Q10. What are the basic differences between RS232, RS485 and RS422 serial transmission protocols?
Simplex can be viewed as a communications "one-way street". Data only flows in one direction. That is to say, a device can be a receiver or a transmitter exclusively. A simplex device is not a transceiver. A good example of simplex communications is an FM radio station and your car radio. Information flows only in one direction where the radio station is the transmitter and the receiver is your car radio. Simplex is not often used in computer communications because there is no way to verify when or if data is received. However, simplex communications is a very efficient way to distributed vast amounts of information to a large number of receivers.
Duplex communications overcome the limits of Simplex communications by allowing the devices to act as transceivers. Duplex communication data flow in both directions thereby allowing verification and control of data reception/transmission. Exactly when data flows bi-directionally further defines Duplex communications.
Full Duplex devices can transmit and receive data at the same time. RS232 is a fine example of Full Duplex communications. There are separate transmit and receive signal lines that allow data to flow in both directions simultaneously. RS422 devices also operate Full Duplex.
Half Duplex devices have the dubious honor of allowing both transmission and receiving, but not at the same time. Essentially only one device can transmit at a time while all other half duplex devices receive. Devices operate as transceivers, but not simultaneous transmit and receive. RS485 operates in a half duplex manner.
Here is the short version of the critical specifications. Unfortunately, these are subject to interpretation by individual manufacturers. That is why RS232 is often regarded as an incredibly un-standard communications protocol.
One important note. You will see that one of the major differences between RS232 and RS422/RS485 is the signaling mode. RS232 is unbalanced while RS422/RS485 is balanced. An unbalanced signal is represented by a single signal wire where a voltage level on that one wire is used to transmit/receive binary 1 and 0: the can be considered a push signal driver. On the other hand, a balanced signal is represented by a pair of wires where a voltage difference is used to transmit/receive binary information: sort of a push-pull signal driver. In short, unbalanced voltage level signal travels slower and shorter than a balanced voltage difference signal.
Q11. What is the difference between baud rate and data rate?
A data transfer rate (or often just data rate) is the amount of digital data that is moved from one place to another in a given time, usually in a second's time. The data transfer rate can be viewed as the speed of travel of a given amount of data from one place to another. In general, the greater the bandwidth of a given path, the higher the data transfer rate.
In telecommunications, data transfer is usually measured in bits per second. For example, a typical low-speed connection to the Internet may be 33.6 kilobits per second (Kbps). On Ethernet local area networks, data transfer can be as fast as 10 megabits per second. Network switches are planned that will transfer data in the terabit range. In earlier telecommunication systems, data transfer was sometimes measured in characters or blocks (of a certain size) per second. Data transfer time between the microprocessor or RAM and devices such as the hard disk and CD-ROM player is usually measured in milliseconds.
In computers, data transfer is often measured in bytes per second. The highest data transfer rate to date is 14 terabits per second over a single optical fiber, reported by Japan's Nippon Telegraph and Telephone (NTT DoComo) in 2006.
BAUD RATE: In telecommunications and electronics, baud (pronounced /bɔːd/ unit symbol "Bd"), is a measure of the symbol rate; that is, the number of distinct symbol changes (signalling events) made to the transmission medium per second in a digitally modulated signal. The term baud rate is also commonly used to refer to the symbol rate.
The baud rate (symbol rate) is distinct from the bit rate, because one symbol may carry more than one bit of information. For example, in modems, where bandwidth efficiency is important, it is commonly arranged for one symbol to carry 3 or more bits. So a 3000 bit per second modem, which is transmitting symbols that each carry 3 bits, should be described as operating at 1000 baud. Conversely, direct-sequencespread spectrum operation requires many symbols to carry only one bit.
Unfortunately, this distinction is not widely understood. Early modems operated only at one bit per symbol, and so baud rate and bit rate for those devices were equivalent. This has led many to believe the two terms to be synonymous, which they are not.
Conveying more than one bit per symbol has advantages. This reduces the time required to send a given quantity of data, and allows modern modems, FDDI and 100/1000 Mbit/s Ethernet LANs, and so on, to achieve high data rates. An optimal symbol set design must take into account channel bandwidth, desired information rate, noise characteristics of the channel and the receiver, and receiver and decoder complexity. A typical 2400 bit/s modem actually transmits at 600 baud (600 symbol/s), where each quadrature amplitude modulation symbol carries four bits of information. And further, 1000 Mbit/s Ethernet LAN cables use multiple wire pairs and multiple bits per symbol to encode their data payloads. Specifically, 1000BASE-T uses 4 wire pairs and 2 data bits per symbol to get a symbol rate of 125MBaud.
Conversely, representing one bit by many symbols has the advantage of overcoming signal noise, particularly radio jamming, hence is commonplace in military radio, despite the disadvantage of using more bandwidth to carry the same bit rate.
Baud is a measurement of transmission speed in asynchronous communication. Because of advances in modem communication technology, this term is frequently misused when describing the data rates in newer devices.
Traditionally, a Baud Rate represents the number of bits that are actually being sent over the media, not the amount of data that is actually moved from one DTE device to the other. The Baud count includes the overhead bits Start, Stop and Parity that are generated by the sending UART and removed by the receiving UART. This means that seven-bit words of data actually take 10 bits to be completely transmitted. Therefore, a modem capable of moving 300 bits per second from one place to another can normally only move 30 7-bit words if Parity is used and one Start and Stop bit are present.
If 8-bit data words are used and Parity bits are also used, the data rate falls to 27.27 words per second, because it now takes 11 bits to send the eight-bit words, and the modem still only sends 300 bits per second.
The formula for converting bytes per second into a baud rate and vice versa was simple until error-correcting modems came along. These modems receive the serial stream of bits from the UART in the host computer (even when internal modems are used the data is still frequently serialized) and converts the bits back into bytes. These bytes are then combined into packets and sent over the phone line using a Synchronous transmission method. This means that the Stop, Start, and Parity bits added by the UART in the DTE (the computer) were removed by the modem before transmission by the sending modem. When these bytes are received by the remote modem, the remote modem adds Start, Stop and Parity bits to the words, converts them to a serial format and then sends them to the receiving UART in the remote computer, who then strips the Start, Stop and Parity bits.
The reason all these extra conversions are done is so that the two modems can perform error correction, which means that the receiving modem is able to ask the sending modem to resend a block of data that was not received with the correct checksum. This checking is handled by the modems, and the DTE devices are usually unaware that the process is occurring.
By striping the Start, Stop and Parity bits, the additional bits of data that the two modems must share between themselves to perform error-correction are mostly concealed from the effective transmission rate seen by the sending and receiving DTE equipment. For example, if a modem sends ten 7-bit words to another modem without including the Start, Stop and Parity bits, the sending modem will be able to add 30 bits of its own information that the receiving modem can use to do error-correction without impacting the transmission speed of the real data.
The use of the term Baud is further confused by modems that perform compression. A single 8-bit word passed over the telephone line might represent a dozen words that were transmitted to the sending modem. The receiving modem will expand the data back to its original content and pass that data to the receiving DTE.
Modern modems also include buffers that allow the rate that bits move across the phone line (DCE to DCE) to be a different speed than the speed that the bits move between the DTE and DCE on both ends of the conversation. Normally the speed between the DTE and DCE is higher than the DCE to DCE speed because of the use of compression by the modems.
Because the number of bits needed to describe a byte varied during the trip between the two machines plus the differing bits-per-seconds speeds that are used present on the DTE-DCE and DCE-DCE links, the usage of the term Baud to describe the overall communication speed causes problems and can misrepresent the true transmission speed. So Bits Per Second (bps) is the correct term to use to describe the transmission rate seen at the DCE to DCE interface and Baud or Bits Per Second are acceptable terms to use when a connection is made between two systems with a wired connection, or if a modem is in use that is not performing error-correction or compression.
Modern high speed modems (2400, 9600, 14,400, and 19,200bps) in reality still operate at or below 2400 baud, or more accurately, 2400 Symbols per second. High speed modem are able to encode more bits of data into each Symbol using a technique called Constellation Stuffing, which is why the effective bits per second rate of the modem is higher, but the modem continues to operate within the limited audio bandwidth that the telephone system provides. Modems operating at 28,800 and higher speeds have variable Symbol rates, but the technique is the same.
Q12. What is asynchronous serial transmission?
Asynchronous transmission allows data to be transmitted without the sender having to send a clock signal to the receiver. Instead, the sender and receiver must agree on timing parameters in advance and special bits are added to each word which are used to synchronize the sending and receiving units.
When a word is given to the UART for Asynchronous transmissions, a bit called the "Start Bit" is added to the beginning of each word that is to be transmitted. The Start Bit is used to alert the receiver that a word of data is about to be sent, and to force the clock in the receiver into synchronization with the clock in the transmitter. These two clocks must be accurate enough to not have the frequency drift by more than 10% during the transmission of the remaining bits in the word. (This requirement was set in the days of mechanical teleprinters and is easily met by modern electronic equipment.)
After the Start Bit, the individual bits of the word of data are sent, with the Least Significant Bit (LSB) being sent first. Each bit in the transmission is transmitted for exactly the same amount of time as all of the other bits, and the receiver “looks” at the wire at approximately halfway through the period assigned to each bit to determine if the bit is a 1 or a 0. For example, if it takes two seconds to send each bit, the receiver will examine the signal to determine if it is a 1 or a 0 after one second has passed, then it will wait two seconds and then examine the value of the next bit, and so on.
The sender does not know when the receiver has “looked” at the value of the bit. The sender only knows when the clock says to begin transmitting the next bit of the word.
When the entire data word has been sent, the transmitter may add a Parity Bit that the transmitter generates. The Parity Bit may be used by the receiver to perform simple error checking. Then at least one Stop Bit is sent by the transmitter.
When the receiver has received all of the bits in the data word, it may check for the Parity Bits (both sender and receiver must agree on whether a Parity Bit is to be used), and then the receiver looks for a Stop Bit. If the Stop Bit does not appear when it is supposed to, the UART considers the entire word to be garbled and will report a Framing Error to the host processor when the data word is read. The usual cause of a Framing Error is that the sender and receiver clocks were not running at the same speed, or that the signal was interrupted.
Regardless of whether the data was received correctly or not, the UART automatically discards the Start, Parity and Stop bits. If the sender and receiver are configured identically, these bits are not passed to the host.
If another word is ready for transmission, the Start Bit for the new word can be sent as soon as the Stop Bit for the previous word has been sent.
Because asynchronous data is “self synchronizing”, if there is no data to transmit, the transmission line can be idle.
Simplex can be viewed as a communications "one-way street". Data only flows in one direction. That is to say, a device can be a receiver or a transmitter exclusively. A simplex device is not a transceiver. A good example of simplex communications is an FM radio station and your car radio. Information flows only in one direction where the radio station is the transmitter and the receiver is your car radio. Simplex is not often used in computer communications because there is no way to verify when or if data is received. However, simplex communications is a very efficient way to distributed vast amounts of information to a large number of receivers.
Duplex communications overcome the limits of Simplex communications by allowing the devices to act as transceivers. Duplex communication data flow in both directions thereby allowing verification and control of data reception/transmission. Exactly when data flows bi-directionally further defines Duplex communications.
Full Duplex devices can transmit and receive data at the same time. RS232 is a fine example of Full Duplex communications. There are separate transmit and receive signal lines that allow data to flow in both directions simultaneously. RS422 devices also operate Full Duplex.
Half Duplex devices have the dubious honor of allowing both transmission and receiving, but not at the same time. Essentially only one device can transmit at a time while all other half duplex devices receive. Devices operate as transceivers, but not simultaneous transmit and receive. RS485 operates in a half duplex manner.
Here is the short version of the critical specifications. Unfortunately, these are subject to interpretation by individual manufacturers. That is why RS232 is often regarded as an incredibly un-standard communications protocol.
One important note. You will see that one of the major differences between RS232 and RS422/RS485 is the signaling mode. RS232 is unbalanced while RS422/RS485 is balanced. An unbalanced signal is represented by a single signal wire where a voltage level on that one wire is used to transmit/receive binary 1 and 0: the can be considered a push signal driver. On the other hand, a balanced signal is represented by a pair of wires where a voltage difference is used to transmit/receive binary information: sort of a push-pull signal driver. In short, unbalanced voltage level signal travels slower and shorter than a balanced voltage difference signal.
Q11. What is the difference between baud rate and data rate?
A data transfer rate (or often just data rate) is the amount of digital data that is moved from one place to another in a given time, usually in a second's time. The data transfer rate can be viewed as the speed of travel of a given amount of data from one place to another. In general, the greater the bandwidth of a given path, the higher the data transfer rate.
In telecommunications, data transfer is usually measured in bits per second. For example, a typical low-speed connection to the Internet may be 33.6 kilobits per second (Kbps). On Ethernet local area networks, data transfer can be as fast as 10 megabits per second. Network switches are planned that will transfer data in the terabit range. In earlier telecommunication systems, data transfer was sometimes measured in characters or blocks (of a certain size) per second. Data transfer time between the microprocessor or RAM and devices such as the hard disk and CD-ROM player is usually measured in milliseconds.
In computers, data transfer is often measured in bytes per second. The highest data transfer rate to date is 14 terabits per second over a single optical fiber, reported by Japan's Nippon Telegraph and Telephone (NTT DoComo) in 2006.
BAUD RATE: In telecommunications and electronics, baud (pronounced /bɔːd/ unit symbol "Bd"), is a measure of the symbol rate; that is, the number of distinct symbol changes (signalling events) made to the transmission medium per second in a digitally modulated signal. The term baud rate is also commonly used to refer to the symbol rate.
The baud rate (symbol rate) is distinct from the bit rate, because one symbol may carry more than one bit of information. For example, in modems, where bandwidth efficiency is important, it is commonly arranged for one symbol to carry 3 or more bits. So a 3000 bit per second modem, which is transmitting symbols that each carry 3 bits, should be described as operating at 1000 baud. Conversely, direct-sequencespread spectrum operation requires many symbols to carry only one bit.
Unfortunately, this distinction is not widely understood. Early modems operated only at one bit per symbol, and so baud rate and bit rate for those devices were equivalent. This has led many to believe the two terms to be synonymous, which they are not.
Conveying more than one bit per symbol has advantages. This reduces the time required to send a given quantity of data, and allows modern modems, FDDI and 100/1000 Mbit/s Ethernet LANs, and so on, to achieve high data rates. An optimal symbol set design must take into account channel bandwidth, desired information rate, noise characteristics of the channel and the receiver, and receiver and decoder complexity. A typical 2400 bit/s modem actually transmits at 600 baud (600 symbol/s), where each quadrature amplitude modulation symbol carries four bits of information. And further, 1000 Mbit/s Ethernet LAN cables use multiple wire pairs and multiple bits per symbol to encode their data payloads. Specifically, 1000BASE-T uses 4 wire pairs and 2 data bits per symbol to get a symbol rate of 125MBaud.
Conversely, representing one bit by many symbols has the advantage of overcoming signal noise, particularly radio jamming, hence is commonplace in military radio, despite the disadvantage of using more bandwidth to carry the same bit rate.
Baud is a measurement of transmission speed in asynchronous communication. Because of advances in modem communication technology, this term is frequently misused when describing the data rates in newer devices.
Traditionally, a Baud Rate represents the number of bits that are actually being sent over the media, not the amount of data that is actually moved from one DTE device to the other. The Baud count includes the overhead bits Start, Stop and Parity that are generated by the sending UART and removed by the receiving UART. This means that seven-bit words of data actually take 10 bits to be completely transmitted. Therefore, a modem capable of moving 300 bits per second from one place to another can normally only move 30 7-bit words if Parity is used and one Start and Stop bit are present.
If 8-bit data words are used and Parity bits are also used, the data rate falls to 27.27 words per second, because it now takes 11 bits to send the eight-bit words, and the modem still only sends 300 bits per second.
The formula for converting bytes per second into a baud rate and vice versa was simple until error-correcting modems came along. These modems receive the serial stream of bits from the UART in the host computer (even when internal modems are used the data is still frequently serialized) and converts the bits back into bytes. These bytes are then combined into packets and sent over the phone line using a Synchronous transmission method. This means that the Stop, Start, and Parity bits added by the UART in the DTE (the computer) were removed by the modem before transmission by the sending modem. When these bytes are received by the remote modem, the remote modem adds Start, Stop and Parity bits to the words, converts them to a serial format and then sends them to the receiving UART in the remote computer, who then strips the Start, Stop and Parity bits.
The reason all these extra conversions are done is so that the two modems can perform error correction, which means that the receiving modem is able to ask the sending modem to resend a block of data that was not received with the correct checksum. This checking is handled by the modems, and the DTE devices are usually unaware that the process is occurring.
By striping the Start, Stop and Parity bits, the additional bits of data that the two modems must share between themselves to perform error-correction are mostly concealed from the effective transmission rate seen by the sending and receiving DTE equipment. For example, if a modem sends ten 7-bit words to another modem without including the Start, Stop and Parity bits, the sending modem will be able to add 30 bits of its own information that the receiving modem can use to do error-correction without impacting the transmission speed of the real data.
The use of the term Baud is further confused by modems that perform compression. A single 8-bit word passed over the telephone line might represent a dozen words that were transmitted to the sending modem. The receiving modem will expand the data back to its original content and pass that data to the receiving DTE.
Modern modems also include buffers that allow the rate that bits move across the phone line (DCE to DCE) to be a different speed than the speed that the bits move between the DTE and DCE on both ends of the conversation. Normally the speed between the DTE and DCE is higher than the DCE to DCE speed because of the use of compression by the modems.
Because the number of bits needed to describe a byte varied during the trip between the two machines plus the differing bits-per-seconds speeds that are used present on the DTE-DCE and DCE-DCE links, the usage of the term Baud to describe the overall communication speed causes problems and can misrepresent the true transmission speed. So Bits Per Second (bps) is the correct term to use to describe the transmission rate seen at the DCE to DCE interface and Baud or Bits Per Second are acceptable terms to use when a connection is made between two systems with a wired connection, or if a modem is in use that is not performing error-correction or compression.
Modern high speed modems (2400, 9600, 14,400, and 19,200bps) in reality still operate at or below 2400 baud, or more accurately, 2400 Symbols per second. High speed modem are able to encode more bits of data into each Symbol using a technique called Constellation Stuffing, which is why the effective bits per second rate of the modem is higher, but the modem continues to operate within the limited audio bandwidth that the telephone system provides. Modems operating at 28,800 and higher speeds have variable Symbol rates, but the technique is the same.
Q12. What is asynchronous serial transmission?
Asynchronous transmission allows data to be transmitted without the sender having to send a clock signal to the receiver. Instead, the sender and receiver must agree on timing parameters in advance and special bits are added to each word which are used to synchronize the sending and receiving units.
When a word is given to the UART for Asynchronous transmissions, a bit called the "Start Bit" is added to the beginning of each word that is to be transmitted. The Start Bit is used to alert the receiver that a word of data is about to be sent, and to force the clock in the receiver into synchronization with the clock in the transmitter. These two clocks must be accurate enough to not have the frequency drift by more than 10% during the transmission of the remaining bits in the word. (This requirement was set in the days of mechanical teleprinters and is easily met by modern electronic equipment.)
After the Start Bit, the individual bits of the word of data are sent, with the Least Significant Bit (LSB) being sent first. Each bit in the transmission is transmitted for exactly the same amount of time as all of the other bits, and the receiver “looks” at the wire at approximately halfway through the period assigned to each bit to determine if the bit is a 1 or a 0. For example, if it takes two seconds to send each bit, the receiver will examine the signal to determine if it is a 1 or a 0 after one second has passed, then it will wait two seconds and then examine the value of the next bit, and so on.
The sender does not know when the receiver has “looked” at the value of the bit. The sender only knows when the clock says to begin transmitting the next bit of the word.
When the entire data word has been sent, the transmitter may add a Parity Bit that the transmitter generates. The Parity Bit may be used by the receiver to perform simple error checking. Then at least one Stop Bit is sent by the transmitter.
When the receiver has received all of the bits in the data word, it may check for the Parity Bits (both sender and receiver must agree on whether a Parity Bit is to be used), and then the receiver looks for a Stop Bit. If the Stop Bit does not appear when it is supposed to, the UART considers the entire word to be garbled and will report a Framing Error to the host processor when the data word is read. The usual cause of a Framing Error is that the sender and receiver clocks were not running at the same speed, or that the signal was interrupted.
Regardless of whether the data was received correctly or not, the UART automatically discards the Start, Parity and Stop bits. If the sender and receiver are configured identically, these bits are not passed to the host.
If another word is ready for transmission, the Start Bit for the new word can be sent as soon as the Stop Bit for the previous word has been sent.
Because asynchronous data is “self synchronizing”, if there is no data to transmit, the transmission line can be idle.
No comments:
Post a Comment
Please leave ur valuable comments.