As it prints, it reduces the amount of characters in its buffer. When the buffer has sufficient space, it sends out a DC 1 code to the computer, which resumes the transmission of data to the printer. The ASCII codes DC 1 and DC3 stand for Device Control 1 and Device Control 3. This method can transmit and receive text files, since the printable ASCII set ranges from 32 to I ~ Binary files (i. e. containing machine code) contain a full range of possible numeric values. This would include the values for the DC 1 and DC3 signals, which means that XON/XOFF is not suitable for transmitting and receiving binary files.
RS232 Pins A summary of the uses of the main pins on a computers serial port is shown in the following table. DTR Data Terminal ready The computer informs the modem that it is powered up and ready to be active, by switching this pin to an ON state. Most modems require to receive this signal before they will operate DSR (Data Set Ready) The modem informs the computer that it is powered up and ready to be active, by switching this pin to an ON state. Most computer ports require this signal before they will operate
CTS (Clear to Send) The modem informs the computer that it is able to accept data for transmission, by switching this pin to an ON state. The computer will not send out data while this pin is OFF RTS (Ready to Send) The computer informs the modem that it wishes to give it data for transmission, by switching by switching this pin to an ON state The modem responds by switching its CTS line ON unless its memory buffer is full, or it is receiving incoming data. TXD (Transmit Data) Carries the data from the computer to the modems RXD pin, to transmit data.
Carries the data from the modem to the computers RXD pin, to receive data. RXD (Receive Data) Receives the data from the modems TXD pin, to receive incoming data. Or Receives the data from the computers TXD pin, to send outgoing data. DCD Data Carrier detect Used by the computer to determine whether the modem has an incoming carrier (i. e. whether the line is idle or not). Some communications packages must detect a DCD signal before they will carry on. This signal can be brought from the modem or can be provided locally by a wraparound.
This connects the computers DCD pin to the computers DTR pin to simulate an idle condition. RI (Ring Indicator) This pin could be used by an auto-answer modem. Its value is raised high when the phone rings. The modem informs the DTE via this change in the RI line and the DTE responds by setting its DTR line high. The modem then answers the call and data is passed from the telephone line to the DTE. * SG (Signal Ground) This pin is used as the reference for all other signal voltages. So, if a pin swings +15 it means that the pin is a 15 volts higher than the voltage on SG.
This pin should not be confused with electric earth or Frame Ground (FG). Error Detection Modem communication over the normal telephone network is always prone to losses due to poor line conditions. As transmission rates become faster, the losses are increased. If an interference pulse occurred on a line a 28,800bps system, the transmission would be affected 24 times more badly than a 1,200bps system, since 24 times more data will have been transferred during that time. Serious attention needs to be paid to detecting and correcting such errors.
The parity bit system described earlier is only a rudimentary check and only applies to ASCII files. Since most files are not plain ASCII, they will require to use all eight bits of the byte and there will be no parity bit. CRC Checking Data is transmitted in blocks or packets with a checksum created using the CRC (Cyclic Redundancy) method. When the data is compiled into a block prior to transmission, a mathematical formula using polynomial codes) is applied to the data to produce a check number that is unique to the data in the block. These check digits are then transmitted along with the data.
The receiver stores the incoming block of data in a buffer for examination. The same formula is applied to the data in the buffer d it should produce the same answer as that stored in the check bytes. If the computed CRC figure accords with the stored CRC figure, the data in the buffer is fit to be passed on and an ACK signal is returned to the transmitting end to acknowledge the receipt of a block in good condition. If there has been any corruption of the data in the block, or even any corruption of the check bytes, then the formula produce answers that do not match.
In this case, the device will request that the block of data be retransmitted This is done by returning a NAK signal to the transmitting end. A number of different block transmission techniques and error detection methods are in common use they are referred to as File Transfer Protocols and include: XModem XModem sends a 128 byte block of data plus one checksum byte constituted from the sum of all the ASCII codes the block. It then waits for an acknowledgement that it has been received. The waiting time reduces the average data transfer average.
All errors are given ten retries before abandoning the file transfer. It also has a ten-second timeout; it sends a NAK signal if has not received any incoming signal after a ten second wait. A variation known as Xmodem/CRC sends the same 128 byte block but sends two CRC bytes. This increases the redundant bytes to be transmitted but it improves the reliability of the system and is used in preference to Xmodem where it is available. Kermit Kermit is a long established system used with mini-computers and mainframes as well as PCs. It also uses a 128 block and it provides a header with control information.
Although it is widely available and reliable, it is a slow method. It is probably best used with noisy lines. Ymodem This method still uses 128 byte chunks but it sends eight of these chunks before sending a two-byte CRC code that covers the entire 1024 data bytes. The method also pads out the data block until it is exactly 1024 bytes. This is fast since most of the transmission is made up of real data and there is only two bytes of redundant information. However, on a noisy line, corruption of a single bit results in the entire 1024+2 bytes being re-sent.
YModem is faster than XModem on good lines but worse on noisy lines. It also allows multiple files to be transferred in the one operation. The user selects the group of files to be transferred and the system sends them all during the one long operation. This is sometimes referred to as Ymodem/Batch. Zmodem The above systems send their block and then wait for an ACK or a NAK from the receiving end before sending another block. They are working in half-duplex mode, resulting in wasted waiting time. Many modem methods do not wait for a response between sending blocks.
Using a full duplex system, it is still able to receive the ACKs and NAKs when they arrive but further transmissions can be undertaken in the meantime. When an ACK arrives it knows that a particular block has been received successfully, while an incoming NAK indicates an unsuccessful transmission of a block. The system keeps track by using a windowing system. Each outgoing packet is given a packet number along with the transmission. The incoming ACK or NAK will also have a packet number attached. In the event of a NAK the system knows which packet to re-transmit.
The difference between the packet currently being sent and the most recent incoming ACK or NAK is known as the window. Since a single byte can contain 256 different numbers, there is a limit to the range of available packet numbers and older numbers are reused after they have accomplished a successful transmission. This means that the window covers a small range of the available numbers and is always changing. This gives the name sliding windows protocol; it is also implemented in the Sliding Windows Kermit and Xmodem systems. Xmodem, Ymodem and Kermit are used at speeds below 96,00bps.
V42 is used at higher speeds and is implement in a ROM chip in the modem. The V42 standard encompasses the MNP 4 proprietary standard developed by Microcom. It is likely that future extensions of the V42 standard will move away from MNP 4 in favour of LAP-M (Link Access Protocol for Modems) scheme as already recommended by CCITT. The MNP 5 standard combines the MNP 4 error correction facilities with data compression. Like MNP 4 it is most commonly implemented in hardware although some communication packages implement it in software. Standards.
Created by CCITT, The International Telegraph and Telephone Consultative Committee, working under the ITU (International Telecommunications Union) which is organised by the United Nations. These standards are divided into a number of groups, including: V-series, dealing with telephone circuits X-series, dealing with data networks 0-series, dealing with digital networks (digital exchanges, multiplexing, PCM, etc. ) I-series, dealing with ISDN (see later) V Standards There is a wide range of definitions and the most common ones are shown in the table.
The V standards get their name from the first letter of the word vitesse, the French for speed although not all V standards are concerned with transmission rate. V24, for example, specifies the serial port standard and V42 and V42bis cover error correction. The bis added to a V number means that it is the second version of the standard. Dataflex, a large UK modem manufacturer, produces VFC models, also known V. Fast Class models, working at 28,800bps. These were introduced before the ratification of the V34 standard and there are some handshaking differences between their specifications. V.
Fast modems can communicate with other VFC models at the top rate but with lA ~ ~t, ~k~1t lflO/ ~ V3 5 models, they can only communicate at 14,400 bps in about 10% of cases Modem Standards Specification Operation Circuit Max Bit Rate Modulation Technique Bell 103 Full Duplex Two Wire Switched 300 Frequency Shift Keying V. 21 Full Duplex Two Wire Switched 300 Bell 202 Half Duplex Two Wire Switched 1,200 Conditioned Lease 1,800 Bell 201 Half Duplex Two Wire Switched 2,400 V. 26ter Full Duplex Two Wire Switched 2,400 Bell 212 Full Duplex Two Wire Switched 1,200 Phase Shift Keying V. 22bis Full Duplex
Two Wire Switched 2,400 Quadrature Phase Shift Keying V. 27 Full Duplex Four Wire Leased 4,800 Half Duplex Two Wire Switched 4,800 V. 29 Full Duplex Four Wire Leased 9,600 Quadrature Amplitude Modulation Half Duplex Two Wire Switched 9,600 V. 32bis Full Duplex Two Wire Switched 14,400 V. 34 Full Duplex Two Wire Switched 28,800 Bell 103 Full Duplex Two Wire Switched 28,800 V. 42 V42bis Data Comp V90 56,600 Even Parity The sending device counts the number of one bits in the character to be Panty Bit transmitted. If the number of these data bits is even, as in the top diagram, the parity bit is set to zero.
If the number of one bits is odd, as in the lower diagram, 1001 1 10 the parity bit is set to one. Thus the total number of ones in the byte will Parity Bit always be even, no matter how many ones are in the character being transmitted0100101 If any of the data bits or parity bits is accidentally altered during transmission, the receiving device can detect the problem by counting the number of bits. If the total is not an even number, there has been corruption of the data. This provides an elementary check for data errors. Odd Parity Here, the sending device counts the number of one bits in the byte to be transmitted.
If the number of these data bits is even, as in the top diagram, the parity bit is set to one. If the number of one bits is already odd, as in the lower diagram, the parity bit is set to zero. In this way, the total number of ones1 in the group is always maintained at an odd value. Note Parity checking is a useful facility but it is not foolproof. For example, two bits in a byte both being altered from zero to one would produce a correct parity check although the data in the byte had been corrupted. In practice, larger blocks of data are examined for corruption (see section later on Error Detection).