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DATAPRO

Data Networking

2740

Standards

Electronic Industries
Association (EIA)
RS-232-C Interface
Standard
Synopsis

In this report:

Editor's Note

Mechanical
Characteristics ..........

Interchange
Circuits ......................

Standard Interfaces
for Selected
Configurations ..........

Summary of
Electrical
Characteristics ..........

(

The subject of this report is considered as a mature standard. No significant developments are anticipated,
but because of its importance in the
industry, coverage is being continued.
Copies of the RS-232-C standard can
be obtained from the Electronic Industries Association, Engineering
Department, 2001 I Street NW,
Washington, DC 20006. A companion publication, "Application Notes
for EIA Standard RS-232-C," is also
available.

Report Highlights
RS-232-C is a set of specifications
that applies to the transfer of data
between data terminal equipment
(DTE) and data circuit-terminating
equipment (DCE). It defines the interface circuit functions and their
corresponding connector pin assignments. It is the most common DTE
to DCE interface used in the United
States. Full- or half-duplex operations are supported for synchronous
or asynchronous transmissions at
speeds up to 20K bps.

-By Algis V. Salciunas
Product Manager

1991 McGraw-Hili, Incorporated. Reproduction Prohibited.
Oatapro InformatIOn ServlOes Group. Delran NJ 08075 USA

MAY 1991

2740
Standards

Analysis
The Electronic Industries Association (EIA) Standard RS-232-C defines the electrical and mechanical characteristics of the interface for connecting
data terminal equipment (DTE) and data circuitterminating equipment (DCE) using serial binary
data communications. In the United States, RS232-C is the most widely used DTE to DCE interface. It applies to all classes of service: private line,
dial-up, point-to-point, multipoint, switched, nonswitched, two-wire, and four-wire service. Asynchronous and synchronous data transmission is
supported at speeds up to 20K bps in full- or halfduplex mode. RS-232-C is a single-ended or unbalanced interface; all of the interchange signals share
a common electrical ground.
RS-232-C is functionally compatible with the
International Telegraph and Telephone Consultative Committee (CCITT) Recommendation V.24.
Another similar interface, ISO IS211 0, has been
adopted by the International Organization for
Standardization (ISO). Both the EIA RS-232-C and
the ISO IS2II 0 specify a 25-pin connector. Pin
incompatibilities exist, however, because of the
diagnostic features outlined in RS-232-C.
RS-232-C coexists with another EIA standard
for DTE to DCE interface, RS-449, which specifies
requirements for expanded transmission speeds,
longer cable lengths, and additional functions.
Equipment meeting RS-232-C standards can be
made compatible with equipment meeting RS-449
standards by using an interface converter.
As the terms relate to this interface, data terminal equipment (DTE) is business machine hardware such as teleprinters, CRTs, front-end ports,
CPUs, etc.; data circuit-terminating equipment
(DCE) is hardware such as modems, limited distance data sets, or the data service units (DSUs).

Electronic Induatrl. .
Aaaoclation (EIAI
RS·I32-C Interface
Standard

Data Networking

12 pins on the bottom row to prevent improper
connection. The male connector is always associated with the DTE, and the female is always associated with the DCE. The cable is provided by the
DTE. The use of short cables, each less than 50 feet
or 15 meters, is recommended. Use oflonger cables is permissible, however, provided that load
capacity requirements are met. Longer cables are
used with the length restricted by the data rate (in
synchronous applications, the clock leads generally
run twice as fast as the data leads), and by the environment. Proximity to heavy rotating machinery
or other noisy/radiating devices will limit the practical cable length.
Pin assignments are explicit and unalterable,
unless unassigned. Special functions not specifically defined should be allotted to unassigned pins.
For example, pin 25 (unassigned) is sometimes
used to "busy out" a connected modem that is associated with a switched application. This causes
the modem to go "off hook," thus preventing an
incoming call from being connected/answered. Because some pins are unassigned, and because all
the functions defined by RS-232-C are not necessarily required for a specific application, all 25 pins
are not usually used. Contact the DTE vendor to
determine the specific configuration.

Interchange Circuits
An interchange circuit is defined as a circuit between the data terminal equipment and the data
circuit-terminating equipment. RS-232-C describes
the four categories of interchange circuits that apply generally to all systems. They are ground or
common return, data, control, and timing circuits.
Twenty-two circuits are specified by possible functions, and three are unassigned. See Table 1.
Ground Circuits

The ground interchange circuits are Protective
Ground (AA) and Signal Ground (AB). Protective
Ground is electrically bonded to the equipment
frame; it can be connected to an external ground
(e.g., the ground pin of an electrical power plug).
Signal Ground establishes a common reference for
all interchange circuits, except Protective Ground.

Mechanical Characteristics
The physical connection between DTE and DCE is
made through plug-in, 25-pin connectors. The connectors are keyed with 13 pins on the top row and
MAY 1991

Data Circuits

Two types of data circuits are described. A primary
channel is a data transmission channel that has the
@ 1991 McGraw-Hili, Incorporated. Reproduction Prohibited.
Datepro Information Sarvices Group. Delran NJ 08075 USA

Electronic Indu.trl••
A••ociatlon lElA)
RS-232-C Interface
Standard

Data Networking

2740

Standards

Table 1. EIA RS·232·C Interface Connector Pin Assignments
Data Signal
from
Circuit DCE DTE

Pin Number

Function

1
2
3
4

Protective ground
Transmitted data
Received data
Request to send
Clear to send
Data set ready
Signal ground/common return
Received line signal detector
Reserved for data set testing
Reserved for data set testing
Unassigned
Secondary received line signal detector
Secondary clear to send
Secondary transmitted data
Transmission signal element timing (DCE)
Secondary received data
Receiver signal element timing (DCE)
Unassigned
Secondary request to send
Data terminal ready
Signal quality detector
Ring indicator
Data signal rate selector
Transmit signal element timing (DTE)
Secondary clear to send

6
7
8
9
10
11
12
13
14
15
16
17

18
19
20
21
22
23
24
25

AA
BA
BB
CA
CB
CC
AB
CF

Control Signal Timing Signal
from
from
CCITT
Equivalent·
DCE DTE
DCE DTE

101
103
104
105
106
107
102
109

X
X
X
X
X
X

122
121
118
114
119
115

X
X

SCF
SCB
SBA
DB
SBB
DO

X
X
X
X

120
108.2
110
125
111/112
113

X
X

SCA
CD
CG
CE
CH/CI
DA
SCB

X
X
X

X
X

X
DTE = Data Terminal Equipment

DCE = Data Communication Equipment

Pin Number Assignments

Equivalents are outlined in CCITT Recommendation V.24.

(

highest signaling rate of all channels sharing a common interface connector.
The secondary channel has a lower signaling
rate than the primary channel in a system where
two channels share a common interface connector.
RS-232-C defines two types of secondary channels:
auxiliary, whose direction of transmission is independent of the primary channel and is controlled
by the appropriate set of secondary control interchange circuits; and backward, whose direction of
transmission must always be opposite that of the
primary channel.
Secondary channels are established because
in some communications systems, greater channel
efficiency can be achieved by using a lower speed

@ 1991 McGraw-Hili. Incorporated. Reproduction Prohibited.
Datapro Information Services Group. Delran NJ 08075 USA

subchannel to carry control responses. Signals usually flow in opposite directions to one another: primary in one direction at a higher speed, and
secondary in the reverse direction at a lower speed.
RS-232-C accommodates primary and secondary arrangements with separately defined secondary interchange circuits for Transmitted Data,
Received Data, Request to Send, Clear to Send,
and Received Line Signal Detector. Secondary circuits may be independent of other interchange circuits in terms of direction and speed. The
secondary interchange circuits, however, are additional circuits that function in the same manner as
their basic counterparts and follow the same conditions that govern corresponding basic circuits (see
Tables 1,2, and 3). For the DTE to transmit data
on the secondary channel, an ON condition must
MAY 1991

2740
Standards

Electronic Industllea

Data Networking

Auocilltlon (EIA)
RS·232-C Inte~
Standard

be present on the following four circuits: SCA (Secondary Request to Send); SCB (Secondary Clear to
Send); CC (Data Set Ready); and CD (Data Terminal Ready).
When the data circuits are idle, they are in a
mark hold condition (binary "ones"). Data is
transferred in bipolar fashion. A binary "one"
(mark) is represented by a voltage more negative
than -3 volts; a binary "zero" (space) is represented by a voltage more positive than + 3 volts.
Control Circuits

Control signals are used to enable and disable data
transmission and reception, and to indicate the
operational status and condition of the DTE and
DCE. A control function is considered to be in the
ON condition when the voltage is more positive
than + 3 volts; it is considered to be in the OFF
condition when the voltage is more negative than
- 3 volts. These control circuits include Request to
Send (RTS), Clear to Send (CTS), Data Set Ready
(DSR), Data Terminal Ready (DTR), Received Line
Signal Detector (carrier detect, CD), and Ring (RI).
Assuming a half-duplex transmission between
DTE/DCE-1 and DTE/DCE-2 is run over the public switched network, the control sequence might
be as follows:
1.

All control circuits OFF.

DTE/DCE-1 places a call to DTE/DCE-2;
DCE-1 turns DSR ON to DTE-1, DTE-l turns
DTR ON to DCE-I.

DCE-2 turns RI ON to DTE-2.

DTE-2 turns DTR ON to DCE-2.

DCE-2 goes "off hook" and turns DSR ON to
DTE-2.

DTE-2 turns RTS ON to DCE-2.

DCE-2 turns CTS ON to DTE-2 (ENQ transmitted), DTE/DCE-2 turns OFF RTS and
CTS.

DCE-l turns CD ON to DTE-I.

DTE-l decodes ENQ, turns ON RTS.

10.

DCE-1 turns ON CTS; DTE-1 transmits response.

• Conditions the local DCE for data transmission.
• Controls the direction of transmission on halfduplex channels, maintains transmit, and inhibits receive.
• Maintains the DCE in the transmit mode on
duplex or one-way only channels; otherwise,
holds it in nontransmit.
• RTS transition (OFF to ON) causes the DCE to
enter transmit mode and turn CTS ON.
• RTS transition (ON to OFF) causes the DCE to
complete the data transmission, enter the receive or nontransmit mode, and turn CTS OFF.
2. Clear to Send (CTS)-Circuit CB
• Indicates to the DTE that data can be transmitted (CTS ON) or cannot be transmitted (CTS
OFF).
• CTS ON is delayed until the remote DCE is initialized.
• When RTS is not implemented, CTS must be
ON continuously.
3. Data Set Ready (DSR)-Circuit CC
• Indicates the local data set status.
• ON indicates that the data set is connected to
the communications channel ("Off Hook" or
switched service) and is not in test, talk (alternative voice/data), or dial mode. The ON condition of this circuit should not be interpreted
as either the status of any remote station equipment or an indication that a communication
channel has been established to a remote data
station.
• ON indicates completed timing functions, if
any, required for call establishment (switched
service).
• ON indicates completed transmission of a discreet answer tone, the duration of which is controlled by the local data set.
• OFF, occurring during transmission, indicates
that the connection has been lost. A new transmission must be originated.

The four basic control signals relating to transmission are:

4. Data Terminal Ready (DTR)-Circuit CD

1. Request to Send (RTS)-Circuit CA

• Used to control switching ofthe DCE to the
communications channel.

MAY 1991

,1991 McGraw-HUI, Incorporated. Reproduction Prohibited.
Datapro Information Services Group. Delran NJ 08075 USA

Data Networking

Electronic Industries
Associlition (EIAI
RS·232·C Interface
Standard

• The ON condition prepares the data set to connect and maintain connection to the communications channel.
• With automatic answering equipment, connection to the communications channel occurs
when both the Data Terminal Ready and the
Ring Indicator circuit are in the ON condition.
The OFF condition of the Data Terminal Ready
circuit does not disable the Ring Indicator signal.
• In switched systems, Data Set Ready must be
OFF before Data Terminal Ready is turned ON
again.
Other control interchange circuits include the following.
The Ring Indicator, Circuit CE, indicates that
a ring (call) signal is being received on the communications channel through the switched network.
The circuit is in an ON condition during ringing
and in an OFF condition between rings or when
ringing is not present.
The Received Line Signal Detector, Circuit
CF, presents an ON condition to indicate that data
signals are being received, and that they are acceptable for demodulation. An OFF condition indicates that data signals are not being received, or
that they are not acceptable for demodulation. The
OFF condition causes the Received Data (Circuit
BB) to be held in a binary one (marking) condition.
The Signal Quality Detector, Circuit CG, indicates whether there is a high probability of error in
received data. The ON condition is maintained,
unless a high probability of error has occurred.
Probable error is indicated by the OFF condition.
This condition can be used to effect retransmission.
. Data Signal Rate Detector, Circuit CH or CI'
IS used to select between two signaling rates when
the data set or data terminal accommodates dual
rates. The ON condition selects the higher rate or
range of rates. Circuit CH indicates that the data
terminal provides the selection signal. Circuit CI
indicates that the data set provides the selection
signal. Selection is between two asynchronous rates
or between two synchronous rates.

(

Timing Circuits

Two circuits are used to provide Transmitter Signal Element Timing.

1991 McGraw·HIII, Incorporated. Reproduction Prohibited.
Datapro Information Services Group. Delran NJ 08075 USA

2740

Standards

Circuit DA provides signal element timing
information to the transmitting signal converter,
the DCE. The ON-OFF transition indicates the
occurrence of the center of the data element. The
DTE normally provides timing information on this
circuit when the DTE is powered ON. If Circuit
CA (Request to Send) is OFF, timing information
on Circuit DA may be withheld by the DTE for
short periods.
Circuit DB provides signal element timing to
the DTE. A data signal on Circuit BA (Transmitted
Data) is provided from the DTE in which the transition between signal elements occurs at the time of
the OFF-ON transition in Circuit DB. If Circuit
CC (Data Set Ready) is OFF, it is permissible for
the DCE to withhold timing information for short
periods. The performance of maintenance tests
within the DCE, for example, may require the
withholding of timing information.
Receiver Signal Element Timing, Circuit DD,
provides timing to the data terminal from the data
set. The ON-OFF transition indicates the center of
the received data elements.

Standard Interfaces for Selected
Configurations
EIA RS-232-C describes a selected set of data
transmission configurations or interfaces. A provision is made for the addition of custom configurations. Determining factors in the selection of an
interface type are whether the DTE transmits, receives, or does both; whether the mode of operation is half or full duplex; and whether a secondary
channel is used. The interface designations do not
relate to which terminal originates or answers the
call, but rather to the data transmitted.
RS-232-C defines selected interface types by
letter designation. These types are described in Table 2, where the direction of data transfer pertaining to the interface is stated (function), and the use
of Request to Send and Received Line Signal Detector interchange circuits is stipulated (comment).
This list indicates that interfaces A and B, which
are one-way only transmissions, differ only in
terms of the use ofRTS. Interface D is normally
employed with half-duplex operation using R TS,
and interface E is normally employed with fullduplex operation without using RTS. When interface D is used in full-duplex operation, however,
R TS is used with special significance.
MAY 1991

2740
Standards

Electronic Induatrt. .
_ _elation lElA)
RS-232-C Interface
Standard

Data Networking

Table 2. Selected Configuration Interfaces
Interface
Type

Function

Comment

Transmits only
Transmits only
Receives only
Transmits or Receives (HOX)

RTS not used
RTS used
RLSO used
RTS used·; RLSO used

Transmits and Receives (FOX)
Primary Transmits only
Secondary Receives only
Secondary Transmits only
Primary Receives only
Primary Transmits only
Secondary Receives only
Secondary Transmits only
Primary Receives only

RTS not used: RLSO used
RTS used·
Secondary RLSO used
Secondary RTS used·
RLSO used
RTS not used
Secondary RLSO used
Secondary RTS not used
RLSO used

Primary Transmits only
Secondary Transmits or Receives (HOX)
Primary Receives only
Secondary Transmits or Receives (HOX)
Primary Transmits or Receives (HOX)
Secondary Transmits or Receives (HOX)
Primary Transmits or Receives (FOX)
Secondary Transmits or Receives (FOX)
Primary Transmits or Receives (FOX)
Secondary Transmits or Receives (FOX)

RTS used·
Secondary RTS and RLSO used
RLSO used
Secondary RTS and RLSO used
RTS used; RSLO used
Secondary RTS used
RTS used·; RSLO used
Secondary RTS used·
RTS not used; RLSO used
Secondary RTS not used;
Secondary RLSO used

o
o
F

J
K
L
L
M

Circuits specified by suppliers

HDX-Half-duplex channel.
FDX-Full-duplex channel.
RTS-Request to Send signal.
RLSD-Received Line Signal Detector.
"Indicates the Request to Send Circuit in a one-way (transmit) of duplex arrangement where RTS might not ordinarily be expected.
It might be used to Indicate a nontransmlt mode to the DCE or to permit the DCE to remove a line signal or to send synchronization signals as required.

Interfaces E, F, G, and H define two-way
transmission, where both the primary and secondary directions are one-way only.
Interfaces J, K, L, and M define less restrictive primary and secondary arrangements.
Interface Z simply allows a special arrangement to be established.
The complete relationship of interchange circuits to standard interface types is depicted in Tables 3a, 3b, and 3c.

Summary of Electrical Characteristics
The EIA RS-232-C standard prescribes bipolarvoltage serial data transmission between communicating devices. Within the EIA RS-232-C standard,
transmitted data is represented by the mark condition for binary one and the space condition for binary zero. A data signal on an interchange circuit is

MAY 1991

in the mark condition when the voltage at the interface point is more negative than - 3 volts with
respect to signal ground (Circuit AB). When the
data signal at the interface point is more positive
than + 3 volts, with respect to signal ground, the
data signal is in the space condition. The area between - 3 and + 3 volts is the transition region. In
the transition region, the signal state is not defined.
On timing or control interchange circuits, the
signal is considered OFF when the voltage at the
interface point is more negative than - 3 volts with
respect to signal ground. It is considered ON if the
voltage at the interface point is more positive than
+ 3 volts with respect to signal ground. The function is not defined for voltages in the transition
region between - 3 and + 3 volts.
Mandatory Interchange Circuit conditions
follow (see Figure 1):

1991 McGraw-Hili, Incorporated. Reproduction Prohibited.
Datapro Information Services Group. Delran NJ 08075 USA

Electronic Industrl. .
Association lElA)
RS·232-C InterfaceStandard

Data Networking

2740

Standards

Table 3a. Required Interchange Circuits for Standard Interface Types A, B, C.
D. and E
Interchange Circuit
AA
AB

Protective Ground
Signal Ground

BA
BB

Transmitted Data
Received Data

CA
CB
CC
CD
CE
CF
CG
CH/CI

Request to Send
Clear to Send
Data Set Ready
Data Terminal Ready
Ring Indicator
Received Line Signal Detector
Signal Quality Detector
Data Rate Selector

DA/DB
DO

Transmitter Timing
Receiver Timing

Interface Type

b
b

b
b
b
b

b
b

b
b
b

s
s

b-Basic interchange circuits required.
s-Requlred for switched network operation.
t-Required for synchronous operation.

Table 3b. Required Interchange Circuits for Standard Interface Types F, G, H,
and I

Interchange Circuit

Interface Type

AA
AB

Protective Ground
Signal Ground

BA
BB
SBA
SBB

Transmitted Data
Received Data
Secondary Transmitted Data
Secondary Received Data

CA
SCA
CB
SCB
CC
CD

Request to Send
Secondary Request to Send
Clear to Send
Secondary Clear to Send
Data Set Ready
Data Terminal Ready

CE
CF
SCF
CG
CH/CI

Ring Indicator
Received Line Signal Detector
Secondary RLSD
Signal Quality Detector
Data Rate Selector

DA/DB
DO

Transmitter Timing
Receiver Timing

b
b
b

b
b

s
s
b

b
b

b-Basic interchange circuits required.
s-Requlred for switched network operation.
t-Required for synchronous operation.

• Open circuit driver voltage (YO), with respect
to signal ground, must not exceed 25 volts with
respect to ground.

(

• The potential at the interface point must not be
less than 5 volts nor more than 15 volts in magnitude when the terminator resistance (RL) is

1991 McGraw-Hili. Incorporated. Reproduction Prohibited.
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between 3000 and 7000 ohms, and the terminator open voltage (EL) is zero.
• The effective shunt capacitance (CL), associated
with the terminator, must not exceed 2500 picofarads at the interface point.

MAY 1991

Data Networking

Electronic Industries
AssocIation (EIA)
RS·232-C Interface
Standard

2740

Standards

Table 3c. Required Interchange Circuits for Standard Interface Types '"" K, L,
andM
Interchange Circuit

Interface Type

Protective Ground
Signal Ground

AB
BA
BB
SBA
SBB

b
b
b

b
b
b
b

b
b

b
b
b
b
b

b
b
b

s
s

Transmitted Data
Received Data
Secondary Transmitted Data
Secondary Received Data

CB
SCB
CC
CD

Request to Send
Secondary Request to Send
Clear to Send
Secondary Clear to Send
Data Set Ready
Data Terminal Ready

b
b
b
b
b

CE
CF
SCF
CG
CHICI

Ring Indicator
Received Line Signal Detector
Secondary RLSD
Signal Quality Detector
Data Rate Selector

DA/DB

Transmitter Timing
Receiver Timing

SeA

b
b

b-Baslc Interchange circuits required.
s-Requlred for switched network operation.
t-Required for synchronous operation.

Send (SCA), where implemented, are used to de·
tect the power-off condition or the disconnection
of the interconnecting cable.
Certain limitations apply to all interchange
signals (data, control, and timing):

• The open circuit terminator voltage (EL) must
not exceed 2 volts.
Request to Send (CA), Data Set Ready (CC), Data
Terminal Ready (CD), and Secondary Request to

• All interchange signals entering transition must
proceed to the opposite signal state, and may
not reenter the transition region until the next
significant change in signal condition.

Figure 1.
Interchange Equivalent Circuit

'\. ~

• The direction of voltage must not change while
in the transition region.

m
j
V

a
t

'b -

Open circu~ driver Voltage.

AO -

Driver internal DC resistance.

Co - Total effective capackance associated with the driver.

MAY 1991

V1 -

Voltage at interface point

CL -

Total effective espaounce associated with the terminator.

'\. -

Terminator load DC resistance.

EL -

Open circuk terminator voltage (bias).

• The time required for a control signal to pass
through the transition region must not exceed
one millisecond.
• The time required for a data or timing signal to
pass through the transition region must not exceed one millisecond or 4 percent of the normal
duration of a signal element on that interchange
circuit, whichever is the lesser.
• The maximum instantaneous rate of voltage
must not exceed 30 volts per microsecond .•

1991 McGraw-Hili, Incorporated. Reproduction PrOhibited.
Datepro Irrformation Services Group. Delran NJ 08075 USA

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