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
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
- 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
- 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
- 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.