Classic Computer Magazine Archive A.N.A.L.O.G. ISSUE 62 / JULY 1988 / PAGE 26

Tips on Modem Use

Modems aren't particularly difficult to set up and use. On the other hand, it's relatively easy to foul up your modem's operation through carelessness or lack of knowledge. This article will serve as a guide in setting up and using your modem. If you observe the procedures and precautions mentioned herein, you'll save yourself a lot of trouble later on.

by Michael A. Banks

by the Book

    As with any electronic device, it is important that you connect your modem properly and use it under practical operating conditions. Study the manual that comes with your modem to assure proper connection, and consult with your computer store and the modem manufacturer's customer-support department if necessary.

and Cables

    The cables (and the connectors used with those cables) that connect a computer's serial port with a modem are obviously very important elements in the data transfer chain. Like serial ports, connectors and cables used with serial ports must conform to the RS-232C standard.

    There are two types of RS-232 connectors-nine- and 25-pin-and these may be male or female. Nine- and

With some equipment you may 
find that you have what is called
a "gender problem,"                  

25-pin connectors are known as DB-9 and DB-25 connectors, respectively. Each type has numbered pins (very important if you intend to make your own cables-saves a lot of messing around with a continuity tester).
    DB connectors can be found on the serial port of your micrcomputer and modem, and at either end of the connecting cable. (A connecting cable is typically a "ribbon cable"-a flat cable with multiple connectors.)


    DB-25 and DB-9 connectors can be used at opposite ends of a cable if necessary (as when a computer's serial port has a DB-9 connector and its modem has a DB-25 connector). All that's required for the connection to be successful is that the pins on each connector be properly wired (i.e., the wire on each numbered pin on the DB-9 connector should be connected to the correspondingly numbered pin on the DB-25 connector).

and Cable

    Incidentally, there's a standard that dictates that the female version of a DB connector should be used only on modems, while the male version should be used on computer serial ports. Thus, a "standard" RS-232C cable will have a male connector on one end (to connect with the modem), and a female connector on the other end (to connect with the computer).
    Unfortunately, not all manufacturers follow this standard regarding the gender of their serial ports. So, with some equipment you may find that you have what is called a "gender problem." (No sex-change jokes, please-this is serious stuff!) When this is the case, you'll have to buy or make an appropriate cable with both female or both male connectors. Or, you can obtain what are called "gender changes" to change the "sex" of one end of the cable.


    Before plugging a telephone line's modular plug into a modem, make sure of the plug's type. Most modems are designed with modular jacks, but modular (also designated "RJ") jacks and plugs come in more than one variety. Some are cross-compatible, and some aren't.
    Generally, a home or single-line business telephone system uses RJ-11 plugs, and these present no problem-even if the plugs are set up to provide dial light power to a "Trimline" phone or other lighted-dial telephone sets. (The only danger in using an RJ-11 plug that provides power for a lighted dial with a modem is if the modem is set up to operate with RJ-12 or RJ-14 plugs. See below for more information on these plugs.)
    RJ-41 and RJ-45S plugs are also "safe" to use with most all modems; the exceptions may be modems which have RJ-12 or RJ-14 plugs. See your modem's documentation for details.
    If your telephone system is a multipleline or "key" telephone system, you must have a modem that is capable of interfacing with RJ-12 or RJ-13 plugs (such as a Hayes Smartmodem 2400). The modem you use must also be software-switchable to RJ-12/RJ-14 operation.

Getting "Set"
DIP Switch

    Most modems have user-accessible DIP switches (although the relative accessibility varies from modem to modem). DIP switches are used to set various attributes of a modem, such as whether it waits for a carrier detect before going online, etc.
    Some software packages require that certain modem attributes be set to a specific state. If your software has decent documentation, it will tell you which states must be set; in which case all you have to do is refer to your modem's documentation to find out which DIP switches are used to set the attributes in question. (Some software manuals will even tell you how to set each DIP switch on the more popular modern brands. Too, some modem manuals provide specific instructions on DIP switch settings for certain software packages.)


    Local telephone company regulations may vary, but in general the following rules are in effect:
    Your telephone company should be notified that you will be connecting an FCC-registered device to your telephone line before you connect it, and that you will be disconnecting the modem when you disconnect it permanently.
    You cannot connect a direct-connect modem to a pay telephone, nor to a party line.

and Heat

    Don't use a modem as a bookshelf or repository for other materials. While some external modems are designed to serve as a resting place for a telephone set, they aren't designed to be smothered by papers, disks, etc. A modem's electronic components generate heat, which must be dissipated; too much heat buildup can interfere with proper operation of the modem. Therefore, heat vents-as well as most of the top of the modem-should not be covered.

Use the

    If your modem is equipped with a power switch, use it to turn the modem off and on. Leaving the switch in the "ON" position and just plugging and unplugging the modem's power supply is not a good idea; this can occasionally create power surges or current overload.

Don't use a modem as a
bookshelf or repository for other

When changing the battery in a battery-powered modem, the power switch should be in the "OFF" position, for the dame reasons.


    Don't plug your modem into an overloaded or faulty circuit. Aside from the fire hazard this creates, overloaded circuits often have low voltage, and low voltage can cause excess heat and poor performance in your modem. (Overloaded circuits are typically those with too many electrical devices plugged into them.)


    Surge protectors (also called "spike protectors") are an excellent investment. The purpose of a surge protector is to protect an electronic device from surges in a power or telephone line. Such surges are common during thunderstorms and during periods when electrical power consumption is particularly heavy. Power-line surge protectors come in a variety of styles, but all operate in the same manner. Placed in the circuit between your computer and/or modem and the wall outlet, they contain capacitors which absorb and then bleed off excess power. Note that power-line surge protectors come in several configurations. Some are simply small cylinders or cubes and offer only one receptacle. Others are large rectangular boxes which mount on the wall in place of the wall receptacle's cover. These usually offer more than one receptacle. Some of the better surge protectors not only provide protection against power surges, but also filter "line noise," and provide a circuit breaker for protection against current overload. Telephone-line surge protectors operate on the same principle as power-line surge protectors. Installed between a modem and its telephone line, a telephone-line surge protector absorbs then slowly discharges potentially damaging voltage spikes.


    Never use your modem during a severe thunderstorm, nor at any time you observe lightning. Lightning is a guaranteed source of power surges in both AC power lines and telephone lines and, unless you have a surge protector on both your modem's telephone line and power line, there's an excellent chance that your modem and computer will he "zapped" by a current surge. (Even with surge protectors, there's no guarantee that lightning won't damage your equipment.)

    There are several excellent books on using modems that expand upon these topics with technical information. These include:
    The Modem Book, by Michael A. Banks (Brady Books, 1988); Understanding Data Communications, by George E. Friend, et. al., (Howard W. Sams & Co., 1987); and Communications and Networking for the IBM PC & Compatibles, by Larry Jordan and Bruce Churchill (Brady Books, 1987).
    Check with your local computer store for information on ordering these books.