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Modems and Modem Initialization Strings

Ports, and a Note on Line Noise

A Universal Modem Initialization String?

First, choose a modem. Each modem and software combination has a different set of options, but most of these are to perform specialized functions. Most modems will respond well to the following initialization string:
AT&F&C1&D2
which is modemspeak for:

AT
'yo, modem!'
&F
'give me the factory settings'
&C1
'give my software a DCD signal to indicate presence of data carrier'
&D2
'Hang up when my software gives you a data terminal ready (DTR) signal'
Other signals you can give include interrogation signals, to tell you what's stored in your modem's memory. This can be used to identify the specific model you have. Typical commands are:
AT&V
most modems with Rockwell Chipsets use this.
ATi0 - ATi13
Our USR 14.4 and 33.6 faxmodems go up to ATi7.
AT%Q
(some modems with Rockwell chipsets) - tests phone line quality. There are several ways to accomplish this, depending on the particular modem.
Some modems use both sets of commands. Expect to see "templates" which list all settings.

Modem communications programs which identify your modem use these templates for that purpose. You can usually find this information in the Windows95 control panel under 'modems' 'diagnostics' (click on modem) 'more info'

Once you have this information, you can search the Web for the ideal initialization string for your software and modem combination. It is possible to use multiple initialization strings, and in fact that is how a fax program shares a computer with a data connection (internet or BBS).



Port Settings

This may be selected by your software, in which case you won't have to worry about it.

If you have an internal modem running at 14,400 baud or higher, you will get the best efficiency if you choose at least the next higher setting, and preferrably at least double the modem speed. For example if you have a 14,400 modem, select a port speed such as 19000 or 38800. (Note that the port settings beyond 9600 generally don't match common modem speeds. Do not use 14400 as a port speed.)

If you have an external modem and do not have what is known as a 16550 UART, you may need to set the port speed to 9600 or slower. If you have a standard 2400 baud modem, you may have to set your port speed to match that of the modem.



Additional Settings

Once you have gotten your modem to operate with reasonable stability, it's time to mess things up! Here are a few options:

On programs which list 'optional' or 'additional' commands (e.g., 'extra settings' in Windows95), the 'AT' is normally left off.

atW2
Reports carrier (DCE) speed, rather than your computer's port speed (DTE) at which it talks to your modem. That will let your program report the connect rate. Modems with Rockwell chipsets.
atS11=50
s11 normally sets the touch tone speed. You can usually speed calls by changing to a lower number (shorter tones). If you set this too fast (eg., 35) the phone company will probably not recognize the call.
atT
touch tone dialing
disable MNP5 (varies - see documentation)
Disable MNP5 compression. MNP5 is a data compression protocol which tends to slow down downloads of most large files, including image files and zip files. The reason is that these files are typically already compressed. (SCN's dialup lines have this protocol disabled.)

Typical settings are:
AT %C2 - Rockwell chipsets (%C0 disables all data compression; %C2 only disables MNP5.)
AT &K3 (or &K0) - USR (&K0 disables all file compression; &K3 only disables MNP5.)
Set maximum connect speed (DCE).
See (Modem Init. Strings) - Max Speed to answer the obvious question Why would someone deliberately downgrade their modem? The reason is that you can increase your data transfer rate by doing so. If you note significant "stalls" when downloading, then your modem is sending NAKs (say again)...

You can also write some of these options to your modem's non-volatile memory. If you do so, first type the 'atZ' command to start with the existing memory configuration and not your program's initialization string. A typical sequence would be something like:

atZ <enter>
atW2 T <enter>
at&W <enter>

That will allow your communications program to call up that configuration, but only if the program calls the "atZ" command instead of the "at&F" command.



Phone Line Quality and Line Noise

Factors involved in phone line quality include:

Obviously some of these things are beyond the user's control, short of obtaining a T-1 connection. Some things can be controlled, however. Most people won't need to do anything (except maybe the Slick 50 56-KFlex Oil Treatment). If you are getting more-or-less consistent connections, you are probably okay. If your connect speed is 2400 below the maximum connect speed, consider it an enhancement to stability. (If you start with a 2400 baud modem and connect at 2400 below that, it will be very stable.)

You may find that the signal levels are not high enough to generate a maximum speed connection. That's usually not a major disadvantage, provided that the signal is clean. My own connection was at a consistent low level but also consistently clean. I'll take that over fast and dirty any day, and besides, think how much harder it is for my dog to eavesdrop on my conversations!



Testing Telephone Line Noise

This can be done with some modems. See www.scn.org/help/linenoise.html for specific information.


Initialization String for U.S.Robotics Sportsters (also good for Winmodems)

The best one we found is at www.allegro.com/sportster.html

In addition, see SpamCop FAQ: Why does my modem disconnect when I try to submit spam?



Problems With Particular Modems



Winmodems and Plug 'n Play Modems

Problems with Winmodems can usually be resolved by updating .inf files. Often the original computer manufacturer will have a different update from the chipset or modem manufacturer. Use the newer one. Some of these can even do .v92 (call waiting).

It may be necessary to uninstall unnecessary hardware (on Windows; not physically), then install the Winmodem, then re-install the other hardware. In any case, installing stuff in Windows is best left to people knowledgable in installing stuff in Windows.

Plug 'n Play modems can sometimes be manually configured. (see below)



Problems with 3Com (U.S. Robotics & Sportster) Modems

These are subject to numerous problems ranging from reporting RING when dialing a telephone to the Sportster Pausing Bug and "Spiraling Death Syndrone". Most of these relate to their use of a single-processor design, and their particular implementation of V.42 compression. You may have to disable V.42, which leaves MNP4 if you have an affected USR/3Com modem. (MNP4 is error correction; not MNP5 which is generally undesirable.) It is reported that Ascend Maxen includes a workaround from Rockwell to fix it from their end. For more information on USR/3Com bug fixes, see
www.scn.org/help/pause.html (here at SCN) or
US Robotics Sportster 56k Voice External - Disconnect Problems, at Christian Wagner's Hong Kong website.

SpamCop FAQ: Why does my modem disconnect when I try to submit spam?

Two problems apply to particular applications:

Rumour has it that 3Com, after acquiring U.S. Robotics, decided to discontinue the Sportster line. I have not seen any information as to whether they are also dropping the design "features" of the Sportster. Information from 3Com in this regard will be helpful.



Com Port Locks up in Windows ("Waiting for Com Port")

This usually happens when a non-windows program is using the com port and you try to activate a Windows com program.
Fix: Certain software, when started from its icon will retrieve the com port, even if it can't access it when already opened. (Bitware has this capability, but check its .ini files and delete the unused stuff before using it - it's full of cute surprises. I changed its defaults to my own defaults.)

If you have this problem on a random basis, look for IRQ conflicts.


Which Modem Shall I Get?

For better information on selecting a modem, see www.teleport.com/~curt/modems.html or
www.aimnet.com/~jnavas/modem/faq_f.htm

First, it's always more economical to buy less than the fastest modem available. Unless you need and expect to use the highest speed, you will get good connections with a slightly slower modem. As of this writing, we are seeing a bit of a limit on the physical capability of modems which means that it is unlikely that you will actually get much more than the 30K from your connections on voice telephone lines.

If I had a choice between a 33.6K and a 56K modem, I would choose the 33.6K modem!

It is possible to exceed the 33.6 limit, but your throughput suffers -- you are actually slowed down. See (Modem Init. Strings) - Max Speed. If you want to connect faster, you need to get rid of your ordinary 'POTS' phone line.

Obtain an internal modem unless you know you have the proper high speed ports to drive a high speed modem.



$$$$

I expect prices for 33.6 and 56 modems to level off to about $50 by the end of '98 and slowly creep down to about $30. New externals are about $5. more, plus the cable. The used market for 14.4's is about $15. or less. ($ = USD)

Look for the following features, as possible options:

The above options (except the sound card) are usually available at low cost.


Fax Considerations

In the U.S. junk faxes are illegal, but overseas users will find the 'delete' key sufficient reason to receive faxes on a computer.
If you are intending to use the fax capability on an ongoing basis or in a business, get a modem with a Rockwell chipset. Some people recommend modems with dual fax class capability, but I see very fast handshaking with ordinary Class 1 connections, and my older software package (Faxworks Pro v.1) only needed +fclass=1 added to the init string.
Fax/Voice provides an ability for selectively answering voice or fax when unattended.
"Extended Distinctive Ring" permits the fax modem to automatically pick up even after the call is answered. This allows you to:
  • Answer the phone in person and force the fax to pick up the call without running to the computer
  • Answer the phone and allow the fax to respond to a hailing signal without picking up the computer.
  • Allow a mechanical answering machine to pick up voice calls and automatically pick up fax calls. If the answering machine is connected through the modem's telephone port, the modem will disconnect the answering machine on receipt of a fax.
Extended Distinctive Ring converts different tones such as the fax hailing signal (long beeps) to distinctive RING codes as if you had distinctive ring. Ordinary distinctive ring is no longer in common use in telephone service.
USRobotics Modems - see Fax termination bug. Some USR modems require +fclass=2.0 rather than +fclass=2 in the init string.
Older software - I found it useful to add +fclass=1 (or 1.0 or 2) to the init string.
Linux - see appropriate HOWTO's.


About COM Ports and IRQ Settings (PCs)

If you don't already have a second COM port, then a factory default setting of COM 2, IRQ 3 should work. If not, then you will need to select an unused COM port and also select an unused IRQ port. Here are typical choices:

COM 1 - IRQ 4 - often the mouse
COM 2 - IRQ 3
COM 3 - IRQ unassigned; usually set at 4 or 5
COM 4 - IRQ unassigned; usually set at 3 or 7
COM 5 - IRQ unassigned
COM 6 - IRQ unassigned
COM 7 - IRQ unassigned
COM 8 - IRQ unassigned

So you see, COM 3 and 4 may be sharing an IRQ setting with COM 1 and 2, and will need to be changed.

If something is connected to the second port, then you will have a conflict if both items share the same IRQ setting. You need a unique IRQ setting for each different device. Unfortunately, you have to pick an IRQ setting which you are not using for something completely different. Most modem programs accept IRQ settings up to 9, although some modems will go up to IRQ 15. Here are conventional IRQ assignments:

IRQ 0 - timer
IRQ 1 - keyboard
IRQ 2 - programmable interrupt controller
IRQ 3 - COM 2 or 4
IRQ 4 - COM 1 or 3 or mouse
IRQ 5 - LPT 2 (parallel port) or sound card
IRQ 6 - floppy controller
IRQ 7 - LPT 1 (parallel port - printer)
IRQ 8 - clock
IRQ 9 - sound card or CD-ROM
IRQ 10 - reserved
IRQ 11 - reserved or CD-ROM
IRQ 12 - reserved or mouse
IRQ 13 - math coprocessor
IRQ 14 - fixed disk
IRQ 15 - reserved
Each computer setup is different. If you have MSD or another diagnostics program, you should be able to identify the occupied interrupts on your computer. In any case, the preferred IRQ are the unused ones of 3, 4, 5, 7, 9. If those aren't open, look into moving the sound card (why else are all IRQ's occupied?) to another port acceptable to your software.

This is for the PC bus. On the IEEE bus, the interrupts will be virtual devices, and will probably be handled by Windows98. Devices will be given a unique tty indentifier by Linex. Macs and other systems have their own protocols.



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