Data circuit interfaces, physical layer.
The most well-known DTEDCE standard was developed by EIA/TIA (Electronic
Industries Association/Telecommunication Industry Association) and identified by the
familiar EIA-232 (Ref. 5). At this writing the F version is current. We suggest drop-
ping the letter after 232 and just add latest version. EIA-232 is essentially equivalent to
ITU-T Recs. V.24 and V.28 and ISO IS2110.
EIA-232 is applicable to the DTEDCE interface employing serial binary data inter-
change. It defines signal characteristics, the mechanical interface and gives functional
descriptions of the interchange circuits. This standard is the old workhorse of data commu-
nications and is applicable to data rates up to 20 kbps and for synchronous/asynchronous
serial binary data systems. EIA/TIA has subsequently published many other standards for
the DTE/DCE interface. Unlike EIA-232, these standards only provide the mechanical
and functional interfaces. The electrical interface is left to still other documents, typically
EIA-422 and EIA-423.
We think the following concept is crucial for the reader's appreciation of data-link
physical layer. It is from Section 2.1.3 of EIA-232 (Ref. 5).
For data interchange circuits, the signal shall be considered in the marking condition when the
) on the interchange circuit, measured at the interface point, is more negative than
minus three volts with respect to circuit AB (signal ground). The signal shall be considered in
the spacing condition when the voltage
is more positive than plus three volts with respect
to circuit AB.
. . . The region between plus three volts and minus three volts is defined as
the transition region. The signal state is not uniquely defined when the voltage
) is in this
During the transmission of data, the marking condition is used to denote the binary
state ONE and the spacing condition is used to denote the binary state ZERO.
Besides EIA-232 there are many other interface standards issued by EIA, CCITT, U.S.
federal standards, U.S. military standards, and ISO. Each defines the DTEDCE interface.
Several of the more current standards are briefly described in the following.
EIA-530 (Ref. 7) is a comparatively recent standard developed by the EIA. It provides
for all data rates below 2.1 Mbps and it is intended for all applications requiring a balanced
It can also be used for unbalanced operation.
Let us digress for a moment. An unbalanced electrical interface is where one of the
signal leads is grounded; for a balanced electrical interface, no ground is used.
EIA-530 applies for both synchronous and nonsynchronous (i.e., startstop) operation.
It uses a standard 25-pin connector; alternatively it can use a 26-pin connector. A list of
interchange circuits showing circuit mnemonic, circuit name, circuit direction (meaning
toward DCE or toward DTE), and circuit type is presented in Table 10.2.
EIA-530 is more properly called ANSI/EIA/TIA-530-A.