Home > Articles

This chapter is from the book

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.

Pearson IT Certification Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Pearson IT Certification and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Pearson IT Certification products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by Adobe Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.pearsonitcertification.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020