module Code: HCA3110

Module Cohort: BEngEE/10A/PT

Title: “Embedded Distributed Systems- Distributed processing in a box”

Academic Year 2010 – Semester 1 Assignment1

Submission and Presentation Date: 17.11.10

Name: KHODABUX Yaasir (100372)

BOODOO Sameer (100367)

PARAOUTY Shameer (100379)TABLE OF CONTENTS

Prepared by: Paraouty, Boodoo, Khodabux Page 1


1.0 OVERVIEW OF DISTRIBUTED PROCESSING............................................................................................3

1.1 Definition of embedded distributed system......................................................................................3

1.2 Applications of Distributed processing..............................................................................................3

1.3 Advantages and disadvantages of Distributed processing systems...................................................4

1.5 Selected Real-time example..............................................................................................................4

2.0 Communication of the PLC and the LAN network.................................................................................7

2.1 Hardware Components of a PLC System............................................................................................7

2.2 The communication component of the system.................................................................................8

2.3 The existing components of the system............................................................................................9

2.31 SOFREL PLC S50 Remote Terminal Unit and the SCADA software................................................9

2.32 Connectivity of the PLC and other related RTUs.........................................................................12

2.33 Communication Protocols..........................................................................................................14

3.0 The S50 System components and specification...................................................................................17

3.1 Specification of the PLC...................................................................................................................17

3.11 Additional boards.......................................................................................................................18

3.12 Communications boards...........................................................................................................18

3.2 Detailed view of the CPU motherboard...........................................................................................19

3.21 Serial daughterboards and Connectivity linked..........................................................................20

3.22 Connection using the Serial communication..............................................................................21

4.0 Important review of distributed processing........................................................................................23

4.1 Synchronous and asynchronous transmissions...............................................................................23

5.0 References:..........................................................................................................................................25



1.0 OVERVIEW OF DISTRIBUTED PROCESSING

1.1 Definition of embedded distributed system

A distributed system is a collection of independent computers that appear to the users of the system as a single coherent system. In another word it can also be defined as having more than one processor interconnected together used for performing one particular task or even several one.

In Distributed Processing, the controller elements are not the core location but rather the system is distributed throughout the system with each sub-system controlled by one or more processors and eventually the entire system is connected by a network for communication and monitoring purposes.

1.2 Applications of Distributed processing

The world wide web – information, resource sharing Clusters, Network of workstations Distributed manufacturing system (e.g.,automated assembly line) Network of branch office computers -Information system to handle automatic

processing of orders Network of embedded systems New Cell processor (PlayStation 3)



1.3 Advantages and disadvantages of Distributed processing systems

Advantages Description

Economics Less costly when intended to further expand the system (save Cost).

Speed Several nodes connected and no centralized domain for storage of data (several memories per nodes available).

Inherent distribution All the workstations are connected together and each

Reliability System is much more reliable since if one station failed there are still others from which information can be fetched from.

Incremental growth Easier to add other processor or systems.

Disadvantages

Software Many software is needed

Network Virus on network

More components to fail

1.5 Selected Real-time example

The wastewater system at Grand Baie Treatment Plant

The Grand Baie Treatment Plant is one of the three main Wastewater Treatment Plant found in Mauritius. The wastewater is treated up to the tertiary level at the Treatment Plant. Wastewater is collected from the Grand Baie Area which consists of inhabitation, bungalows, restaurants and hotels. There are a total of 19 Nos Pumping stations for



this purpose. They are all connected to a SCADA system posted at the Treatment Plant for Supervision and Emergency control. They communicate to the SCADA system through telemetry system and uses Dial-up communication.

At a pumping station there is an ultrasonic level transmitter for measuring the level (thus flow) of wastewater in the pump well. Normally float switches are used to run or stop a pump, but in case the run float switch has failed, the transmitter can also directly start a pump to prevent overflow.

Brief description of the Pumping Station



The wastewater is pumped from the wet well of the PS to the Treatment Plant. Actually there are two pumps installed in each of the Pumping Station which provide flow to the TP. The Pumps is being monitored from the monitor at the TP, in the SCADA room by two operators. This is realized by the used of PLC system and SCADA system. These are linked either through cables or wireless telemetry system.

The most important part of this system in our case is the communication type, the method used for the monitoring and controlling of the pumps on the PS. Actually the SCADA room is comprised of two Personal computer for data acquisition and monitoring by using distributed SCADA processing system i,e the pumps and other equipment can be controlled from any of the two monitor.

Distributed Processing involved in the automated wastewater system

As already described above the data acquisition, monitoring and controlling of the different component present are done by using the Programmable Logic Controller (PLC). The SCADA system is also being implemented for supervisory purposes. The data collection and processing, when performed by an individual controller, can burden the processor’s scanning time, consume large amount of memory, and complicate the control logic program and thus by using distributed processing minimizes these drawbacks.

Distributed control applications allocate control functions, once performed by asingle controller, among several controllers; this eliminates dependence on a single controller and improves performance and reliability. However to use the distributed processing approach, a LAN and the PLCs attached to it must provide the following functions:

communication between PLCs upload capability to a host computer from any PLC download capability from a host computer to any PLC reading/writing of I/O values and registers to any PLC monitoring of PLC status and control of PLC operation



2.0 Communication of the PLC and the LAN network

This chapter gives us a clear explanation on how the communication between the different hosts

is being performed. To have a better understanding, let us consider some important aspect in our

system first. Below is some basic explanation which will help us throughout this chapter.

2.1 Hardware Components of a PLC System

Central Processing Unit (CPU)CPU – Microprocessor based, may allow arithmetic operations, logic operators, block memory moves, computer interface, local area network, functions, etc.CPU makes a great number of check-ups of the PLC controller itself so eventual errors would be discovered early.

System Busses

The internal paths along which the digital signals flow within the PLC are called

busses.

The system has four busses:

- The CPU uses the data bus for sending data between the different elements,

- The address bus to send the addresses of locations for accessing stored data,

- The control bus for signals relating to internal control actions,

- The system bus is used for communications between the I/O ports and the I/O unit.

Memory

System (ROM) to give permanent storage for the operating system and the fixed data used by the

CPU.

RAM for data. This is where information is stored on the status of input and output devices and

the values of timers and counters and other internal devices. EPROM for ROM’s that can be

programmed and then the program made permanent.



I/O Sections

Inputs monitor field devices, such as switches and sensors.

Outputs control other devices, such as motors, pumps, solenoid valves, and lights.

Power Supply

Most PLC controllers work either at 24 VDC or 220 VAC. Some PLC controllers have electrical

supply as a separate module, while small and medium series already contain the supply module.

Programming Device

The programming device is used to enter the required program into the memory of the processor.

The program is developed in the programming device and then transferred to the memory unit of

the PLC.

2.2 The communication component of the system

The figure above best described the communication process between the different workstations. As illustrated, there is two substation and a master station from which all the monitoring and controlled is being carried out. The two substations mainly consist of the following component which makes the distributed processing possible:



PLC – this is where all the sensors and transducer are connected from the I/O module such as the Variable Speed Drive for speed control of the pump, the float switches for the Start and stop of the pump, the temperature sensor of the pump, the moisture detector of the pump, the pressure sensor for measuring of pump pressure, running of the generator and change-over module and contactors for the opening and closing of the gate valve and manual operation of the pump.

The autodialer and the PSTN Telco line – used in case of emergency. Data logger – This stored all the events occurred throughout the day and are

being transmitted to the SCADA found at the PCs for every Station. PC – this is where the SCADA software is installed for the monitoring of the data

from the Pumping station. The processor of the CPU communicates to the PLC processor either by a serial or parallel communication depending on the set-up.

Router – used for communication between the PC between the different Workstations.

Printer – this print the report for the events occurred throughout specified days.

2.3 The existing components of the system

The main important components of our distributed system is the PLC, the SCADA software, the PC and the Router for communication purposes. The system being a real one consist of real and existing components as listed below.

2.31 SOFREL PLC S50 Remote Terminal Unit and the SCADA software

Description

The SOFREL S50 Remote Terminal Unit is dedicated to the remote control and management of geographically widespread or isolated facilities.

RTU S50 contains the following plug-in boards:

• A Power Supply unit;



• A CPU board with memory, serial interface, i/o daughterboards, etc.

• Input/output boards;

• modem boards for communication over dial-up/dedicated lines;

• Serial interface boards;

• A voice board.

Function and specifications of the PLC

S50 is a self-contained RTU package allowing you:

• To continuously check the operational status of your remotely controlled facility;

• To be automatically notified of a failure or fault condition;

• To locally or remotely control your equipment;

• To log, record, and process operating data collected from your facility.

RTU S50 meets the requirements of any combination of separate or interconnected sites. RTU S50 offers a comprehensive set of features and tools combining high performance, open communications, flexibility, and ease-of-use. S50 is a fully user-configurable RTU that can be run on a PC running the SOFTOOLS software, either locally or remotely.

RTU S50 lets you view and print:

• Mimic diagrams of the remotely controlled facility;

• Alarm and event logs;

• Historical records of operating data;

• Operating data reports.

In local mode, S50 connects to:

• A PC running SOFTOOLS;



• PLCs (Programmable Logic Controllers)

• a local display (direct viewing, alarm acknowledgement, etc.)

• one or several badge readers (access control),

In remote mode, S50 can communicate with:

• A PC running SOFTOOLS;

• A standard DTMF telephone (using SOFREL ‘VOICE SERVER’);

• Pagers;

• Mobile telephones;

• Other SOFREL RTUs (via SOFREL ‘INTER-RTU’ connections);

• A SOFREL CS100 front-end processor, and

• SCADA Master Stations and supervisory control host devices.

Communications media used by RTU S50 include:

• The Public Switched Telephone Network, referred to as ‘Dial-up phone network’.

• Mobile telephone networks;

• Radio links;

• Dedicated lines

• RS-232 and RS-485 serial links.



2.32 Connectivity of the PLC and other related RTUs.

The PLC S50 communicates with the SCADA master stations or supervisory control host computers. SCADA master stations or supervisory control hosts are used to centralize RTU calls and poll RTUs automatically. The data collected by the RTUs are logged to disk for retrieval and further processing using popular spreadsheet packages (EXCEL, and the like). Data may be displayed in graphic or tabular format (mimic diagrams, trend charts, tabular reports, etc.). Many commercially available process control systems are compatible with S50.

Communication with the HMI display

Information may be viewed directly on the remote display connected to the S50 terminal and the following operations may be carried out:

• viewing the alarm log,

• Global alarm acknowledgement,

• Consultation of datapoint groups,

• positioning setpoints (DO, AO),

• initializing counters (CI)



Remotely controlled facility

Communication with other RTUs

Using ‘Inter-RTU’ links, RTU S50 can directly call one or more SOFREL RTUs to transfer data and control remote sites.

Over a radio link, a single frequency can be used. RTU S50 can also serve as a “relay station” for communicating with other stations.

Radio communication

The S50's radio functions offer a wide range of possibilities; they are compatible with traditional radio sets as well as with the new radio sets that do not require a license for use.

• The « Radio relay» function makes it possible for the Master station to communicate with Station B via Station A, in which case the latter behaves transparently.



• With the « Configuration download », station configurations can be downloaded by radio (Reading/Writing), including for the relayed stations.

• The « Spontaneous call » function means that in the event of an alarm or event, an instant values message can be sent directly to the Master station or to another RTU (relayed or not).

Communication with other PLC and Interfacing with Printers

S50 can interact with PLCs or supervisory control software systems using the ‘SERIAL connection’ function supporting various communications protocols. Similarly the RS 232 can be used for interfacing the Printer with the RTUs.

2.33 Communication Protocols

Communicating with Master Stations

RTU S50 supports major PLC communications protocols in addition to SOFREL’s proprietary SOFBUS. SOFBUS is based on the industry-standard MODBUS protocol complemented with several layers allowing:

• Communications over any type of media (fixed or mobile phone network, radio, etc.),

• Transmission of time-stamped values (historical measurement data and alarms,etc.), in point-to-point mode;



• Transfer of data to a “relay station” (for radio networks).

Communicating with PLCs

RTU S50 can serve as a SCADA interface for monitoring and controlling one or more PLCs. Supported protocols include:

• MODBUS, in MASTER mode;

• MODBUS, in SLAVE mode; and

• TELEMECANIQUE, in UNI-TELWAY mode.

RTU S50 handles:

• Two RS-232C/RS-485 serial links for communicating with ten MODBUS slaves; and

• One RS-485 serial link for communicating with ten TELEMECANIQUE (UNITELWAY) servers.

Communications between S50 type RTUs

Using the ‘INTER-SITE feature, RTU S50 can exchange snapshot data directly with one or more other SOFREL RTUs.

This allows S50 to send its own control/setpoint and status data to remote sites and retrieve the process data from these sites.

Inter-site data exchange is performed through polls or can be initiated by DI state changes.

Over a RADIO link, RTU S50 can spontaneously transmit current data to other stations, using a single frequency for several stations. S50 can also be used as a “relay station” to pass data along other stations.

INTER-SITE connections can be simultaneously performed via several communications boards, which may be linked up to external modems (for radio or mobile phone communications).



3.0 The S50 System components and specification



3.1 Specification of the PLC

The communication must be there for the distributed processing to take place. So far we have been acquaint with different communication and connectivity which can be performed by the PLC , PC and the SCADA system. Technically speaking, the PC and the PLC are two separate stand alone embedded components. For the communication to be successful there must be a proper synchronization between them.

RTU S50 consists of several electronic boards, which can be easily removed or inserted into their slide rails. The chassis must be housed in a metal enclosure for wall mounting, or snapped into a DIN rail within an equivalent metal cabinet.



3.11 Additional boards

Various add-ins can be plugged onto the CPU:

• One SERIAL daughterboard for serial RS-232 / RS-485 connections

• One or two I/O daughterboards among the following options:

- One ‘FAULT option’ board;

- One or two ‘Digital Output option’ boards (STOR/DO),

- One or two ‘Analog Input option’ boards (EANA/AI).

3.12 Communications boards

For data exchanges, S50 handles up to four communications boards:

• One DIAL-UP MODEM board;



• One to four DEDICATED LINE MODEM boards;

• One to four SCOM boards fitted with the ‘SERIAL’ daughterboard;

• One VOICE board for restoring messages over the phone.

3.2 Detailed view of the CPU motherboard



3.21 Serial daughterboards and Connectivity linked

The CPU board consists of serial daughterboards which are used for the communication of the different components of the system. Below is some brief description of the linked process. The SERIAL-A daughterboard provides an RS-232C link and a nonisolated RS-485 link. The RS-485 is used for communication between the PLC itself while the RS 232 is used for connection between the PLC and the PC .

RS-232 transmission

An RS-232 link typically requires a twisted-pair cable with a shield or screen for each pair of stranded wires.

The cable must be connected to earth on only one side. The maximum length for an RS-232 cable is 15 meters.

RS-485 transmission

An RS-485 link typically requires a shielded or screened twisted-pair cable. The cable must be wired to earth on one side only. Polarities must be observed.

The RS-485 link for communicating with S50 I/O is not electrically isolated. The following rules are to be applied:

• The RS-485 bus must have a line structure (i.e., no ring, no star topology);

• Branches from the main bus must be kept as short as possible (less than 10 meters);

• The bus must be properly terminated to prevent reflections and suppress interference.

A common method is to install a jumper-selectable 120-ohm terminating resistor on the device located at the end of the bus line. To do this, set jumper S2 to ‘A’ on the CPU motherboard, and jumper S3 to ‘B’ on an I/O board.



3.22 Connection using the Serial communication

Connecting two RTUs via RS 485

The diagram above illustrates how the communication is being performed between two RTUs found in different Pumping stations by using the RS 485 cable.





4.0 Important review of distributed processing

4.1 Synchronous and asynchronous transmissions

Synchronous and asynchronous transmissions are two different methods of transmission synchronization. Synchronous transmissions are synchronized by an external clock, while asynchronous transmissions are synchronized by special signals along the transmission medium.

Like any data transfer methods, Serial Communication also requires coordination between the sender and receiver. For example, when to start the transmission and when to end it, when one particular bit or byte ends and another begins, when the receiver's capacity has been exceeded, and so on. Here comes the need for synchronization between the sender and the receiver. A protocol defines the specific methods of coordinating transmission between a sender and receiver. Let us take an example. A serial data signal between two PCs must have individual bits and bytes that the receiving PC can distinguish. If it doesn't, then the receiving PC can't tell where one byte ends and the next one begin or where one bit ends and begins. So the signal must be synchronized in such a way that the receiver can distinguish the bits and bytes as the transmitter intends them to be distinguished.There are two ways to synchronize the two ends of the communication namely synchronous and asynchronous. The synchronous signaling methods use two different signals. A pulse on one signal line indicates when another bit of information is ready on the other signal line. The asynchronous signaling methods use only one signal. The receiver uses transitions on that signal to figure out the transmitter bit rate (known as auto baud) and timing, and set a local clock to the proper timing, typically using a PLL to synchronize with the transmission rate. A pulse from the local clock indicates when another bit is ready. That means synchronous transmissions use an external clock, while asynchronous transmissions are use special signals along the transmission medium.( Refer Fig1.a)Asynchronous communication is the commonly prevailing communication method in the personal computer industry, due to the reason that it is easier to implement and has the unique advantage that bytes can be sent whenever they are ready, an no need to wait for blocks of data to accumulate. Mode of connection: -In a simple connection, the hardware configuration is such that only one-way communication is possible. For example, from a computer to printer that cannot send


http://en.wikipedia.org/wiki/Synchronization#Communication

http://en.wikipedia.org/wiki/Data_transmission


status signals back to the computer. In a half-duplex connection, two-way transfer of data is possible, but only in one direction at a time. But in a full-duplex configuration, both ends can send and receive data simultaneously, which technique is common in our PCs.

Synchronous transmission - a brief explanation: In synchronous transmission, the stream of data to be transferred is encoded and sent on one line, and a periodic pulse of voltage which is often called the "clock" or "strobe" is put on another line, that tells the receiver about the beginning and the ending of each bit (or byte). In general, such synchronous transmission protocols are used for all the types of parallel communications. For example, in a computer, address information is transmitted synchronously, i.e.: the address bits over the address bus, and the read strobe in the control bus.Synchronization can also be embedded into a signal on a single wire. In differential Manchester encoding, used in video-tape systems, each transition from a low to high or high to low represents a logical zero. A logical one is indicated when there are two transitions in the same time frame.



5.0 References:

Lecture notes

http://www.sofrel.com/

http://arxiv.org/abs/1010.4065

http://www.lesman.com/unleashd/catalog /sensors_multiranger.html


Embedded_system_design_assignment 2

Documents

plc system

wastewater system

manufacturing system

entire system

s50 system components

single coherent system

network of workstations

lan network