The Online World

by Odd De Presno

Dumb or intelligent modem? ————————————— Some modems are operated with switches or buttons on a panel. They do not react to commands from your computer. We call them dumb. You must dial numbers manually, and press a key on the modem, when you hear the tone from a remote modem. Only when the modem is connected to the remote modem, can you ask your communications program to take over. We call those modems 'intelligent' that can react to commands from your computer. Most of them react to commands according to the Hayes standard. Buy intelligent, Hayes-compatible modems - even when other standards may seem better. Most of today's communication programs are designed to be used by such modems.

Note: Buy modems that use the Hayes extended command set.

When a popular communications program, like Procomm and Crosstalk, tells the modem to "dial a number" or "go on hook," then the Hayes- compatible modem will do just that. When you press ALT-H in Procomm, the modem will disconnect from the remote modem. If you press ALT-D followed by the number "2," Procomm will locate the number to an online service in your telephone directory, and dial that number. When the connection with the remote modem has been established, your modem will report back to you with a message like CONNECT 2400. This tells that a connection has been set up at 2400 bps. If I select "k" from a menu provided by my communications program's command scripts, then my system will retrieve today's business news from Tokyo and put them up on my screen. In the process, my system tells the modem to do several things, including "call a number," "speed 2400 bps," "redial if busy," "go on-hook when done." The only thing that I have to do, is press "k". The communications program and the modem will do the rest. Automatic communication is impossible without an intelligent modem.

The Hayes standard ————————— The U.S. company Hayes Microcomputer Products, Inc. pioneered command-driven modems. Their Smartmodem became a success, and "Hayes compatibility" a standard for intelligent modems. Today, it is as unimportant to buy a Hayes modem to get access to Hayes commands, as to buy an IBM PC to run PC software. Automatic dialing (autodial) was one of Smartmodem's important features. The modem could call a number and prepare for data communication, once a connection had been set up. If the line was busy, it could wait a while and then redial. The operator could work with other things while waiting for the equipment to be ready for communication. The modem had automatic answer (autoanswer), i.e., when someone called in, the modem could take the phone off hook and set up a connection with a remote modem. The modem enabled a connected PC to act as an electronic answering machine. Hayes-compatible modems can report call progress to the local screen using short numeric codes or words like CONNECT, CONNECT 1200, CONNECT 2400, NO CARRIER, NO DIALTONE, BUSY, NO ANSWER, RING etc. There can be small differences between such modems. The message DIALTONE on one modem may be DIAL TONE on another. Most of the main progress messages, however, are the same across brands. The old Smartmodem had switches used to configure the modem. Most modern Hayes-compatible modems come without switches and have more commands than their ancestor. Today's Hayes-compatible modems have a core of common commands, the "real" Hayes-commands, and several unstandardized additional commands. Here is an example:

A standard on the move ——————————— On the Quattro SB2422 modem, 2400 bps speed without automatic speed detection is set by the command "AT&I1." The equivalent command on Semafor's UniMod 4161 is "AT+C0". Automatic detection of speed is a feature that lets the modem discover the speed of the remote modem to set its own speed at the same level. (Other modems may use different commands to set this.) When I want Procomm to call a bulletin board, it first sends a sequence of Hayes commands to the Semafor modem. The purpose is to "configure" the modem before calling. It sends the following:

AT S0=0 +C0 S7=40 S9=4 &D2

The cryptic codes have the following meaning:

AT "Attention modem. Commands following.." S0=0 No automatic answer +C0 No automatic speed detection (fixed speed) S7=40 Wait 40 seconds for an answer tone from the remote modem. S9=4 Wait 4/10 seconds for detection of carrier &D2 Go on-hook if the DTR signal is being changed.

If this command is sent to the Quattro modem, it will reply with "ERROR". The code "+C0" must be replaced with an "&I1". The rest of the commands are the same. (Note: when a modem responds with "ERROR," it has usually rejected all commands sent to it!) This setup is held in the modem's memory when Procomm sends its dialing command: ATDT4737031378. AT stands for ATtention, as above. DT stands for Dial Tone. Here, it is used to dial the number 4737031378 using tone signaling (rather than pulse dialing).

The modem cable ———————- If you have an external modem, you must connect your computer to the modem with a cable. Some modems are sold without a cable. This cable may be called a serial cable, a modem cable, a RS232C cable, or something else. Make sure that you buy the correct cable for your system. Make sure that the connectors at each end of the cable are correct. If a male connector (with pins) is required in one end and a female (with holes) in the other, do not buy a cable with two male connectors. Some connectors have 9 pins/holes, while others have 25 or 8- pin round plugs (Apple computers). Use a shielded cable to ensure minimal interference with radio and television reception. At this point, some discover that there is no place on the PC to attach the cable. Look for a serial port at the rear of your machine, labeled MODEM, COMMUNICATIONS, SERIAL, or with a phone symbol. If you find no suitable connector, you may have to install an asynchronous communication port in the box.