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Troubleshooting Modems

A section on troubleshooting modems has to be subdivided into two segments:

  • External modems

  • Internal modems

You should check an internal modem using the same basic sequence as any other I/O card. First, check its hardware and software configuration, check the system for conflicts, and check for correct drivers.

Improper software setup is the most common cause of modems not working when they are first installed. Inspect any cabling connections to see that they are connected correctly and functioning properly, and test the modem's hardware by substitution.

If an external modem is being checked, it must be treated as an external peripheral, with the serial port being treated as a separate I/O port.

Modem Symptoms

Typical symptoms associated with modem failures include the following:

  • There is no response from the modem.

  • The modem does not dial out.

  • The modem does not connect after a number has been dialed.

  • The modem does not transmit after making connection with a remote unit.

  • The modem does not install properly for operation.

  • Garbled messages are transmitted.

  • The modem cannot terminate a communication session.

  • The modem cannot transfer files.

COM Port Conflicts

Every COM port on a PC requires an IRQ line to signal the processor for attention. In most PC systems, two COM ports share the same IRQ line. The IRQ4 line works for COM1 and COM3, and the IRQ3 line works for COM2 and COM4. This is common in PC compatibles. The technician must ensure that two devices are not set up to use the same IRQ channel.

If more than one device is connected to the same IRQ line, a conflict occurs because it is not likely that the interrupt handler software can service both devices. Therefore, the first step to take when installing a modem is to check the system to determine how its interrupts and COM ports are allocated.

To install a non-PnP device on a specific COM port (for example, COM2), you must first disable that port in the system's CMOS settings to avoid a device conflict. If not, the system might try to allocate that resource to some other device because it has no way of knowing that the non-PnP device requires it.

Windows Modem Checks

In Windows 9x, you can find the modem configuration information by navigating to Control Panel, Modems. The Modems Properties dialog box has two tabs—the General tab and the Diagnostics tab. The Diagnostics tab, shown in Figure 3.13, provides access to the modem's driver and additional information.

Figure 3.13Figure 3.13 The Windows 9x Diagnostics Tab of the Modems Properties dialog box.

In Windows XP, the Diagnostics tab for the modem is available by clicking the Properties button on the Modems tab of the Phone and Modem Options dialog box. The Query Modem button on this tab can be used to perform low-level tests on the modem.

The Hayes AT Command Set

The Hayes command set is based on a group of instructions that begin with a pair of attention characters, followed by command words. Because the attention characters are an integral part of every Hayes command, the command set is often referred to as the AT command set.

AT commands are entered at the command line using an ATXn format. The Xn nomenclature identifies the type of command being given (X) and the particular function to be used (n).

Except for the ATA, ATDn, and ATZn commands, the AT sequence can be followed by any number of commands. The ATA command forces the modem to immediately pick up the phone line (even if it does not ring). The Dn commands are dialing instructions, and the Zn commands reset the modem by loading new default initialization information into it. After a command has been entered at the command line, the modem attempts to execute the command and then returns a result code to the screen. Table 3.2 describes the command result codes.

Table 3.2 AT Command Result Codes

Using the AT Command Set

At the command line, type ATZ to reset the modem and enter command mode using the Hayes-compatible command set. You should receive a 0, or OK response, if the command was processed. A returned OK code indicates that the modem and the computer are communicating properly.

You can use other AT-compatible commands to check the modem at the command-prompt level. The ATL2 command sets the modem's output volume to medium, to ensure it is not set too low to be heard. If the modem dials, but cannot connect to a remote station, check the modem's Speed and DTR settings. Change the DTR setting by entering AT&Dn.

  • n = 0—The modem ignores the DTR line.

  • n = 1—The modem goes to async command state when the DTR line goes off.

  • n = 2—A DTR off condition switches the modem to the off-hook state and back into command mode.

  • n = 3—When the DTR line switches to off, the modem is initialized.

If the modem connects, but cannot communicate, check the character-framing parameter of the receiving modem, and set the local modem to match. Also, match the terminal emulation of the local unit to that of the remote unit. American National Standards Institute (ANSI) terminal emulation is the most common. Finally, match the file transfer protocol to the other modem.

During a data transfer, both modems monitor the signal level of the carrier to prevent the transfer of false data due to signal deterioration. If the carrier signal strength drops below a predetermined threshold level, or is lost for a given length of time, one or both modems initiate automatic disconnect procedures.

Use the ATDT*70 command to disable call waiting if the transmission is frequently garbled. The +++ command interrupts any activity the modem is engaged in, and brings it to command mode.

Modem Hardware Checks

Modems have the capability to perform three different kinds of self-diagnostic tests:

  • The local digital loopback test

  • The local analog loopback test

  • The remote digital loopback test

If transmission errors occur frequently, you should use the various loopback tests to locate the source of the problem. Begin by running the remote digital loopback test. If the test runs successfully, the problem is likely to be located in the remote computer.

If the test fails, run the local digital loopback test with self-tests. If the test results are positive, the problem might be located in the local computer. On the other hand, you should run the local analog loopback test if the local digital test fails.

If the local analog test fails, the problem is located in the local modem. If the local analog test is successful, and problems are occurring, you should run the local analog test on the remote computer. The outcome of this test should pinpoint the problem to the remote computer or the remote modem.

If the modem is an internal unit, you can test its hardware by exchanging it with a known-good unit. If the telephone line operates correctly with a normal handset, only the modem, its configuration, or the communications software can be causes of problems. If the modem's software and configuration settings appear correct and problems are occurring, the modem hardware is experiencing a problem and it is necessary to exchange the modem card for a known-good one.

With an external modem, you can use the front panel lights as diagnostic tools to monitor its operation. You can monitor the progress and handling of a call, along with any errors that might occur.

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