Lucent

Lucent modems come in two varieties, the Win modem and the non-Win modem. The Win modems have a different set of commands they take from the non-Win modems. It is generally said that of the different Win modems, the Lucent is the only one that is really good. They are reliable and upgrades are freely available from Lucent, and are generally very standard. Lucent modems support both K56Flex and V.90.

Rockwell (Conexant)

Like the Lucent, this modem also comes in the two varieties, Win and non-Win which take different commands from the other. The Rockwell non-Win modems work pretty good, but the Win modems do not perform as well as the Lucent Win Modems and cannot be as easily upgraded. Rockwell Modems support both K56Flex and V.90. This is the most popular chip that most brand name modems use. Recently Rockwell changed their modem divisions name to Conexant.

U.S. Robotics (3com)

These chips are usually in U.S. Robotics modems (now owned by 3com), and are also in Telepath modems on Gateway computers. Only a hand full of other manufacturers use these chips. You will find these chips in both Win and non-Win modems, but they all use the same commands. The U.S. Robotics chip supports both X2 and V.90. They tend to connect quicker than other modems but are not always as reliable.

PC Tel

This is one of the worst chips. They only seem to be used on Win Modems and they connect slower and with less reliability than any of the above chips. They support both V.90 and K56Flex, but their K56flex implementation is very poor and works worse than any of the previously mentioned modems. Unfortunately, because they are so cheap, they have become popular in many computers, including most Compaq computers. These modems have also been seen under the name "AMR" before which, apparently, stands for "Audio Modem Riser", which is a small card plugged into the motherboard, not in a standard slot.

Motorola

Not all Motorola modems use the Motorola chip, older Motorola 56k Modems used the Rockwell chip. These chips are not as bad as PC Tel, but not as good as any of the other previously mentioned chips. They only seem to be used on Win Modems. Although the documentation says that they support K56Flex, it is widely reported that they only properly support V.90. They use the same commands as the Rockwell Win Modem.

Creative Labs

Not all Creative Labs modems use the Creative Labs Chip, many of them use the Rockwell chip. The Creative chip seems to not work as well as the Rockwell, Lucent or U.S. Robotics chips. They use commands that are similar to Rockwell's commands but not the same. They do not support K56Flex and only seem to be used in Win Modems.

ESS

Haven't been able to find much information about this chip, only that it seems to be identical to the Creative Labs chip as far as commands and compatability, etc. are concerned.

Cirrus Logic

Couldn't find much information on this chip either. It doesn't seem to be as good as the Rockwell, Lucent and U.S. Robotics chips. It uses commands that are similar to the Rockwell. They also only appear to be used in Win Modems.