Adding modems

Connecting a modem to your computer

You can obtain suitable modem cables from most computer stores and suppliers. You cannot connect modems using the three-wire cables that are often used to connect terminals to the computer. To connect a modem to a 25-pin serial port, pins 2, 3, 7, 8, and 20 must be wired straight through (meaning the pins are connected: pin 2 to pin 2, pin 3 to pin 3, and so on). In addition, pins 4 and 5 must be connected straight through if RTS/CTS flow control is used. If you are unsure what to use, a cable that connects all pins straight through should work correctly. See the serial(HW) manual page for details on 9-pin connections.

A COM port on a computer is usually DTE (Data Terminal Equipment) type, and a modem port is usually DCE (Data Communications Equipment) type, so that a straight-through cable is suitable for connecting the two. However, COM ports on some serial expansion boards are DCE type. If this is the case, you need a null-modem cable to connect a modem. Check your hardware documentation if you are unsure.

NOTE: When you are hooking up your modem (or any other serial device), ensure that serial wires connected to your computer are not left hanging. An unterminated line connected to your computer can considerably reduce system performance. Always unplug a modem cable at the computer end, not at the modem end.

Physical connections between a device and the system may depend on the hardware configuration. For specific information about connecting a modem, refer to the hardware manuals provided with the modem and with your computer.

Your modem may have DIP switches to configure various settings when it is first powered on. One of the settings may allow you to choose whether the modem uses the default factory settings in read-only memory (ROM) or settings you have written to non-volatile memory (NVRAM). If so, select DIP switch and NVRAM settings that are close to the normal operating state of the modem. If necessary, you can normally override these settings by sending commands to the modem.

The factory default settings are normally suitable for installing the modem. Do not adjust these settings until you have tested that the modem operates successfully with your system.

Though unlikely, you may have to physically configure the modem to allow dial-in connections as follows:

Set the modem to auto-answer. A few modems do not have this setting and can only make outgoing calls.
Set the modem so that it does not answer when DTR (Data Terminal Ready) is low, and so that it drops the current connection when DTR goes from high to low.
Set CD (Carrier Detect) to go high when a carrier is present, low when a carrier is not present.
Turn off echoing of characters sent to the modem.
Select quiet mode so that the modem sends no response codes.