Tech Files

Troubleshooting a Modem

Modems are pretty robust beasts, but most techs get the call at one time or another.

Phone: Brrrrring! Brrrrring!
Tech: Hello?
Caller: Help! I can't get on the Internet!
Tech: I'd be a heck of a ride if you could!
Caller: Huh? No, I mean can't connect with my modem.

At this point, you'd follow pretty standard troubleshooting techniques, asking if the modem ever worked, if there's a dial tone, checking Device Manager, and so forth. If you can't solve the issue over the telephone, then a house call is in order to determine if the modem is alive or dead.

You need two tools to determine the physical health of a modem, knowledge and Modem Doctor software. You should know at least the standard modem commands, often called AT commands. And you can download Modem Doctor.

Modem Commands

Modems have many functions, and there must be a way to speak to a modem to tell it which function to perform (answer the phone, perform data compression, and so on). These commands are known collectively as the modem command set. You'd think that there would be a standard for modem commands, but there isn't. Modem commands vary from manufacturer to manufacturer, although a few front-runners tend to control things. The number one name in modem command sets is certainly Hayes. Hayes developed the famous AT command set in the early 1980s and released it to the public. All modem manufacturers use the majority of these commands.

AT commands are typed at a command prompt. To use these commands, you have to make sure the communication software is loaded and that you are either in terminal mode or at a terminal screen. Unless your modem is set up to auto-connect, which is the online mode, it should be in command mode, which is where you use the AT commands. Although there are hundreds of these commands, the table below lists only the ones that technicians actually use.

AT	Your modem should respond with an OK, letting you know that your modem is plugged in and turned on.
ATE1	Echo your command on the screen.
ATE0	Turns off the echo to the screen. Some modems will not run correctly with the echo on, so make sure you turn it off when done with it.
ATH1	This is taking the phone off the hook. You should get a reply of OK or 0 back from the modem, a dial tone and an OH indicator on the modem, if it's an external modem.
ATM1	Turns the speaker on, this is for the dial tone. ATL0 is the lowest volume. ATL2 is the medium volume.
ATM0	Turns the speaker off.
ATD	This command will take the phone off the hook as ATH1 did. It will also dial a number if you include it with your command, i.e. ATDT2222222. The T is for tone. Put a P in its place for a pulse phone. If you include a W (ATDTW), it will wait for a dial tone before dialing. If you include a (,) anywhere after the command, it will pause before continuing to dial. For instance, you may need a 9, for a outside line.
ATQ0	This enables result codes. This aids in troubleshooting problems. If you typed an ATV1 prior to this command, you will get back verbose result codes. (OKBUSYCONNECT 2400,9600COMPRESSION:V.42bis). If you type ATV, or ATV0, than you will get number or short codes: i.e. 0, 12,10.
ATQ1	Disables result codes.
ATH,ATH0	Makes the modem hang up.
ATZ	This resets your modem to a predefined state. You can reconfigure your own profile for the modem for resetting. If it wasn't set previously, then it will reset to the factory's setting.

Table 1: AT Commands

Modem Doctor

A broken modem is probably the most frustrating repair problem in the PC world. But by following these steps, you can make the problem much easier (I didn't say easy, just easier) to repair.

Is the Modem Using a Non-Conflicting COM Port and IRQ?

To find out, I turn to a very powerful shareware program called Modem Doctor, made by Hank Volpe. This program will query all your COM ports looking for a modem, as you can see in the figure below. Modem Doctor does an excellent job of detecting COM ports and determining whether there is a modem at a particular COM port.

Figure 1: Modem Doctor looking for a modem

Click for larger image.

Many technicians make the mistake of not considering a COM port or IRQ conflict unless the modem or some other hardware has just been installed. Do not do that! Many different cards can be changed by software or corrupted files. Assume nothing!

Has the COM Port or Modem Failed?

Modem Doctor does far more than just verify a COM port's IRQs. Many times a technician wants to know if the modem or the COM port is still good. That is where Modem Doctor shines. Modem Doctor will test your modem thoroughly and give you a complete description of its quality.

Modem Doctor will perform two different loopback tests. First it will perform a Local Digital loopback , which verifies that the system's digital signal can be read by the COM port. Then it will perform a Local Analog loopback , which sends a signal through the COM port to the modem that converts the signal to analog and then loops it back through the system. Local Analog loopbacks are the preferred test because they test both the COM port and the modem. They require an analog loopback plug, however, available at any electronics store.

The one nice thing about modems is that they never get sick; they just die. Modem Doctor will let you know without a doubt if your modem is alive or dead. The figure below shows a modem passing with flying colors. Note the AT commands used. If Modem Doctor says your modem is okay, then it's time to look at the software.

Figure 2: A diagnostic display of Modem Doctor