The immediate problem could be caused by the Zoom MODEM, serial port, MODEM cable, telephone line, COMPUSERV, or the configuration of the MODEM/port (see FAQ below for troubleshooting). Try the external MODEM on another ISP or BBS (or are they a thing of the past now). You can try it on another ISP without signing-up if you know the phone number. Many ISPs list dial-up numbers on their web sites (many don’t, but you must have a friend who doesn’t use COMPUSERV). In fact, just about any phone number can be used to test the ability of the MODEM to pick-up the line, detect a dial tone, and dial. Use the Windows Hypterminal, which you will probably have to install with Add/Remove programs in the Control Panel. The MODEM should connect 9ta an ISP), handshake, and you should receive a prompt to login. If that works, try the same thing on COMPUSERV. If you have a phone system other than a regular old phone with a direct connection to phone line, that is probably the problem. The internal MODEM may be causing a resource conflict problem with the serial port used for the Zoom MODEM, try pulling it. Try disabling COM1: on the motherboard if you do not need it for a serial mouse. I stopped using COMPUSERVE many years ago… Too expensive, but I suppose you have one of those multi-year contracts that are sold with computers? LarryQ. How can I test the serial port and cable connected to an external MODEM?
A. There may be a diagnostic program that came with the MODEM, if that is what you are connecting to. A MODEM that uses the Hay's AT command set (most do) can be tested by simply echoing an AT command at the DOS prompt and redirecting it to the COM port, e.g.,
C:\>ECHO ATDT > COM1
Should cause a MODEM on COM1: to pickup the phone line and you should hear dial tone from the MODEM.
C:\>ECHO ATH > COM1 USING ECHO
...should cause it to hang-up, killing the dial tone.
MODEM Doctor is a useful MODEM diagnostic shareware program that will check both ports and MODEMs.
Windows 9x/Me has a MODEM Diagnostic at Start, Settings Control Panel, Modems, Select the MODEM, Diagnostics tab, select the port/MODEM, More Info. In Windows 2000: Start, Settings, Control Panel, Modems tab, Properties, Diagnostics, Query Modem.
MODEMs can be tested by dialing a local Internet Service Provider with Windows Hyperterminal (I used to dial a local bulletin board). You can pretty much tell whether the serial port and cable are working or not by typing something in the Windows HyperTerminal and looking at the LEDs on the MODEM. AT commands can be issued from HyperTerminal as well. The MODEM should respond to AT with OK. ATIn should displays the MODEM's manufacturer information for n = 1 through 7. This provides information such as the port speed, the result of a checksum test, and the model information. Check the manufacturer's documentation for the expected results.
MS Diagnostics (MSD) that comes with Windows 95 and MS-DOS 6.X is useful for determining if a serial port is responding (it is also on the Windows 98, 98 Second Edition, and Windows Millennium Edition CD's at D:\tools\oldmsdos where D: is your CD-ROM drive letter).
A loopback test, which requires a loopback plug/adapter, is a very good test of a serial port. Several commercial diagnostics programs (e.g., AMIDiag) come with a loopback plug. When a loopback test is run data is sent out the serial port’s transmit pin to the loopback plug which sends the data back to the port’s receive pin. This test assures the 1488 and 1489 buffer chips or equivalent are working and they are the chips that are usually damaged by a close lightning strike, etc. Scott Mueller’s Upgrading and Repairing PCs has more info and instructions for making a loopback plug. Just search Google for the book. Every shop should have a copy. However, the best and easiest to read shop reference on serial ports and cables I know of is Data Communications for Microcomputers by Elizabeth A. Nichols, Joseph C. Nichols, and one of my business acquaintances from the old days, Keith R. Musson, McGraw-Hill 1982. I think it is out of print.
An RS-232 breakout box and a multimeter are useful tools for troubleshooting serial ports and cables.
