The electrical characteristics of a serial communication connection are specified by various interfacing standards, one of which is the RS-422 standard used in all Macintosh computers. This standard is an enhancement of the RS-232 standard, with electrical characteristics modified to allow higher transmission rates over longer lines. Although the electrical voltage differences can be critical at times and should therefore not be ignored, most of the terminology and concepts remain the same across these two standards. For purposes of this discussion, it is convenient to treat these two standards as a single entity.

The specifications of the RS-422 and RS-232 interfacing standards are contained in documents available from the Electronic Industries Associations (EIA). The specifications cover several aspects of the connection between data terminal equipment and data communication equipment. These aspects include the electrical signal characteristics, the mechanical description of the interface circuits, and the functional description of the circuits.

The principal interface signals specified by the EIA are described in the following list. The term data terminal equipment (DTE) is used to describe the initiator or controller of the serial connection, typically the computer. The term data communication equipment (DCE) describes the device that is connected to the DTE, such as a modem or printer.

The RS-422/RS-232 signals are described below. For specific information about how these signals are used in Macintosh computers.

  • Data Terminal Ready (DTR). The DTR signal indicates that the DTE (that is, your computer) is ready to communicate. Disserting this signal causes the DCE to suspend transmission. The DTR signal is the most important control line for a modem, because when it is disserted, most modem functions cease and the modem disconnects from the telephone line. In Macintosh computers, the DTR signal is connected to the CTS signal, discussed next.
  • Request to Send (RTS) and Clear to Send (CTS). The RTS signal was originally intended to switch a half-duplex modem from transmit to receive mode. The computer would send an RTS signal to the modem and wait for the modem to respond by asserting CTS. Since most communications between microcomputers are full duplex nowadays, RTS/CTS handshaking is not often used in its original form. Rather, in most full-duplex modems, the CTS signal is permanently asserted, and the RTS signal is not used. In Macintosh computers, the CTS signal is connected to the DTR signal.
  • Data Set Ready (DSR). The DSR signal is not used by Macintosh computers and is usually permanently asserted on microcomputer modems. It was intended to signal the computer that the modem had made a proper connection to the telephone line and received an answer tone from the modem on the other end. Modern modems communicate this information by sending messages to the computer.
  • Transmitted Data (TD). The TD signal carries the serial data stream from the DTE to the DCE. The EIA specifications dictate that the DTR, RTS, CTS, and DSR signals must be asserted before data can be transmitted, but this requirement is not strictly followed in the computer industry.
  • Received Data (RD). The RD signal is the counterpart of the TD signal, and carries data from the DCE to the DTE. Although the EIA specifies that this signal be in the mark state when no carrier is present, this requirement is rarely adhered to.
  • Data Carrier Detect (DCD). Macintosh computers do not use the DCD signal. In systems that use the signal, it is asserted by the DCE when a carrier signal is received.
  • Ring Indicator (RI). Macintosh computers do not use the RI signal. In systems that use the signal, it is asserted by the DCE when the telephone line is ringing.

As you can see, implementations of the RS-422/RS-232 interface do not always correspond to the specifications set forth by the EIA. This is especially true when the DCE is not a modem.