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Last updated: 5/28/01

Q.  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.,


Should cause a MODEM on COM1: to pickup the phone line and you should hear dial tone from the MODEM.


...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.

Q. How can I quickly run a speed test on MODEM?

A.  Search google.com with modem speed test to find various web sites that test MODEM speeds.

Q.  My AOpen FM56-P modem auto selects IRQ0 and COM3. No way to get around these settings. Running on ASUS P2B-F AGP Motherboard. Best speed is 33.6K. Somehow V90 is not working. Can you help?

A.  A MODEM using IRQ0 (the one used by the System Timer) is a problem I haven't seen before. Try forcing the MODEM to use another COM port. Power-down, pull the MODEM, power-up, go into the CMOS Setup, Go into Plug 'n Play, Set it to Manual, Assign, IRQ 3 to Plug 'n Play from Legacy/ISA, go into Integrated Peripherals, disable Serial Port 2, Change Serial Port 1 to 3BC/IRQ 4 (to stop the MODEM from using it), power-down, plug-in the MODEM, power-on. The idea here is to block COM3 and , hopefully, push the MODEM into COM2: where it belongs.

How do you know the MODEM isn't or trying to talk V.90?  Theoretically, you have to be within four miles of the telephone switch (exchange - not the ISP) to go from analog at 33.6 to digital at something less than 56K. V.90 MODEMs can not operate at 56K. It is illegal. The fastest they can go is 53K. I have never seen one go faster than 49K and my store is less than a mile from the telephone switch. I have seen them go faster than 33.6 at distances greater than four miles, however. Is your ISP set-up for V.90?  Are you using the right phone number for your ISP's digital dial-up?  Some ISP's have different phone numbers for analog and digital connections. What is the maximum speed showing in the MODEM properties?  It should be 115,200.

Q.  My Aopen FM56-ITU Modem keeps disconnecting within 30 seconds to about 2 minutes when online. I have no trouble with my US Robotics modem. I try the FAQ's and newsgroups on Aopen's web site and see the same complaint from other people. Please tell me how to fix this problem.

A.  My guess is you have a bad MODEM, IRQ conflict, noisy phone line, or a compatibility problem with your ISP--in that order. Try a dial-up to another provider. Your ISP is running V.90 or K56flex, right? X2 will work with a US Robotics MODEM, but it will not work a FM56-ITU. If your ISP has not updated to V.90 by now I would get a new one.  I have had very good luck with the FM56-ITU MODEM.

Q.  I have a  RCVDL56ACFW/SP MODEM and wondered if you know of where I can get the Windows NT 4.0 drivers for it?

A.  That is the Rockwell chip on the MODEM.  You will need the MODEM Manufacturer and Model.  With that info you can probably locate the manufacturer on our Manufacturer Links page or with a search engine, and download the drivers from the manufacture's web site.  How to Identify a Motherboard has some tips which can be used to identify a MODEM.

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