PassMark ModemTest allows users to test their dial-up modem by performing low-level testing independent of your Internet Service Provider and TCP/IP settings.

It allows the modem to be tested in addition to the phone line the modem is connected to. ModemTest provides a method for system integrators and computer repair centers to test both internal and external modems. By process of elimination home, users can also use ModemTest to help isolate Internet connection problems with a faulty modem, or phone line or their ISP service. ModemTest logs any data corruption and incorporates a modem speed test which reports the initial connection speed and the actual throughput.

Two different test methods are provided. The first test method uses two telephone lines and two modems in a master/slave configuration. The master dials the phone number of the slave modem, and after a successful connection, data packets are exchanged between the master and slave. The second test method requires only a single modem and no telephone line. A local loopback is done on the analog side of the modem, ModemTest then sends data packets which are echoed back by the modem. In both cases, ModemTest communicates to the modem using a COM port. (In the case of Internal modems or USB modems, this is a virtual COM port and not a physical plug on the machine).

Before sending any data, ModemTest first checks the settings on the serial port and then on the modem itself. Any data received is verified to be sure that it is complete and without errors. A log of the activity is kept in the right-hand section of the window. Different types of activity are highlighted in different colors. Any errors are displayed in red. This log can be saved to disk if required.

Using the file transfer options, ModemTest also assists in modem compliance testing against the International Telecommunications Union ITU v.56 ter standard. (c.f. Test procedure for evaluation of 2-wire 4-kHz voiceband duplex modems, ITU-T v.56 later, version 08/96, sections 6.1 and 6.2).

PassMark ModemTest was released as freeware in 2012 as dial-up modems have mostly become a thing of the past. All license restrictions were removed.