Types of communications links: Copper wire, optical fiber, RF (radio-frequency).
DCS (Distributed Control System): Hardware and software based process control and data acquisition system that controls many processes through a plant by allowing PLCs that are distributed through the plant to communicate with each other.
SCADA (Supervisory Control and Data Acquisition): System that pulls info from remote terminal units out in plant and/or remote facilities back to a centralized master station where they can be monitored and control. It’s different from a DCS as SCADA normally receives the info at a centralized location.
Deterministic: There can be guarantee of a critical message being delivered within a determined time.
Non-deterministic: There cannot be guarantee of a critical message being delivered within a determined time.
Deterministic/Non-deterministic: Since packets of data that travel through Ethernet can take variable paths, there are variable times from the sending node to the receiving node.
Seven Layers of OSI Model (Please Do Not Throw Sausage Pizza Away): 1. Physical 2. Data Link 3. Network 4. Transport 5. Session 6. Presentation 7. Application
Troubleshooting Communication System: Ensure all equipment is powered, check all cables and connections, use fault isolation lights or codes, start to isolate parts of the network to determine location of fault(s).
Noise: Caused by any device that produces spikes in voltage or current.
Sources of Electrical Noise:Large electrical motors being turned on, fluorescent lighting tubes, lightning strikes, welding equipment, high voltage surging due to electrical faults.
Four Forms of Coupling of Electrical Noise Into the Communication Circuits: Impedance coupling, electrostatic coupling, magnetic or inductive coupling, radio frequency radiation.
Minimize Noise From Coupling to Communication Cables Depending On The Type Of Noise: Impedance: Run a balanced circuit with separate returns for each signal. Electrostatic: use shielded cables; remove from the source noise; reduce the noise; twisting of the single wires. Magnetic: twist the signal wires. RF: use properly grounded shielded cable; use capacitors to shunt the noise to ground.
Grounding Importance For Communication Circuits: Minimizes electrical noise in the system, reduces the effects of fault or earth loop currents on the instrumentation system, minimizes high voltage on equipment due to electrical faults
What is EIA-232 and why was it developed? Standard interface for Data Terminal Equipment (DTE) developed for communications between a DTE and a modem (DCE or Data Communications Equipment)
Define and Explain DTE: DTE is Data Terminal Equipment such as a computer terminal or printer.
Define and Explain DCE: DCE is Data Communications Equipment or Data Circuit Terminating Equipment such as a modem.
Difference between DTE and DCE: DTE and DCE are equipment that communicate with serial binary data. A DTE device communicates with a DCE device that communicates to another DCE device that communicates to a DTE device. In short, DTE > DCE > DCE > DTE or Computer > Modem > Modem > Computer.
BPS and Baud Rates: Per the book on NRZ coding, 1 baud equals 1 bit per second.
BPS and Baud Rates: Bit rate is number of bits (0’s and 1’s) transmitted per unit time while Baud Rate is the number of signal units transmitted per unit time that is needed to represent those bits. Bit Rate = Baud Rate x the number of bit per baud
Commonly Used Connector Types for EIA-232: DB-9 (commonly called a 9-pin) and DB-25 (commonly called a 25 pin). DB-9 is the industry standard to reduce the wastage of pins.
Least Number of Conductors Required for EIA-232 Communications: Three and they are 1. Transmitted Data 2. Received Data 3. Signal/Common Ground
What is the maximum number of transceivers you can have on a EIA-485 line? Up to 32 standard transceivers. Up to 64 transceivers with devices equivalent to ½ standard device. Up to 128 transceivers with devices equivalent to ¼ standard device.
What can be used if you need more than the maximum number of devices? A repeater
Explain what reflections or ringing is? Reflections, also called ringing, are caused by signal reflecting off the end of the wire and corrupting the signal.
How is ringing detected? By using an oscilloscope across the line.
How is ringing reduced or eliminated? By placing a 120 ohm terminating resistor at the end of each line.
What is a current loop? A current signal rather than a voltage signal employing separate pairs of wires for the transmitter current loop and receiver current loop.
How is data transferred on a current loop? A current level of 4 mA – 20 mA (or 0 mA – 60 mA) is used to indicate logic 1 and 0.
Describe what a serial interface converter is. Many industrial devices still use EIA-232 ports. It is necessary to use converters to interface a device to other physical interface standards.
Why are serial interface converters used? They help newer RS-422 and RS-485 devices interact with older RS-232 ports.
List two features of using opto-isolation for serial interface convertors. 1. Operating over longer distances 2. Reduce noise from surrounding lightning 3. Protect against high-voltage spikes, surges, and ground loops.
What is the main benefit of using fiber cable over copper cable when distance is not an issue? Fiber cables are not subject to electromagnetic interference.
Describe how the light is able to stay in the middle of the cable. The different index of refraction (IOR) between the center core and the outside cladding makes the light stay in the core as it’s reflected off the cladding.
Describe the different layers of construction of fiber cable. The glass fiber (core and cladding) is in the middle. On the outside of the glass is a protective coating and may be 900 micron tight buffer PVC or a 250 micron acrylate colorized coating. The fiber is then placed with other fibers inside a cable that can be constructed for different uses for inside buildings, outside underground, and aerial applications.
Describe both “loose tube” and “tight buffered” fiber. Loose Tube is where multiple fiber strands are placed in a hard plastic tube. Tight Buffered cable is where each strand of glass fiber is surrounded by a PVC jacket like insulation on a copper conductor.
What is the overall diameter of the glass (the core and the cladding)?125 microns
What is the overall diameter of the glass with coating (loose tube fiber)? 250 microns
What is the overall diameter of a tight buffered fiber? 900 microns
Typically how are the different frequencies of light indicated in fiber optic systems? By their wavelength.
Indicate the different core sizes for both single mode and multi mode fibers AND what their typical uses are. 50 and 62.5 microns is used for multimode applications limited by shorter distances. The larger the core the shorter the distance the light can travel. 8.5 micron (called 9 micron) core is used for long distance singlemode applications.
What are the common pieces of test equipment used to test and troubleshoot fiber? A power source, a power meter, an OTDR, a visible light source (continuity tester)
What company created Modbus protocol? Gould Modicon now Schneider Electric
Is Modbus an open or proprietary standard? Open
Why doesn’t Modbus specify a particular cable to use? Modbus is a protocol only. No physical (OSI layer 1) interface has been defined.
What is the difference between Modbus ASCII and RTU transmission modes. ASCII is typically used for testing and has a typical message that is twice as long as the length of an equivalent RTU message. RTU is compact and faster in a hexadecimal format and is used for normal operations.
Which Modbus transmission mode is preferred? RTU
What types of links is Modbus used over? EIA-232, EIA-485, TCP/IP over Ethernet
What is the most important troubleshooting tool for Modbus? The instruction manuals of various components involved
In the “Function Code 01” how is the response of both coils on indicated? • By the hexadecimal coil data 03 • 03 Hex is 11 in binary • 1 = On and 0 = Off
In the “Function Code 02” how is the response of both coils on indicated? • By the hexadecimal input data 02 • 02 Hex is 10 in binary • 1 = On and 0 = Off • The least significant bit is off and the next bit is on
In Modbus, what does slave and master mean? There is one master that queries all of the slave (devices). The slaves can only talk to a master.
CH 7, CH 9
How many devices can you have on a Modbus Plus segment? Up to 64 devices per network segment.
If you have more than the maximum number of devices, what would you use to connect those additional devices and how? You would connect multiple networks together using a bridge plus device
What is the reason Modbus Plus was developed to improve the Modbus protocol? To overcome the “single-master” limitation prevalent in the Modbus protocol
Is Modbus Plus an open or proprietary standard? Proprietary standard
What is the physical medium used for Modbus Plus? Twisted pair shielded cable which is 3-wires (one pair and a shield). The pair is usually made up of a 24 AWG wire.
What company created the Data Highway Plus protocol? Allen Bradley
What three layers of the OSI model does Data Highway Plus use? 1) physical 2) Data Link, 7) Application
What is the physical medium used for Data Highway Plus? Twin axial cable with three conductors
In Data Highway Plus, what is the benefit of each node being able to become a master? Efficiency. Once the node becomes a master, it can immediately transmit without checking for permission to transmit. This avoid network collisions
When using Modbus Plus, can you have duplicate addresses? No
CH 10, CH 11 (Week 7)
What does the acronym HART stand for? Highway Addressable Remote Transducer
When devices are communicating with the HART protocol, explain how the analog signals is carried The analog signal is a continuous value between 4-20 mA
When devices are communicating with the HART protocol, explain how the digital signals is carried The digital signal uses a FSK modulation to very between two frequencies, 2200 Hz = 0 and 1200 Hz = 1. Since the average current change between the digital signal is zero (AC) this doesn’t affect the DC signal
Example of an analog HART device Temperature transmitter/sensor —- also flow sensor – 4 mA may indicate no flow and 20 mA may indicate maximum flow. Using only 4-20 mA only one device can communicate the analog flow rate between no flow and max flow, such as 68% flow.
What OSI layers does the HART digital transmission use? 1) Physical, 2) Data Link and 7) Application
What does AS-i stand for? Actuator Sensor Interface
What type of cabling is required for AS-i? Two-wire untwisted, unshielded flat cable. For high noise environments there is a special shielded cable available.
What type of devices communicate using AS-i Binary sensors and actuators
What topology can AS-I be connected in All of the major topologies such us bus, ring, tree, star, etc.
When troubleshooting AS-I problems, what is a monitor used for? To diagnose and log operational, master and cable errors as well as maintenance related slave, master, and cable errors. Professor’s Answer: to capture and analyze the traffic on the AS-i bus
Midterm Study Guide
The Midterm is made up of 12 short answer questions worth 10 points each and one 30 point essay (150 to 300 words) for a total of 150 points. The questions will come from the readings in chapters 1 through 11 with many mirroring what was discussed in the weekly discussions and the weekly quizzes. Be sure to know the following:
• EIA-232 – Know what it is, how it works, and connector types
EIA-232 is a standard interface for Data Terminal Equipment (DTE) developed for communications between a DTE (computer terminal or printer) and a DCE (Data Communications Equipment) such as a modem. The commonly used connector types for EIA-232 are DB-9 (9-pin) and DB-25 (25-pin) with DB-9 being the industry standard to reduce the wastage of pins.
• Current Loop – How it works
Commonly used interface technique: by using a current signal rather than a voltage signal employing a separate pair of wires for the transmitter current loop and receiver current loop. A current level of 4 mA – 20 mA (or 0 mA – 60 mA) is used to indicate logic 1 and 0.
• Serial Interface Convertors – why do we use them
They help newer RS-422 and RS-485 devices interact with older RS-232 ports.
Permits “multidrop” network connection on 2 wires and allows reliable serial data communication for:
• Distances up to 1200 m (same as EIA-422)
• Data rates up to 10 Mbps (same as EIA-422)
• Up to 32 line drivers on the same line
• Up to 32 line receivers on the same line
• Maximum bit rate at 4000 ft is 96 kbps
Maximum number of transceivers:
Up to 32 standard.
Up to 64 with devices equivalent to ½ standard.
Up to 128 with devices equivalent to ¼ standard.
• Know what ringing is and how to eliminate it
Reflections (ringing) are caused by signal reflecting off the end of the wire and corrupting the signal. It’s detected using an oscilloscope across the line. It’s eliminated by placing a 120 ohm terminating resistor at the end of each line.
• Basic protocol troubleshooting steps
Ensure all equipment is powered, check all cables and connections, use fault isolation lights or codes, start to isolate parts of the network to determine location of fault(s). Look at instruction manuals of various components involved.
• Explain electrical noise and remedies
Noise is caused by any device that produces spikes in voltage or current such as large electrical motors being turned on, fluorescent lighting tubes, lightning strikes, welding equipment, high voltage surging due to electrical faults. Remedies: grounding
Impedance Noise: run a balanced circuit with separate returns for each signal.
Electrostatic Noise: use shielded cables, remove from the source noise, reduce the noise, twisting of the single wires
Magnetic Noise: twist the signal wires
RF Noise: use properly grounded shielded cable, use capacitors to shunt the noise to ground
• HART – know how both analog and digital signals work
(Highway Addressable Remote Transducer) Hybrid protocol that offers a reliable solution for plants that operate on traditional 4-20 mA analog wiring and are in need of digital communication. Allows digital information to be simultaneously communicated with the 4-20 mA signal. In short, offers digital transformation for analog systems.
The analog signal is a continuous value between 4-20 mA. The digital signal uses a FSK modulation to vary between two frequencies: 2200 Hz equals 0, 1200 Hz equals 1. Since the average current change between the digital signal is zero (AC) this doesn’t affect the DC signal. Analog HART device = temperature transmitter/sensor.
OSI layers – 1) physical 2) Data Link 7) Application
• AS-i – required cabling and types of devices
(Actuator Sensor Interface) – cabling: two-wire untwisted, unshielded flat cable that serves as both communications link and power supply. For noisy environments, there is a special shielded cable available. Types of devices: binary sensors and actuators
Topology: all major topologies: bus, ring, tree, star, etc.
Monitor: used to diagnose and log operational, master, cable errors by capturing and analyzing the traffic on the AS-i bus
• Modbus and Modbus Plus
and their differences
The Modbus protocol was created by Gould Modicon now Schneider Electric.
Modbus Modbus Plus
Protocol Only Software & Hardware
Serial (Master to Slave) Token Passing
Open Standard Proprietary Standard
RS232 2 devices and up to 247 with fiber 64 devices on a network segment
RS485/RS422 multidrop 32 devices sing netw
19.2 Kbaud speed 1Mbaud speed (1 Mbps)
RS-485 4000 ft distance RS-485 1500 ft distance
• Data Highway Plus
Allen Bradley, three layers used (1) Physical (2) Data Link (7) Application
Physical medium is twin axial cable with three conductors. Benefit of each node being able to become a master is efficiency. Once the node becomes a master, it can immediately transmit without checking for permission to transmit avoiding network collisions.
• Fiber Optics –
Construction of cable and its use
The glass fiber (core and cladding) is in the middle (125 microns). On the outside of the glass is a protective coating and may be 900 micron tight buffer PVC or a 250 micron (loose tube) acrylate colorized coating. The fiber is then placed with other fibers inside a cable that can be constructed of different uses for inside buildings, outside underground, and aerial applications. Core sizes: 50 or 62.5 microns for multimode limited by shorter distances. 9 microns used for singlemode. The larger the core, the shorter the distance the light can travel.
• OSI Layers
Please Do Not Throw Sausage Pizza Away
1) Physical 2) Data Link 3) Network 4) Transport 5) Session 6) Presentation 7) Application
• Know the different types of communication links
Copper wire, optical fiber, RF (radiofrequency)
• There will also be a question about your HMI programming experience.