Programmable logic controllers by w. bolton pdf

Programmable logic controllers Item Preview. EMBED for wordpress. Want more? Advanced embedding details, examples, and help! In this straightforward, easy-to-read guide, Bill Bolton has kept the jargon to a minimum, considered all the programming methods in the standard IEC - in particular ladder programming, and presented the subject in a way that is not device specific to ensure maximum applicability to courses in electronics and control systems.

Now in its fourth edition, this best-selling text has been expanded with increased coverage of industrial systems and PLCs and more consideration has been given to IEC and all the programming methods in the standard. The new edition brings the book fully up to date with the current developments in PLCs, describing new and important applications such as PLC use in communications e.

Ethernet an extremely popular system , and safety in particular proprietary emergency stop relays now appearing in practically every PLC based system. The coverage of commonly used PLCs has been increased, including the ever popular Allen Bradley PLCs, making this book an essential source of information both for professionals wishing to update their knowledge, as well as students who require a straight forward introduction to this area of control engineering.

Complete answers are provided in the back of the book. Programmable logic controllers -- 2. Input-output devices -- 3. As a result the size of the alternating voltage induced in one coil is greater than that in the other. The difference between the two secondary coil voltages, i. The output from the LVDT is an alternating voltage. This is usually converted to an analogue d. The capacitance will change if the plate separation changes, Figure 2.

All these methods can be used to b changing the area of overlap, give linear displacement sensors. The change in capacitance has to be c moving the dielectric converted into a suitable electrical signal by signal conditioning.

Input—output devices 27 2. The fractional change in resistance is proportional to the fractional change in length, i. For metals the gauge factor is about 2 and for semiconductors about Metal resistance strain gauges are in the form of a flat coil in order to get a reasonable length of metal in a small area. Often they are etched from metal foil Figure 2. The change in resistance of the strain gauge, when subject to strain, is usually converted into a voltage signal by the use of a Wheatstone bridge Figure 2.

A problem that occurs is that the resistance of the strain gauge also changes with temperature and thus some means of temperature compensation has to be used so that the output of the bridge is only a function of the strain.

This can be achieved by placing a dummy strain gauge in an opposite arm of the bridge, that gauge not being subject to any strain but only the temperature Figure 2. Strain gauge Output voltage Dummy gauge d. This not only gives temperature compensation but also gives a much larger output change when strain is applied.

The following paragraph illustrates systems employing such a form of compensation. By attaching strain gauges to other devices, changes which result in strain of those devices can be transformed, by the strain gauges, to give voltage changes. They might, for example, be attached to a cantilever to which forces are applied at its free end Figure 2. The voltage change, resulting from the strain gauges and the Wheatstone bridge, then becomes a measure of the force. The output from the gauges, and associated Wheatstone bridge, then becomes a measure of the pressure.

The diaphragm type consists of a thin disc of metal or plastic, secured round its edges. When there is a pressure difference between the two sides of the diaphragm, the centre of it deflects. The amount of deflection is related to the pressure difference. This deflection may be detected by strain gauges attached to the diaphragm see Figure 2. When a piezoelectric crystal is squeezed, there is a relative displacement of positive and negative charges within the crystal and the outer surfaces of the crystal become charged.

Hence a potential difference appears across it. This has a built-in vacuum on one side of the diaphragm and so the deflection of the diaphragm gives a measure of the absolute pressure applied to the other side of the diaphragm. The output is a voltage which is proportional to the applied pressure with a sensitivity of 0.

Other versions are available which have one side of the diaphragm open to the atmosphere and so can be used to measure gauge pressure; others allow pressures to be applied to both sides of the diaphragm and so can be used to measure differential pressures. A typical form involves a diaphragm or bellows which moves under the action of the pressure and operates a mechanical switch.

Diaphragms are less sensitive than bellows but can withstand greater pressures. Thus a Diaphragm commonly used method of determining the level of liquid in a tank is to pressure gauge measure the pressure due to the liquid above some datum level Figure 2. Often a sensor is just required to give a signal when the level in some Liquid container reaches a particular level.

A float switch that is used for this purpose consists of a float containing a magnet which moves in a housing Figure 2. As the float rises or falls it turns the reed switch on or sensor off, the reed switch being connected in a circuit which then switches on or off a voltage. This pressure difference can be monitored by means of a diaphragm pressure gauge and thus becomes a measure of the rate of flow.

The term non-volatile is used because the memory has to retain certain parameters when the power supply is removed. Because the elements are processor-based devices, such sensors can be programmed for specific requirements.

For example, it can be programmed to process the raw input data, correcting for such things as non-linearities, and then send the processed data to a base station.

It can be programmed to send a warning signal when the measured parameter reaches some critical value. The IEEE The standard requires the non-volatile EEPROM embedded memory to hold and communicate data which will allow a plug-and-play capability.

It thus would hold data for the identification and properties for the sensor and might also contain the calibration template, so facilitating digital interrogation. Generally, the digital signal from an output channel of a PLC is used to control an actuator which in turn controls some process.

The term actuator is used for the device which transforms the electrical signal into some more powerful action which then results in the control of the process. The following are some examples.

When a current passes through a solenoid a magnetic field is produced and this can then attract ferrous metal components in its vicinity.

One example of such an actuator is the relay, the term contactor being used when large currents are involved. The result is that much larger currents can be switched on. Thus the relay might be used to switch on the current to a motor. The valve may be used to control the directions of flow of pressurised air or oil and so used to operate other devices such as a piston moving in a cylinder. Pressurised air or hydraulic fluid is inputted from port P, this being connected to the pressure supply from a pump or compressor and port T is connected to allow hydraulic fluid to return to the supply tank or, in the case of a pneumatic system, to vent the air to the atmosphere.

With no current through the solenoid Figure 2. When a current is passed through the solenoid, the spool valve switches the hydraulic fluid or pressurised air to the left of the piston and exhausted from the right. The piston then moves to the right. The movement of the piston might be used to push a deflector to deflect items off a conveyor belt see Figure 1. Directional control valves are described by the number of ports they have and the number of control positions.

The valve shown in Figure 2. A, B, P and T, and two control positions. The basic symbol used on drawings for valves is a square, with one square being used to describe each of the control positions. Thus the symbol for the valve in Figure 2. Within each square the switching positions are then described by arrows to indicate a flow direction or a terminated line to indicate no flow path.

The term single acting cylinder Figure 2. The term double acting cylinder Figure 2. The armature is mounted on bearings and is free to rotate. It is mounted in the magnetic field produced by permanent magnets or current passing through coils of wire, these being termed the field coils. When a current passes through the armature coil, forces act on the coil and result in rotation.

Brushes and a commutator are used to reverse the current through the coil every half rotation and so keep the coil rotating. The speed of rotation can be changed by changing the size of the current to the armature coil. However, because fixed voltage supplies are generally used as the input to the coils, the required variable current is often obtained by an electronic circuit.

This can control the average value of the voltage, and hence current, by varying the time for which the constant d. A PLC might thus control the speed of rotation of a motor by controlling the electronic circuit used to control the width of the voltage pulses. This might be done using a relay.

The diode is included to dissipate the induced current resulting from the back e. Sometimes a PLC is required to reverse the direction of rotation of the motor.

This can be done using relays to reverse the direction of the current applied to the armature coil. For rotation in one direction, switch 1 is closed and switch 2 opened. For rotation in the other direction, switch 1 is opened and switch 2 closed. This uses a permanent magnet for the magnetic field but, instead of the armature coil rotating as a result of the magnetic field of the magnet, the permanent magnet rotates within the stationary coil.

With the conventional d. With the brushless permanent magnet motor, electronic circuitry is used to reverse the current. The motor can be started and stopped by controlling the current to the stationary coil. To reverse the motor, reversing the current is not so easy because of the electronic circuitry used for the commutator function. One method that is used is to incorporate sensors with the motor to detect the position of the north and south poles.

The speed of rotation can be controlled using pulse width modulation, i. Though a. As a consequence, d. Motor Digital Rotation in 2. Thus, if one input pulse produces a rotation of 1. To obtain one motor complete revolution through o, digital pulses would be required.

Object The motor can thus be used for accurate angular positioning. Such a motor is used with computer printers, robots, machine tools and a wide range of instruments where accurate positioning is required. Motor Pulley There are two basic forms of stepper motor, the permanent magnet wheel type with a permanent magnet rotor and the variable reluctance type with a soft steel rotor. The current is supplied from a d. With the currents switched through the coils such that the poles are as shown in Figure 2.

This would be, for Figure 6. If the current is then switched so that the polarities are reversed, the rotor will move a step to line up with the next pair of poles, at angle o and stop there. The rotor is made of soft steel and has a number of teeth, the number being less than the number of poles on the stator. The stator has pairs of poles, each pair of poles being activated and made into an electromagnet by a current being passed through the coils wrapped round them.

When one pair of poles is activated, a magnetic field is produced which attracts the nearest pair of rotor teeth so that the teeth and poles line up. This is termed the position of minimum reluctance. By then switching the current to the next pair of poles, the rotor can be made to rotate to line up with those poles.

Thus by sequentially switching the current from one pair of poles to the next, the rotor can be made to rotate in steps. This combines features of both the permanent magnet and variable reluctance motors. The rotor sets itself in the minimum reluctance position in response to a pair of stator coils being energised. To drive a stepper motor, so that it proceeds step-by-step to provide rotation, requires each pair of stator coils to be switched on and off in the required sequence when the input is a sequence of pulses Figure 2.

Driver circuits are available to give the correct sequencing and Figure 2. Motors are termed unipolar if they are wired so that the current can only flow in one direction through any particular motor terminal, bipolar if the current can flow in either direction through any particular motor terminal. The stepper motor will rotate through one step each time the trigger input goes from low to high.

The motor runs clockwise when the rotation input is low and anticlockwise when high. When the set pin is made low the output resets. In a control system, these input pulses might be supplied by a microprocessor. When an item is loaded onto the conveyor belt, a contact switch might be used to indicate that the item is on the belt and start the conveyor motor.

The motor then has to keep running until the item reaches the far end of the conveyor and falls off into the packaging area. When it does this, a switch might be activated which has the effect of switching off the conveyor motor.

The motor is then to remain off until the next item is loaded onto the belt. Thus the inputs to a PLC controlling the conveyor are from two switches and the output is to a motor. Switch Switch Loading Packaging Figure 2. It might be bricks from the ground level to the height where the bricklayers are working. The lift is to move upwards when a push button is pressed at the ground level to send the lift upwards or a push button is pressed at the upper level to request the lift to move upwards, but in both cases there is a condition that has to be met that a limit switch indicates that the access gate to the lift platform is closed.

The lift is to move downwards when a push button is pressed at the upper level to send the lift downwards or a push button is pressed at the lower level to request the lift to move downwards, but in both cases there is a condition that has to be met that a limit switch indicates that the access gate to the lift platform is closed.

The output from the control system is the signal to control the motor. When there is an input to solenoid A of valve 1, the piston moves to the right and causes the gripper to close. If solenoid B is energised, with A de-energised, the piston moves to the left and the gripper opens.

When both solenoids are de-energised, no air passes to either side of the piston in the cylinder and the piston keeps its position without change. Likewise, inputs to the solenoids of valve 2 are used to extend or retract the arm. Inputs to the solenoids of valve 4 are used to rotate the base in either a clockwise or anticlockwise direction.

Such a resistance change can be transformed by signal conditioning to give suitable on—off signals. A limit switch: i Can be used to detect the presence of a moving part. A thermistor is a temperature sensor which gives resistance changes which are: i A non-linear function of temperature. Such a sensor will need to have a diaphragm with: A A vacuum on one side. B One side open to the atmosphere.

C The pressure applied to both sides. D A controlled adjustable pressure applied to one side. B The change in diameter of the shaft. C The change in angular position of the shaft.

D The absolute angular position of the shaft. Input devices which give an analogue input for displacement are: i Linear potentiometer. In the control positions: i A is connected to T and P to B. A stepper motor has a step angle of 1. This means that: i Each pulse input to the motor rotates the motor shaft by 1. A proximity switch is required for detecting the presence of a non- metallic object. Types of switches that might be suitable are: i Eddy current type. A i T ii T B i T ii F C i F ii T D i F ii F 11 Explain the operation of the following input devices, stating the form of the signal being sensed and the output: a reed switch, b incremental shaft encoder, c photoelectric transmissive switch, d diaphragm pressure switch.

This is based on the use of the 10 digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. With a number represented by this system, the digit position in the number indicates the weight attached to each digit, the weight increasing by a factor of 10 as we proceed from right to left. Hence we have The denary system is just convenient because we have ten fingers. If we had only two then our system for everyday counting would probably have been different. Computers, and hence PLC systems, are based on counting in twos because it is convenient for their system, their two digits being effectively just the off and on signals.

When working with PLCs, other base number systems are also used, e. These are termed binary digits or bits. When a number is represented by this system, the digit position in the number indicates the weight attached to each digit, the weight increasing by a factor of 2 as we proceed from right to left.

For example, with the binary number , the least significant bit is the bit at the right-hand end of the number and so is 0. The most significant bit is the bit at the left-hand end of the number and so is 1.

The conversion of a binary number to a denary number thus involves the addition of the powers of 2 indicated by the number. The conversion of a denary number to a binary number involves looking for the appropriate powers of 2.

We can do this by successive divisions by 2, noting the remainders at each division. The first division gives the least significant bit because we have just divided the 31 by 2, i.

The last division gives the most significant bit because the 31 has then been divided by 2 four times, i. A problem with binary numbers is that a comparatively small number requires a large number of digits.

For example, the denary number 9 which involves just a single digit requires four when written as the binary number The denary number , involving three digits, in binary form is and requires eight digits. When a number is represented by this system, the digit position in the number indicates the weight attached to each digit, the weighting increasing by a factor of 8 as we proceed from right to left.

Thus we have Thus the denary number 15 divided by 8 gives 1 with remainder 7 and thus the denary number 15 is 17 in the octal system.

To convert from octal to denary we multiply the digits by the power of 8 appropriate to its position in the number. For example, the binary number would be written as: 11 Each group is then replaced by the corresponding digit 0 to 7. The binary number is 6, the is 2 and the 11 is 3. Thus the octal number is As another example, the binary number is: Binary 4 7 2 Octal Octal to binary conversion involves converting each octal digit into its 3-bit equivalent.

Thus, for the octal number 21 we have 1 as and 2 as 2 1 Octal number Binary number and so the binary number is When a number is represented by this system, the digit position in the number indicates that the weight attached to each digit increases by a factor of 16 as we proceed from right to left.

To convert from denary numbers into hex we successively divide by 16 and note the remainders. Thus the denary number when divided by 16 gives 9 with remainder 12 and so in hex is 9C. To convert from hex to denary we multiply the digits by the power of 16 appropriate to its position in the number.

To convert binary numbers into hexadecimal numbers, we group the binary numbers into fours starting from the least significant number. Thus, for the binary number we have: 11 Binary number 3 A 6 Hex number For conversion from hex to binary, each hex number is converted to its 4-bit equivalent. Because the external world tends to deal mainly with numbers in the denary system and computers with numbers in the binary system, there is always the problem of conversion.

There is, however, no simple link between the position of digits in a denary number and the position of digits in a binary number.

An alternative method that is often used is the binary coded decimal system BCD. With this system, each denary digit is coded separately in binary. For example, the denary number 15 has the 5 converted into the binary number and the 1 into 1 5 Denary number Binary number to give in BCD the number Table 3. We continue this through the various bits and end up with the Since there is generally a need to handle both positive and negative numbers there needs to be some way of distinguishing between them.

This can be done by adding a sign bit. When a number is said to be signed then the most significant bit is used to indicate the sign of the number, a 0 being used if the number is positive and a 1 if it is negative. Thus a positive binary number of would be written as A negative number of would be written as However, this is not the most useful way of writing negative numbers for ease of manipulation by computers.

The result is thus When we have a positive number then we sign the normal binary number with a 0, i. Fixed point numbers are ones where there is a fixed location of the point separating integers from fractional numbers. Thus, We have, with the eight-bit binary number, four digits before the binary point and four digits after it. When two such binary numbers are added by a computing system the procedure is to recognise that the fixed point is fixed the same in both numbers so we can ignore it for the addition, carry out the addition of the numbers and then insert in the result the binary point in its fixed position.

For example, suppose we want to add Using fixed points does present problems. If we are concerned with very large or very small numbers we could end up with a large number of zeros between the integers and the point, e. Thus, the above number might be written as 0. Likewise, the binary number 0. Such notation has a floating point. With binary numbers the base is understood to be 2, i. Thus a computing system needs, in addition to storing the sign, i. Because with floating point numbers it is possible to store a number in several different ways, e.

This means that they are all put in the form 0. Thus, with binary numbers we have 0. In order to take account of the sign of a binary number we then add a sign bit of 0 for a positive number and 1 for a negative number. Thus the number 0. Unlike fixed point numbers, floating point numbers cannot be directly added unless the exponents are the same.

Thus to carry out addition we need to make the exponents the same. Such signed numbers are referred to as integers with the symbol INT being used with inputs and outputs in programs of such bit words. Decimal fractions are referred to as real or floating point numbers, being represented by the symbol REAL for inputs and outputs in programs. These consist of two bit words and so we might have 1. Time duration, e. Note that is the symbol used to indicate that which follows is a numerical quantity.

Problems 1 Convert the following binary numbers to denary numbers: a , b , c The CPU, however, must have an input of digital signals of a particular size, normally 0 to 5 V. The output from the CPU is digital, normally 0 to 5 V. Thus there is generally a need to manipulate input and output signals so that they are in the required form. It is this in-built facility to enable a range of inputs and outputs to be handled which makes PLCs so easy to use.

The following is a brief indication of the basic circuits used for input and output units. For a PLC input unit, with sourcing it is the source of the current supply for the input device connected to it Figure 4. With sinking, the input device provides the current to the input unit Figure 4. Optoisolators see Section 1. With the a. Individual status lights are provided for each input to indicate when the input device is providing a signal. With a rack mounted system this may be achieved by Selected Multi- mounting a suitable analogue input card in the rack.

So that one analogue plexer output card is not required for each analogue input, multiplexing is generally used Figure 4. This involves more than one analogue input being connected to the card and then electronic switches used to select each input in turn. Cards are typically available giving 4, 8 or 16 analogue Channel inputs. The eight signals then constitute the so-termed digital word corresponding to the analogue input signal level.

The digital output goes up in steps Figure 4. Digital output Digital Bit output 7 6 5 Analogue Analogue- 4 to-digital 3 input converter 2 1 0 0 1 2 a b Analogue input Figure 4. The term resolution is used for the smallest change in analogue voltage which will give rise to a change in one bit in the digital output.

This means that a change of 0. The number of bits in the output from an analogue-to-digital converter thus determines the resolution, and hence accuracy, possible. To illustrate the above, consider a thermocouple used as a sensor with a PLC and giving an output of 0. What will be the accuracy with which the PLC will activate the output device if the thermocouple is connected to an analogue input with a range of 0 to 10 V d. Quite often, sinking input units are used for interfacing with electronic equipment and sourcing output units for interfacing with solenoids.

Figure 4. The input to the converter is a sequence of bits with each bit along a parallel line. A bit in the 1 line gives rise to an output pulse of twice the size of the 0 line pulse.

A bit in the 2 line gives rise to an output pulse of twice the size of the 1 line pulse. A bit in the 3 line gives rise to an output pulse of twice the size of the 2 line pulse, and so on. All the outputs add together to give the analogue version of the digital input.

When the digital input changes, the analogue output changes in a stepped manner, the voltage changing by the voltage changes associated with each bit. Suppose the output range is set to 10 V d. Thus we have: Digital input V Analogue output V 0. Analogue output modules are usually provided in a number of outputs, e. Modules generally have outputs in two forms, one for which all the outputs from that module have a common voltage supply and one which drives outputs having their own individual voltage supplies.

However, many sensors generate analogue signals. In order to avoid having a multiplicity of analogue input channels to cope with the wide diversity of analogue signals that can be generated by sensors, external signal conditioning is often used to bring analogue signals to a common range and so allow a standard form of analogue input channel to be used.

A common standard that is used Figure 4. Thus, for example, a sensor used to monitor liquid level in the height range 0 to 1 m would have the 0 level represented by 4 mA and the 1 m represented by 20 mA. The use of 4 mA to represent the low end of the analogue range serves the purpose of distinguishing between when the sensor is indicating zero and when the sensor is not working and giving zero response for that reason.

When this happens the current would be 0 mA. The 4 mA also is often a suitable current to operate a sensor and so eliminate the need for a separate power supply. Such is the case when a sensor, e. The sensor is connected in a Wheatstone bridge and the out-of-balance potential difference amplified by a differential amplifier before being fed via an analogue-to-digital converter unit which is part of the analogue input port of the PLC. The distant PLCs do not contain the control program since all the control processing is carried out by the master PLC.

Coaxial cable enables higher data rates to be transmitted and does not require the shielding of steel conduit. Fibre-optic cabling has the advantage of resistance to noise, small size and flexibility and is now becoming more widely used. Thus if an 8-bit word is to be transmitted, the eight bits are transmitted one at a time in sequence along a cable.

This means that a data word has to be separated into its constituent bits for transmission and then reassembled into the word when received. Parallel communication is when all the constituent bits of a word are simultaneously transmitted along parallel cables Figure 4.

This allows data to be transmitted over short distances at high speeds. Bits Bits 7 6 5 4 a 3 2 1 b 0 Figure 4. It is much cheaper to run, for serial communication, a single core cable over a long distance than the multicore cables that would be needed for parallel communication. With a PLC system, serial communication might be used for the connection between a computer, when used as a programming terminal, and a PLC.

Parallel communication might be used when connecting laboratory instruments to the system. However, internally, PLCs work, for speed, with parallel communications. Bear in mind that a sequence of words are being sent along the same cable and it is necessary to be able to be able to determine when one word starts and finishes and the next word starts.

This is necessary if, for example, a particular duration transmitted pulse it to be recognised by the receiver as just a single bit rather than two bits. This is commonly done by using two extra signal wires termed handshake wires , one to tell the receiver that the transmitter is ready to send the data and the other to tell the transmitter that the receiver is ready to receive data.

The most common standard serial communications interface used is the RS Connections are made via pin D-type connectors Figure 4. Not all the pins are used in every application. Pins 1 to 13 Pins 14 to 24 Figure 4.

However, not all the bits transmitted can be used for data, some have to be used to indicate the start and stop of a serial piece of data, these often being termed flags, and as a check as to whether the data has been corrupted during transmission.

The parity bit is added to check whether corruption has occurred, with even parity a 1 being added to make the number of 1s an even number. To send seven bits of data, eleven bits may be required. RS is limited over the distances it can be used, noise limiting the transmission of high numbers of bits per second when the length of cable is more than about 15 m. RS can be used for longer distances. This uses a balanced method of transmission.

Such circuits require two lines for the transmission, the transmitted signal being the voltage difference between the two lines. Noise affecting both lines equally will have no effect on the transmitted signal. RS lines can be used for much greater distances than RS The serial data is transmitted by the current being switched on and off, a 0 being transmitted as zero current and a 1 as 20 mA.

Transmitter 20 mA Current Receiver source Figure 4. This was originally developed by Hewlett Packard to link their computers and instruments and was known as the Hewlett Packard Instrumentation Bus. It is now often termed the General Purpose Instrument Bus.

This bus provides a means of making interconnections so that parallel data communications can take place between listeners, talkers and controllers. Listeners are devices that accept data from the bus, talkers place data, on request, on the bus and controllers manage the flow of data on the bus and provide processing facilities. There is a total of 24 lines, of which eight bi-directional lines are used to carry data and commands between the various devices connected to the bus, five lines are used for control and status signals, three are used for handshaking between devices and eight are ground return lines Figure 4.

The commands can be directed to individual devices by placing addresses on the data lines. Device addresses are sent via the data lines as a parallel 7-bit word, the lowest 5-bits providing the device address and the other two bits control information. If both these bits are 0 then the commands are sent to all addresses, if bit 6 is 1 and bit 7 a 0 the addressed device is switched to be a listener, if bit 6 is 0 and bit 7 is 1 then the device is switched to be a talker.

As illustrated above by the function of the ATN line, the management lines each have an individual task in the control of information. The handshake lines are used for controlling the transfer of data. The three lines ensure that the talker will only talk when it is being listened to by listeners. Table 4. This is used to either signify the end of a message sequence from a talker device or is used by the controller to ask a device to identify itself.

When the level is low on this line then the information on the data bus is valid and acceptable. This line is used by listener devices taking it high to indicate that they are ready to accept data.

This line is used by listeners taking it high to indicate that data is being accepted. This is used by the controller to reset all the devices of the system to the start state. This is used by devices to signal to the controller that they need attention.

This is used by the controller to signal that it is placing a command on the data lines. This enables a device on the bus to indicate that it is to be selected for remote control rather than by its own control panel.

When a data word is put on the data lines, NRFD is made high to indicate that all listeners are ready to accept data and DAV is made low to indicate that new data is on the data lines. When a device accepts a data word it sets NDAC high to indicate that is has accepted the data and NRFD low to indicate that it is now not ready to accept data. This then results in NDAC being set low.

The entire process can then be repeated for another word being put on the data bus. This is termed the protocol. Thus one device needs to indicate to the other to start or stop sending data. This can be done by using handshaking wires connecting transmitting and receiving devices so that a signal along one such wire can tell the receiver that the transmitter is ready to send RTS and along another wire that the transmitter is ready to receive, a clear to send signal CTS.

An alternative is to use additional characters on the transmitting wires. When this character is received the end of the data packet has been reached. Once the receiver has processed that data, it can indicate it is ready for another block of data by sending back an acknowledge ACK signal.

The transmitter then waits for an XON signal before resuming transmission. One form of checking for errors in the message that might occur as a result of transmission is the parity check. For example, is even since there is an even number of 1s and is odd since there is an odd number of 1s. To make both these odd parity then the extra bit added at the end in the first case is 1 and in the second case 0, i.

Thus when a message is sent out with odd bit parity, if the receiver finds that the bits give an even sum, then the message has been corrupted during transmission and the receiver can request that the message be repeated. The parity bit method can detect if there is an error resulting from a single 0 changing to a 1 or a 1 changing to a 0 but cannot detect two such errors occurring since there is then no change in parity.

To check on such occurrences more elaborate checking methods have to be used. One method involves storing data words in an array of rows and columns. Parity can then be checked for each row and each column. The following illustrates this for seven words using even parity. Click here to download Wait You will be directed to the download link after the count has ended. Books Electronic. You may like these posts. Post a Comment 0 Comments. Popular Posts. Download Motor Vehicle Mechanic's Textbook pdf.

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