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COMPUTER BITS BY JENNIFER BONNITCHA Talk to the world: modem mysteries revealed Have you been curious about modems but did not know what questions to ask? Like a lot of computer hardware, they are pretty inscrutable devices but very useful nonetheless. In this article, we give some interesting background on the subject and dispel some of the mysteries of Hayes-compatible modems. at least one COM port is standard on personal computers, this simple connection method opens up world-wide communication opportunities. Macintosh users will note that a special port on the rear of the computer has been set aside for a modem. Much like the personal computer, a special cable is required to attach the modem to the port. Most computer users are aware that a modem is a device which enables a computer to connect via a telephone line to similar equipment across town or across the world. The name Modem comes from the terms MOdulation and DEModulation. Modulation (the regulation, adjustment or varying of tone or pitch, amplitude or frequency of a wave by a wave of a lower frequency to convey a signal) is used because some signals cannot be directly sent over some transmission channels. The solution is to modify or modulate a carrier wave to represent the Modem types & connections original digital signal. Demodulation is the reverse process of modulation where the original signal is extracted from the carrier wave. The modem thus modulates digital data into a carrier wave for transmission through an analog system (ie, a telephone line) and then demodulates the carrier wave back to a digital signal at the receiving end for further use by a terminal or computer. The vast majority of modems are external boxes which occupy a small space near your computer. Using the appropriate cable, the modem is connected to a COM or serial port. Since Table 1 : Communications Standards 82 CCITT V21 300 bps full duplex; Australia & Europe CCITTV22 1200 bps full duplex; Australia & Europe CCITT V22bis 2400 bps full duplex; Australia & Europe CCITTV23 75/1200, 1200/75 full duplex; Australia & Europe CCITT V23 1200 bps half duplex; Australia & Europe BELL 103 300 bps full duplex; USA only BELL 212A 1200 bps full duplex; USA only BELL 2400 2400 bps full duplex; USA only SILICON CHIP Modems differ in operating parameters and performance according to their intended application and this can include: (1). Acoustic Couplers: these acoustically connect any telephone handset by cradling the coupler's acoustic transmitter and receiver to the handset. These may be OK for non-critical portable applications but they are now largely superseded by standard modems which are connected directlv to the phone lines. " (2). Modem Eliminators (line drivers): these can substitute for a conventional modem by extending the cable distance between two data terminal equipment devices beyond the 15-metre limitation imposed by the RS-232 interface. (3). "Standard" Modems: These are intended for communication through the public switched telephone network, leased telephone line or both switched and dedicated applications. The acoustic-coupled modem uses the telephone handset and two foam cups into which you insert the earpiece and mouthpiece of the telephone. The main problem with these is that stray sounds in the vicinity of the connecting cups can leak into the Table 2: The Hayes Command Set COMMAND DESCRIPTION A Initiates an answer mode data call Turn carrier off Enable carrier Pause for x seconds (default is usually 2) Forces the modem to re-enter the command state after dialling. Dial the number string "s" Does not echo characters typed when in the command mode Echoes characters typed when in command mode Half-duplex: Echoes characters typed during a data call Full-duplex: Does not echo characters typed during a data call Hangs up Goes off-hook Speaker always off Speaker on until carrier detected Speaker always on Goes back to data (on-line) mode Dials using rotary pulse dialling Enables status messages (Result Codes) Disables or turns off status messages Dials using touch Results Codes as digits Results Codes as words Wait for second dial tone Resets co C1 Ds EO E1 FO F1 HO H1 MO M1 M2 0 p QO 01 T VO V1 w z signal and affect your data. Also, connecting the microphone to the mouthpiece and the speaker to the earpiece is an easy mistake to make and very frustrating. As noted above, this type of modem is now pretty well obsolete. Direct-connect modems plug directly into your telephone or wall jack. Since the acoustic links are eliminated, the signals are cleaner and much more reliable. Auto-answer modems are usually able to auto-dial, thus enabling you to select a phone number or enter it direct from your computer keyboard. This versatile style of modem is the target of most communications software. Data transmission rates The differences between modem types are primarily speed-related. Speed is measured in bits per second (bps) but is generally referred to as baud rate (from Emil Baudot, an early pioneer of telegraphic communication). Originally, the baud described how fast dots and dashes could be sent on telegraph wires. Standardisation exists to the extent that 2400 baud is the most common rate at present. This represents about 230 characters per second. CCITT (International Telegraph and Telephone Consultative Committee) is an international body which establishes and recommends communications standards. The 2400 baud rate is also known as CCITT V22Bis. Other common baud rates are 300 (V21), 1200 (V23) and 9600 (V32). The Bell system has traditionally dominated the US communications industry and while the CCITT standards run parallel to many Bell specifications, the two systems are not fully compatible. Some of the more common standards are listed in Table 1. Low speed modems use bit rates up to 600 bps; medium speed from 1200 to 2400bps; high speed from 3600 to 16K bps and wide band 19.2K bps and faster. The "better" quality modems are capable of accurately sending more data down the phone line. While speed is important, particularly with STD or overseas phone charges to consider, remember that your fast modem needs a modem of equivalent capabilities at the other end to get the most out of it. Where time is money, you could consider a modem with data compression and error detection facilities. The modem analyses the data coming from your computer and sends it out as short sequences of characters to represent a longer information string. Thus, the data is effectively compressed; at the other end a modem which understands the compression method unpacks the compressed data. Communication codes There are three basic communication modes: (1). Simplex: the most elementary approach which provides 1-way communication between two points or transmission in one direction only, thus providing no way of responding - like your radio. (2). Half Duplex: provides transmission in two directions but only in one direction at any one time - like a taxi radio where the driver or controller may both talk but only one at a time. (3). Full Duplex: transmissions can occur in both directions simultaneously - like talking to your mother on the phone. When data is transmitted, it synchronises. or operates in response to the receiving device's clock. Most low to medium speed modems use asynchronous transmission where each information character sent contains one start bit and one or two stop bits to frame the character. Each character or sequence of bits is sent down the line in a strictly-timed rhythm. Thus, both the sending and receiving devices must be "in tune" for the inforDECEMBER 1990 83 if you look is a combination fax and modem. The Sendfax (from Bitzel Products) gives you the best of both worlds with a 2400 baud modem and 4800 baud fax. Load the appropriate software and you are in business. Table 3: Typical Result Codes DIGIT WORD DESCRIPTION 0 OK Command executed 1 CONNECT Connected 2 RING Ringing signal detected 3 NO CARRIER Carrier signal not detected or lost 4 ERROR Illegal command, error in command, command too long, invalid character format 5 CONNECT 1200 Connected at 1200bps 6 NODIALTONE No dial tone so subsequent commands discarded 7 BUSY Busy detected so subsequent commands discarded 8 NOANSWER Silence not detected so subsequent commands discarded 9 CONNECT 1200/75 Connected in 1200/75 mode mation to be received in an intelligible manner. The start and stop bits allow the receiving and transmitting devices to synchronise with each other. Though advantageous at high speed, sophisticated timing circuitry is required at both ends of the transmission. High speed synchronous transmissions also require high cost devices to control the data flow. Using your modem The popularity of bulletin boards, the ability to perform banking transactions and the listing of up-to-date Modem commands share price information, amongst other things, means that today there is virtually no information you can't access with a computer and modem. All kinds ofreally good stuff is available for the asking. A fax machine lets you talk to the world. Basically, it consists of a telephone, scanner, thermal printer and modem. Fax cards are a cost-effective method of sending and receiving information. You can only send the information stored in your computer unless you have access to a scanner, thus this limits to a certain extent the usefulness of these devices. On offer Table 4: Pin Assignments For Connectors PC END D25F 2 3 4 5 6 7 8 12 20 22 84 D9F 3 2 7 8 6 5 1 NA 4 9 SILICON CHIP MODEM END SIGNAL TD RD RTS CTS DSR GND7 CD HS DTR RI DIRECTION To Modem 2 From Modem 3 To Modem 4 From Modem 5 From Modem 6 From Modem 8 From Modem 12 To Modem 20 From Modem 22 D25M Mandatory Mandatory Mandatory Desirable Desirable Modems manufactured by Hayes Microcomputer Products Inc were among the first to employ a modem control language. A modem needs software to "drive" it (much like a fax card requires operating software), providing information via a series of commands to, for example, dial a telephone number, automatically answer a received call and disconnect finished calls. The microprocessor inside the modem enables it to interpret the software commands. As a result, Hayes utilised the term "Smartmodem" and sales boomed. Other vendors subsequently incorporated the same command set into their products and thus helped to give almost total acceptance to one of the few standards in the computer industry. Communications programs were able to be used on modems manufactured by any vendor utilising the "Hayes-compatible" command sets. AT is a mnemonic for ATtention and each command in the Hayescompatible modem must be preceded by the character sequence AT in uppercase notation to gain the modem's attention - thus the AT commands. After the initial AT, the following commands can be in upper or lowercase. Table 2 lists some of the commonly used commands in the Hayes modem command set. Note that it is by no means exhaustive and you should check your modem manual for all the codes it supports. Each command sent to a Hayescompatible modem consists of an attention sequence (uppercase AT), the command and a terminating carriage return. Each command starts with a single letter (eg, Dial, Tone, Pulse) and may have additional characters as well. Some commands use numeric options; if you choose to use just the command letter, the number O (zero) is assumed. You can put more than one modem command on a single line provided any command that dials or answers the phone is last on the line. Secondly, any command which resets the modem also clears the buffer, so no The MegaModem from Avtek obeys the Hayes command set, can run at speeds up to 2400 baud, and features autodialling (tone or pulse) & auto answer. following commands are executed. For example, to command a modem to automatically dial a call, you could enter the following: AT DP9, T1234567 <CR> The AT is the modem attention command while the D tells the modem to initiate dialling using pulse dialling (P). The 9 might be included if the modem's telephone line requires that digit to access an outside line. The comma is useful for making calls through a PABX to ensure sufficient time elapses between tones before continuing. In this example, the command causes the modem to pause for two seconds (the default setting) before continuing to dial. Then the phone number is dialled using tone (T) dialling. Once the number has dialled and your modem connects to another modem, it is said to be in "on-line" mode. The rest of the operation takes place between the two devices. Sometimes you may need to get back to command mode. Depending on the operation in progress, the modem transmits characters and monitors the information. You can interrupt the modem by: (1). a silence of one second or longer; (2). 3 "+" characters - the Escape command; (3). another silence of one second or longer. The modem responds by returning to command mode where you can hang up (ATH), change parameters as required, or return to online mode (ATO). In response to commands, modems can issue result codes. These can be sent as either a single digit or a word code via the V command so they can be interpreted by the operator. You can suppress them altogether via the Q command. As always, check your manual for information appropriate to your modem. Typical Result Codes are shown in Table 3: A table showing the 25-pin serial port pin assignments (IBM/clones) was published in last month's column, while Table 4 shows the pin assignments for the D25M modem end of the serial cable and for the D25F and D9F PC connectors. AT-style PCs using the D9 connector cannot support the HS signal due to insufficient pins on the connector. What next? You can set up your modem to make it answer a phone automatically after a specified number of rings for example. You can learn more about your modem by checking the manual since most manufacturers provide some "extra" features. With patience and practice, you can make your computer talk to the world. 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