PLCs
What is protocol?
A protocol is a set of RULES that defines how the computer will manage their communications. The protocol may specify how data is formatted for transmitting and when and how each node may transmit.
A/D conversion?
A/D conversion if the process of taking an analog value and converting it to a PROPORTIONAL digital value.
By what means does a computer select a byte of information out of a memory chip?
Address
List the three busses found on a computer system.
Address, Data, Control
What is the advantage of parallel communications over serial communications? Disadvantage?
Advantage - more data transmitted at a time. Disadvantage - More susceptible to noise - cabling and connections are more expensive
What are the advantages and disadvantages of ECL.
Advantage: Fast Disadvantage: Power
What is REM?
Allows the processor mode to be changed remotely from the programming or operator station. The available modes are REM/PROG, REM/RUN and REM/TEST.
What is PROG?
Allows the user to edit PLC program or to download programs from disk on the programming terminal to the PLC.
What is a stop bit? (TTL)
Always a mark (1). Ends frame and allows time to recover for next data byte.
The data type INT contains how many bits of memory?
An INT can store up to 16 bits
Step Size formula?
Analog Span / Digital Span = Analog Span / 2^n-1 where n = bit resolution (3 bits)
Define SIGNAL LOSS PER UNIT DISTANCE and give the correct unit?
Antenuation - signal (power) loss - expressed in dB/kilometres
What is Idle?
Assumes a mark (1) when line not in use, also referred to as a Stop.
What is the difference between BPS and BAUD?
BPS - Bits per second, Band - Data transitions per second.
On a controller what type of memory should be used to store control parameter data?
Battery backed up CMOS
What is Bd?
Baud or Baud Rate. The number of discrete signal events per second occurring on a communication channel. Normally this is the same as BPS but often incorrectly used. May very for some MODELM
What is BPS?
Bits Per Second. The number of bits transmitted in on e second. Example: 1200 BPS, 1 bit every 1/1200=843uS
What is Full-Duplex?
Both devices send and receive at the same time. Two channels. This is the same if one was to take simplex connections in the opposite direction to one another.
What is Half-Duplex?
Both devices send and receive information. ONly one can transmit at a time. One channel.
What does the term ANALOG mean when referring to data communications?
Broadband - continuous voltages - sinusoidal wave form
What is Information Level?
Buses are high level, high-speed networks used to connect engineering and plant management offices together. Ethernet is a common example of this type of network.
List the five components of a microcomputer.
CPU, input, output, memory, storage
Describe the characteristics of a modem RS-232C DTE/DCE interface (ie. Voltage levels, pin out ect.)
+15V -> +3V = Space -15V -> -3V = Mark
What voltage range indicates a space for RS-232?
+3 to +15V
Structured Text represents?
- Complex mathematical operations - Specialized array or table loop processing - ASCII string handling or protocol processing
Ladder Logic represents?
- Continuous or parallel execution of multiple operations - Boolean or bit-based operations - complex logical operations
Function Block Diagrams represents?
- Continuous process and drive control - Loop control - Calculations in circuit flow
Sequential Function Charts (SFC) represents?
- High-level management of multiple operations - Repetitive sequences of operations - Batch process
What are Check Sums?
1) Longitudinal Redundancy Checking (LRC) - a parity done on a block of data one bit for each bit position. 2) Logical or Arithmetric Checksums. The data is added together and the result is sent as a data byte at the end (only LSB)
What two things does the the controller memory store?
1) The state of the inputs and outputs 2) The logic program that controls the outputs
Plant systems can be divided into 3 main categories of communication levels?
1) information level networks 2) control level networks 3) field device level networks
List four tasks performed by a discrete input module?
1. Sense when a signal is received from a field device. 2. Convert the input signal to the correct voltage level for the particular PLC. 3. Isolate the PLC from fluctuations in the input signal's voltage or current. 4. Send a signal to the processor indicating which sensor originated the signal.
List the speed and distance limitations for RS-422?
10Mbps -> 10M or 100Kbps 4000 ft
List the speed and distance limitations for RS-449?
10Mbps -> 10M or 100Kbps 4000 ft
Data rate speed?
19200 BPS = 50 ft
List the speed and distance limitations for RS232-C?
20Kbps -> 15M
What is Start bit? (TTL)
A (0) referred to as a space. Starts the timing clock on the transmission so that the first data bit is received in 1.5 bits time.
The data type DINT contains how many bits of memory?
A DINT can store up to 32 bits
The data type SINT contains how many bits of memory?
A SINT can store up to 8 bits
What type of communications uses different frequencies to indicate ones and zeros?
FSK
Describe the signal produced by a Bell 202 modem?
FSK - Frequency shift keying
Define FSK, PSK, ASK?
FSK - frequency shift keying PSK - phase shift keying ASK - amplitude shift keying
RS-232 serial communications always use asynchronous signals methods?
False
What type of hardwire communication media has the highest through put?
Fiber Optic
Forcing Logic Elements?
Forcing can be used to override logic that your program has produced or is using. Forces may be used in the following situations. - test and debug logic - check wiring to output devices - to temporarily keep program or process running when servicing a field device
What is the difference between full and half duplex?
Full- Simultaneous communications in both directions Half - Communication Alternates, one direction at a time.
What does fan out tell us?
How many input and output can drive.
What is Asynchronous Transmission?
In asynchronous transmission, the link doesn't include a clock line, because each end of the link provides its own clock. Each transmitted byte includes a Start bit to synchronize the clocks, the one or more Stop bits to signal the end of the transmitted word (not common).
What is Synchronous Transmission?
In synchronous transmission, all devices use a common clock generated by one of the devices or an external source.
What do the status lights do?
Indicate the PLC's current operating mode.
Why do electrical data signals degrade as the transmission line length increases?
LRC model characteristics of copper wire
When is a modem required to transmit data signals?
Long distances, telephone lines, radio transmission.
What is a Transmission Channel?
Medium which connects the two devices together. Examples: wire, fibre-optic, radio. CONNECTS!!
For radio networks, line of sight must be used for what frequency band?
Microwave
What is Control Level Network?
Networks are high speed networks designed specifically for industrial control applications, and transmit data between controllers and operation stations. Examples of this network type include PROFIBUS FMS and ControlNet.
What is Field Device Level Network?
Networks designed specifically for communications from (and sometimes between) controllers to field devices such as transmitters, sensors and valves. Examples are Hart, Foundation Fieldbus, Honeywell DE, PROFIBUS, DeviceNet and ASi.
List the steps required for a shift register to convert 8 bit parallel data into serial data.
One Parallel Load and then 7 Shifts Out
What is Simplex?
One device sends, the other receives. One direction only. One channel. ONE WAY!!!
What is the purpose of the program counter?
Points to the address of the next instruction.
What does the Input Module do?
Provide an interface and voltage isolation between the input field device signal and the processor. Input modules READ the status of the input device and WRITE data to the processor.
What does the Output Module do?
Provides an interface and voltage isolation from the processor to control an output field device. Output Modules READ data from the processor and WRITE the data to the output device.
Which serial communications standard is the most common?
RS-232E
Which standard(s) use + or - 6 volt levels?
RS-422 and RS-485
Which standard can be used in a network?
RS-485 - Tri state capabilities
Which standard uses balanced inputs and outputs?
RS-485, RS-422
For RS-232C what two signal lines must be crossed to allow two DCE's to communicate with each other?
RX and TX pins 2 and 3
What is analog?
Representative of a value between a range of values: 1-5 V, 4-20 mA, 20-100 kPa
What is digital?
Representative of one of two values: 0 or 1, off or on, low or high
What factor contribute to signal attenuation fiber-optic cables?
Scattering, Absorption, Connection losses and Bending losses.
How is full duplex communications achieved when using the Bell 103 standard?
Separate frequencies
Explain the Serial Asynchronous Data Transmission format?
Serial - one bit at a time. Asynchronous - independent clocks - data framed by a start and stop bit. Start is used to indicate the beginning of the frame.
What is the advantage of a full duplex communication system?
Simultaneous communication between two devices (higher bandwidth-data exchange)
What does the term unbalanced transmission mean?
Single ended - input reference to ground - slower - short distances
What are the disadvantages of unbalanced transmission methods?
Slower and shorter distances - less immunity to noise
What are the differences between static and dynamic RAM?
Static: Made with Flip Flops Dynamic: Made with Capacitors
For asynchronous frame serial communication synchronization is achieved at what time?
Stop to start transition
List two limitations of LCD displays.
Temperature and Viewing Angle
MOV is?
The MOV instruction copies the source to the destination. The source remains unchanged.
What is parallel?
The information is sent on two or more channels. Example: eight channels to transmit a byte in parallel. (less practical over long distances)
What is serial?
The information is sent using one channel. Example: one bit at a time (slower) **Cheaper than parallel and more popular
How do TON timers get reset?
Use the input false condition to reset the timer [RST]
What is a Parity Bit?
Used for error detection. Catches about 30% of errors. Parity can be Even, Odd, Mark, or Space. The following table demonstrates Even and Odd parity.
When is a null modem connection used?
Used to connect two DTE together -
When is a NULL MODEM connection and when is it required?
Used to connected two DTE together - used to cross pins
What is RUN?
When first switching from the Programming mode to Run mode, program begins executing. If switch is in Run mode when the PLC is switched On ( Powered On) program starts executing.
Stranded cable is better than solid cable for transmission lines? Why?
Yes. Skin Effect. As frequency increases signal wants to travel closer to the surface of the conductor
Which three connections are required for RS-232 communications?
pin 2 - TxD, pin 3-RxD and 7-GND
What is CRC?******
CRC Cyclic Redundancy Checking. With this method, a block of data is sent, after that a CRC code is sent (typically 16 bits). This method has a very good change of catching any errors. One of the most popular methods around, although its calculation is difficult.
What is CPS?
Characters Per Second. A transfer rate estimated from the bit rate and length of each character. Often called Data Rate
List two error checking methods used with synchronous communication?
Checksum and CRC
When is a MODEM required and what does it do?
Connect two DTEs over long distances - Modulator Demodulator
What is the Controller Module?
Contain the CPU and memory for the PLC, and control operation of the other modules.
What does UART do?
Converts parallel data from your computer into serial data stream, and vice versa.
D/A conversion?
D/A conversion is the process of taking a digital value and converting it to a PROPORTIONAL analog value.
What does the term DTE and DCE mean? Give examples of each.
DTE - Data terminal equipment. Computer, RTU (remote terminal unit), flow computer, PLC, and DCS (distributed control system) DCE - Data communication equipment (modem)
What is DCE?
Data Communications Equipment. Coverts the signal into a form which can be transmitted over the transmission channel. (MODEM) CONVERTS!!
What is DTE?
Data Terminal Equipment. The equipment which originates or receives the transmission. (Computers, PLCs, Controllers) RECEIVES!!
What is Data bits? (TTL)
Data being transmitted. Usually ASCII. Transmitted LSB first!!!!
What does the term balanced transmission mean?
Differential Inputs - Good common mode noise rejection - faster, longer distances.
What is the purpose of a shield in coaxial cable?
EM ( electromagnetic interference ) shield keeps noise out
What is DTE/DCE Interface?
Electrical connection from DTE to DCE. Examples: RS-232, RS-422 CONNECTION!!
Describe the SCAN cycle of a PLC.
The processor module automatically scans the input module via the backplane of the chassis and sets the corresponding data table bit located in the processor module memory to a 1 or 0.
What 3 items does the Power Supply provide power to?
The processor, communication and I/O and modules.
D/A conversion resolution (step size)
The resolution of a D/A convertor is defined as the smallest change that can occur in the analog output as a result of a change in the digital output.
What is a space?
The transmission of a logical 0. (0V)
What is a mark?
The transmission of a logical 1. (5V)
Error detection methods.
The transmitter may add to data extra information for the receiver to detect errors.
What is the Chassis?
This chassis is the housing which the controller, I/O modules and power supplies are inserted. (4,7,10,13 and 17 slots available).
What is the function of the ALU?
To do math logic and calculations for the CPU.
How do RTO timers get reset?
To reset the accumulated value in RTO, you must use a reset instruction (RES) with the same address.
What is TTL?
Transistor-Transistor-Logic. Internal to the computer.
What is the advantage of quadrature amplitude modulation?
Transmit 4bits/baud - transmit data at higher bit rate.