External modems and TAs communicate with their host computers by means of an RS-232 communications port. The RS-232 standard was developed by the Electronics Industry Association (EIA) for low-speed data communication; the standard defines a series of signals that are sent between two telecommunications devices to indicate line and transmission status. The following table shows the most common signals.
|CTS||Clear to Send|
|DCD||Data Carrier Detected|
|DSR||Data Set Ready|
|DTR||Data Terminal Ready|
|RTS||Request to Send|
|RTSRD||Request to Send/Receive Data|
RS-232 connections can make use of either 25-pin or 9-pin connectors. On many PCs, the end attaching to a modem or TA has a 25-pin connector, while the PC has a 9-pin connector. The following table presents the layout and signals for both.
|Description||Pin Outs on 9-Pin Cable||Pin Outs on 25-Pin Cable||Signal||Direction|
|Transmitted Data||3||2||TD||DTE ->DCE|
|Received Data||2||3||RD||DCE ->DTE|
|Request to Send||7||4||RTS||DTE ->DCE|
|Clear to Send||8||5||CTS||DCE ->DTE|
|Data Set Ready||6||6||DSR||DCE ->DTE|
|Signal Ground (Common)||5||7||-||-|
|Data Carrier Detected||1||8||DCD||DCE ->DTE|
|Data Terminal Ready||4||20||DTR||DTE ->DCE|
|Ring Indicator||9||22||RI||DCE ->DTE|
|Data Signal Rate Detector||-||23||DSRD||DCE<->DTE|
Telephone-Line Basics For Modems
Modem connections to the telephone service are made using two wires (ring and tip) that are used in a standard telephone jack. The wires are named for the plug wires used in the original telephone lines by which telephone operators would manually connect two telephones at the phone company switchboard. There are two versions of the telephone jack:
- Half-duplex: The RJ-11 has only two wires, which make up one line. Therefore, only one signal can be sent or received at a time.
- Full-duplex: The RJ-12 uses four wires to make up two lines; it can be used to simultaneously send and receive.
Most modems offer some form of fax capability, along with software that adds functions beyond the average, small, stand-alone fax machine. Such a modem is usually labeled a fax/modem. They can store faxes, both incoming and outgoing, for reference or online reading. Most allow direct faxing of a document from a word processor, generally by using the print command to send the pages to the modem, where they are converted on the fly to the bitmap form used to send and receive fax transmissions. Many programs let you to automatically attach a predesigned cover sheet with each fax.
Another addition to the basic data out/data in modem is voice mail. Here, the PC and telephone work just like an answering machine. If the phone rings and the modem does not detect either a data or fax tone, it switches modes and streams a recorded message (the outgoing message). The caller can be prompted to record a message for the owner, and in some cases the modem will even forward a pager call or fax with the message contents.