ICA communications -A03
description
Transcript of ICA communications -A03
SOUD1288-‐A03 ICA Communications
T.Littlejohns
30148554
7/5/15
Task 1: Communications terminology
1) Explain how 4-‐20mA current loops are used in an industrial application. �Describe the �advantages and disadvantages �
A 4-‐20mA current loop interface is a standard used for transmitting analog measurement signals between the sensor and measurement device. They are used widely in process automation.
The sensor is connected to process controlling equipment, which reads the sensor value and supplies a voltage to the loop where the sensor is connected, from here it can read the amount of current drawn.
(Acromag 2014).
The supply voltage required for this is in the region of 12 to 24 Volts through a resistor and the measured output is the voltage drop across that resistor converted into current. The typical value of the resistor is 250ohm.
The Advantages of a 4-‐20mA current loop
The most significant advantage of a 4-‐20mA Signal transmission is the current loop’s low sensitivity to electrical noise.
The current loop is designed so that a sensor takes 4 mA (min) and 20 mA (max). The sensor will always pass at least 4 mA current representing a live zero value, allowing controller’s to detect wire breaks if 0mA is detected.
There is improved signal to noise ratio, at low levels -‐ accurately distinguishing low signal levels without added noise or interference.
Many sensor types can be made to be powered from only that loop current, it requires less power allowing greater distances to be achieved (up to 1000m).
The 4-‐20mA current loop continues to be supported by thousands of compatible devices including wireless transducer’s.
The Disadvantages of a 4-‐20mA current loop
A 4-‐20mA current loop consumes high amounts of power compared to other analogue signal types. Supply not isolated from output. Increasing circuit load resistance will reduce the supply voltage available to power the transmitter that is generating the 4-‐20mA signal.
Increased sensor complexity, slower response and less accuracy -‐ particularly if you don't control the value of the 250ohm sensing resistor. 4-‐20mA current loops are not used or suitable for high-‐speed applications, limited data can be transmitted, due to the fact that by definition a current source has very high impedance.
2) Compare the advantages and disadvantages of transmitting signals in analogue and digital � forms
Advantages of digital transmission
Digital amplifiers regenerate an exact signal eliminating errors.
Voice, data and video can all by carried by digital circuits.
Better integration if all signals are in one form. Can integrate voice, video and digital data.
Digital Repeaters take out problems in transmission, therefore transmit longer distances -‐ increasing integrity.
Disadvantages of digital transmission
Digital transmissions require processing capacity to refine digital signals which in itself is an analog signal. It reflects in quantity and quality of computers and connections between them.
Generally digital transmissions require higher running costs with lots of factors to consider such as shutdowns.
Needs sufficient electric power to compensate for loss in voltage etc.
Digital transmissions are vulnerable to cyber attacks.
Not suitable for signals passing through hundreds of meter’s of cable due to the length of wires and loss in voltage signals.
All digital interference that occurs is to some extent unpredictable compared to analog, which can be eliminated with shielding.
Advantages of analog transmission
A good example is a phone system which carries a bandwidth of about 3 kHz. The system is limited to only pass 0-‐3kHz -‐ the spectrum that covers the most important frequencies of human voice.
The voltage drop in the interconnecting wiring does not affect accuracy of the signal which allows the signal transmission to occur over long distances, compared to voltage signals which will always have signal loss related to the length of the wires.
Receiver devices can be connected in series within the loop without a loss of signal.
Disadvantages of analog transmission
An incoming (analog) signal is sampled, its value is determined, and the node then generates a new signal from the bit value, the incoming signal is discarded.
Analog circuits nodes can amplify the incoming signal, including noise, the signal is copied and re-‐copied, or transmitted over long distances, leading to high error rate.
Analogue circuits require amplifiers. Each amplifier adds distortion and noise to the signal creating errors.
3) Explain the differences between a “Smart device” and a “dumb device”
“Smart devices”
A “smart device” is a monitor that also has its own processing power for special features, controls and is connected to the internet or to your wireless home network using Wi-‐Fi or Bluetooth. “Smart devices” are constantly talking to another device, moving data. Some of these devices can be vulnerable to cyber attack as many new smart devices
such as smart watches and fitness tracking devices lack the ability to carry security software. These are commonly called “dumb smart devices” and fall under the internet of things. “Dumb device”
Thermostats are a typically “dumb device” (i.e. not connected to the internet or your home network) with the addition of sensors and CPU processing power installed into the thermostat it turns into a smart device. Its function is heavily dependent on a connection to the Internet.
Another example of a “dumb device” is a dumb phone i.e not a smart phone that can download apps, using data from the internet on a constant basis. Dumb phones can’t download apps and software, they just place calls and text messages maybe with the occasional game of snake on your Nokia 3210.
4) Explain the purpose of the 7-‐layer OSI Communications Mode
An OSI model describes a data flow in a network from the physical connections (lower level) up to the users levels containing applications.
Data is passed from layer to layer, each one communicating with the levels above and below. Each layer is written for efficiency and the software is a streamlined component -‐ referred to as packets of data.
When a layer receives a packet of information the destination address is checked (looking for its own address), each packet is layered with protocols from each of the layers as it’s processed. If it is not there the packet gets passed to the next layer and the process continues.
Layering protocols around package is called encapsulation.
A transport layer is designed to communicate with another transport layer on another computer on the network.
The transport layer has no care for how the communication gets through lower layers from the first computer across media and through the second computers lower levels.
The seven layers are described below.
(Lewis 2004).
Application (Top) layer For applications access network services that directly support software for file transfers, electronic mail, database access. Presentation layer
Takes data from the application layer and converts/translates into an intermediary format. This layer can be used for data encryption by providing services for security and also compresses data so that fewer bits need to be transferred across the network. Session layer Allows two applications on different computers to establish, use and end a particular session. This layer also establishes dialogue between the two computers in that session, also regulating the length of time it transmits for and which side transmits. Transport layer Deals with error recognition and recovery. It can also repackage long messages when necessary breaking the message into small packets for transmission, then once at the receiving end, rebuilds the packets into the original message. Also sends receipt acknowledgments when receiving. Network layer Translates logical addresses and names into physical addresses when addressing messages. It determines the route from the source to the destination computer by switching, re-‐routing and controlling the congestion/traffic of data packets. Data Link layer Takes raw bit packages from the physical layer and places into logical, structured packets for data often classed as frames or encapsulated packets. Then transferring frames from one computer to another without errors. When the frame is sent it waits for an acknowledgment from the receiving computer. Physical (bottom) layer
Takes bits from one computer and transmits to another -‐ also regulating the transmission of a stream of bits over a physical medium and what transmission technique is used to send data over the cable. This layer defines how the network cable is attached to the network adapter.
5) Explain what is meant by synchronous and asynchronous communications and which of these �you find typically used within industrial networks.
Synchronous communications
Synchronous data communications means that all data bits are aligned with the clock at the receiving and transmitting ends therefore transmitting and receiving at the same speeds, all data is sent at one time with no packet switching meaning all data travels on one route to its destination.
Synchronous data communications are typically used over vast cable lengths. The digital telephone network uses synchronous data transmission.
Asynchronous Communications
Asynchronous communications commonly used by terminal, com, and modem programs and are used to communicate with host computers.
Asynchronous means that the data bits are not locked to a clock edge on the receiving end, the bits are timed at the transmitter clock from the sender end meaning it transmits and receives at different speeds, making them synchronous with the source clock. Asynchronous transmissions allow packet switching meaning the packets can take different routes to the same destination.
Asynchronous allows for purchase of greater bandwidth for your money but at a cost of lower upload speeds, this allows for bandwidth sharing i.e. someone surfing the net while another is talking to a colleague on the phone. The most common communication method used in industrial networks is asynchronous transmission such as Ethernet. This has become the standard for industrial networks, linking layers and allowing them to communicate easily with each other while keeping costs down .
Asynchronous Ethernet is generally used over shorter runs of around 100m it is used in a wide range of environments from industrial to office, allowing seamless communications. (Djiev 2014)
6)Explain what is meant by “Channel Arbitration”. Your answer should include two examples as �well as the advantages and disadvantages of each. �
Channel arbitration is used for connections desiring priority access over a communication channel, this includes detecting a connection therefore giving it priority through the communication channel and assigning the connection to a group having specific arbitration parameters.
The arbitration parameters are configured to dynamically change for every packet within the time slice, making it very efficient when managing available channel bandwidth.
Data is then transmitted using the arbitration parameters of the group.
One example is CAN (Controller Area Network) protocol developed by Bosch which is bus based CSMA/CA (carrier sense multiple access, collision avoidance protocol).
CAN messages consist of 6 fields -‐ the first field being a 32-‐bit arbitration field the message identifier, then a control field followed by a data field of between 0-‐64 bits. The first three fields are protected by a 16-‐bit cycle redundancy check (CRC) detecting errors. The fields after this can be used for immediate acknowledgement messages.
CAN itself is a low level message arbitration protocol, that uses logic and assumes a presence of recessive and dominant state on the bus, allowing it to overwrite the recessive state. The dominant state would be coded with 0, and 1 coded into the recessive state.
When a message is intended to be sent by a node it puts the first bit from the arbitration field commonly known as the message identifier onto the channel. The node with 0 in its first identifier always wins therefore making the one at 1 back off.
The process of arbitration continues for all bits in that particular arbitration field. A node for example with all 0’s as its bit pattern has the highest priority message. CAN message priority is therefore determined by the message identifier.
Advantages of CAN
Eliminates large and expensive wiring harness in the automotive industry and automated equipment in factories, the wiring can be done with software reducing labour cost for fault finding in the loom.
CAN is robust. In extremes network errors are very low statistically less than 1 faulty message per century.
Disadvantages of CAN
CAN signal rate decreases when transmission distance increases, steady-‐state losses may become a factor at the longest transmission distances, limiting signal rate as cable length is increased are time varying.
Cable bandwidth limitations, which degrade the signal transition time and introduce ISI (inter-‐symbol interference) therefore reducing the achievable signal rate when transmission distance is increased.
CAN also requires additional software layers on top. CAN is implemented onto inexpensive chips produced by lots of vendors, the chips require an additional layer of software for a fully functional network protocol, typically higher level protocols are added like DeviceNet these are more sophisticated versions for CAN messages and are well suited for use in automation.
Corrigan (2008)
Another example is ARCNET which was developed in the 1970’s for the office automation networks, it is now more commonly used in the automation control industry being well suited to control applications.
The arbitration methods used in ARCNET and the key to its success is its token-‐passing protocol, when using a token passing network a node can only message when it receives the token, at this moment it becomes the MASTER of the network. The size of the message is limited, therefore no one node can dominate the network as it needs to pass on the token as soon as a message is sent making the time performance predictable, allowing for calculation of the time taken to pass the token.
A token or invitation to transmit (ITT) is a sequence that is passed among all active nodes. Once received by the node it has sole right to initiate the transmission sequence or it must pass to its neighbour, the neighbour will have the highest address to the node with the token, this sequence continues serving all nodes equally.
In the transmission process the node with the token becomes the source node, the destination node is whichever node is selected for communication.
ARCNET reconfigures the network if a node is added or removed, for example when a node is added and is not automatically included in the passing or reviving of the token it will jam the network with reconfiguration burst, destroying the token, the process then starts again until a node responds. With ARCNET this is done very quickly without software intervention.
Advantages of ARCNET
Very robust -‐ can span over long distances making it ideal for control technology which requires messages to be delivered quickly and in predictable time.
Packet lengths are variable from 0-‐507 bytes with very little overhead and it has a very quick response time to short messages.
Built in CRC-‐16 error checking.
Protocols are built into the controller chip network configurations, error checking flow control are carried out automatically, eliminating the need for software.
Supports several cable schemes such as fibre optics.
Disadvantages of ARCNET
Designed in the 1970’s no longer commonly used except in control applications.
ARCNET can only send about one-‐third in packets compared to Ethernet. Typically 508 bytes in length this is smaller than the internet’s bare minimum of 512Mbps.
Slower than Ethernet at 2.5Mbps.
The number of ISU (information symbol Unit) can range from 0 to 252 in short packet mode and 256 to 507 in long packet mode. Packets which contain 253, 254 or 255 ISU’s cannot be sent. Packets of this size are called exception packets and must be padded with null data and sent as a long packet.
Distance constraints due to time delay/timeouts of nodes.
(Arcnet 1999 and Thomas 1999)
Task 2: Selecting an industrial network technology
7) Explain five key considerations when deciding on which communications protocol to use for �an industrial control application.
When choosing a networking protocol there are a number of factors you should consider and evaluate for your specific application. 1:Communication type Command or Message-‐Based Communication Command or message-‐based communication is infrequent information transfer triggered by a specific event. Streaming/Buffered Communication When streaming data, large amounts of information are sent continuously but not necessarily in real time. This is useful when you want to send lots of data and you need to capture each specific data point. Process Data Communication Process data communication consists of periodic transfer of the latest values of process variables. This is the most common communication for control applications.
2: System configuration Each type of communication involves targets and hosts these can be arranged in different combinations. (1:1) single target and single host (1:N) single target and multiple hosts (N:1) multiple targets and a single host 3: Performance This will be dictated by your specific requirements of your application. Each different type of communication offers different performance for that application. 4: Ease of use This depends on your previous programming experience for that type of system or application, for example TCP and UDP Internet protocols are low-‐level building blocks offering high flexibility when programming. 5: Supported 3rd party APIs When developing your application to communicate with 3rd party applications developed with C or VB, you will need to use networking protocols that interface with 3rd party APIs. (National Instruments 2013)
8)With reference to these five key considerations, explain the differences between Modbus and Profibus networking protocols.
1:Communication type Modbus was designed as a request-‐response protocol and initially was intended to transfer data over a serial layer. It now includes implementations over serial, TCP/IP, and the user datagram protocol (UDP). The device requesting the information is called the Modbus Master and the devices supplying information are Modbus Slaves. It is a serial transmission technique, which uses master-‐slave arbitration. A master can communicate half-‐duplex style with up to 247 slaves. Profibus is a field bus protocol, it is also a master-‐slave type protocol like Modbus but with an additional token ring protocol to allow for multiple masters. Also, unlike Modbus, all devices go through a startup sequence during which they “join” the network.
(Powell 2013) 2: System configuration The main physical layer for Profibus DP is based on RS-‐485, which Modbus uses, in the case of Profibus, the Profibus specification extends the RS-‐485 specification. Modbus ASCII and RTU both typically use a RS-‐232 or RS-‐485 physical layer, but can also use other physical layers. Recommended Standards (RS) 232 and 485 were established physical layers when Modbus was first developed. 3: Performance Profibus handles large amounts of data at high speeds, and serves the needs of large installations. The physical layer was tightened up to require only two wires, with speeds as fast as 12 megabits per second. Because Modbus uses TCP/IP protocol for all messages it is slow compared to other Ethernet industrial protocols but is still fast enough for monitoring applications. 4:Ease of use Modbus is a very easy simple to use protocol. However, there is a fair amount of variation in the protocol itself and in its physical layer definition, which causes issues in multi-‐vendor applications. Profibus is a very strong protocol that was designed to automate large plants. It works extremely well in multi-‐vendor applications, and has highly detailed diagnostics. Modbus would be the easy solution when connecting a controller to one smart device, in a point-‐to-‐point configuration, where different vendors are included. For circumstances where there are more points, or in a hazardous situation, Profibus is a better answer. 5: Supported 3rd party APIs The Profibus specification also standardized there connectors to be used, this is beneficial when working with 3rd party APIs – wiring is easy and consistent. Modbus SunSpec is an open communications protocol used with Fronius products which inverts into third party systems via Modbus.
Word count
Acromag (2014) Acromag introduction to two wire transmitters [online] Available at: http://www.acromag.com/sites/default/files/Acromag_Intro_TwoWire_Transmitters_4_20mA_Current_Loop_904A.pdf Accessed 5/5/15
Arcnet (1999) Arcnet tutorial [online] Available at: http://www.ccontrols.com/pdf/Tutorial.pdf Accessed 3/5/15
Corrigan S (2008) Controller area network physical layer requirements [online] Available at: http://www.ti.com/lit/an/slla270/slla270.pdf Accessed 3/5/15
Djiev S (2014) Industrial networks for communication and control [online] Available at: http://anp.tu-‐sofia.bg/djiev/PDF files/Industrial Networks.pdf Accessed 5/5/15
Kopetz H (2011) Design principles for distributed embedded applications 2nd Ed. Springer. New York.
Lewis D (2004) James Bond meets the 7 layer OSI model [online] Available at: http://www.lewistech.com/rlewis/Resources/jamesX.aspx Accessed 5/5/15
National instruments (2013) Using the right networking protocol [online] Available at: http://www.ni.com/white-‐paper/12079/en/#toc2 Accessed 2/5/15
Powell J (2013) Profibus and Modbus: a comparison [online] Available at: http://www.automation.com/automation-‐news/article/profibus-‐and-‐modbus-‐a-‐comparison Accessed 6/5/15
Thomas G (1999) Ethernet ARCNET CAN proposed network hierarchy for open control [online] Available at: http://www.ccontrols.com/pdf/ENetwhtppr.pdf Accessed 2/5/15