2740_EIA_RS 232 C 2740 EIA RS

(

In

this report:

Mechanical

Characteristics .......... 2

Interchange

Circuits ...................... 2

Standard Interfaces

for

Selected

Configurations .......... 5

Summary

of

Electrical

Characteristics .......... 6

DATAPRO

Data

Networking

2740

Standards

Electronic Industries

Association

(EIA)

RS-232-C Interface

Standard

Synopsis

Editor's Note

The subject

of

this

report

is

consid-

ered

as

a mature standard.

No

signifi-

cant 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 In-

dustries Association, Engineering

Department, 2001 I Street NW,

Washington,

DC

20006. A compan-

ion publication, "Application Notes

for EIA Standard RS-232-C," is also

available.

-By

Algis

V.

Salciunas

Product

Manager

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 in-

terface circuit functions

and

their

corresponding connector

assign-

ments.

It

is the most common

DTE

to

DCE

interface used

in

the United

States. Full-

or

half-duplex opera-

tions are supported for synchronous

or

asynchronous transmissions at

speeds up to 20K bps.

1

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2

2740

Standards

Analysis

The Electronic Industries Association (EIA) Stan-

dard RS-232-C defines the electrical

and

mechani-

cal characteristics

of

the interface for connecting

data

terminal equipment (DTE) and data circuit-

terminating equipment (DCE) using serial binary

data communications.

In

the United States, RS-

232-C

is

the most widely used

DTE

to

DCE inter-

face.

It

applies to all classes

of

service: private line,

dial-up, point-to-point, multipoint, switched, nons-

witched, two-wire, and four-wire service. Asyn-

chronous and synchronous data transmission is

supported at speeds

up

to

20K bps in full-

or

half-

duplex mode. RS-232-C is a single-ended

or

unbal-

anced interface; all

of

the interchange signals share

a common electrical ground.

RS-232-C is functionally compatible with the

International Telegraph

and

Telephone Consulta-

tive 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 ter-

minal equipment (DTE) is business machine hard-

ware such as teleprinters,

CR

Ts, front-end ports,

CPUs, etc.; data circuit-terminating equipment

(DCE)

is

hardware such as modems, limited dis-

tance data sets,

or

the data service units (DSUs).

Mechanical Characteristics

The

physical connection between

DTE

and DCE is

made through plug-in, 25-pin connectors. The con-

nectors are keyed with

13

pins on the top row and

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Aaaoclation (EIAI

RS·I32-C Interface

Standard

Data

Networking

12

pins

on

the bottom row

to

prevent improper

connection. The male connector is always associ-

ated with the DTE, and the female is always associ-

ated 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 ca-

bles 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 envi-

ronment. Proximity

to

heavy rotating machinery

or

other noisy/radiating devices will limit the prac-

tical cable length.

Pin assignments are explicit and unalterable,

unless unassigned. Special functions not specifi-

cally defined should be allotted

to

unassigned pins.

For

example, pin

25

(unassigned) is sometimes

used to "busy

out"

a connected modem that is as-

sociated with a switched application. This causes

the modem to

go

"off

hook," thus preventing an

incoming call from being connected/answered. Be-

cause some pins are unassigned,

and

because all

the functions defined by RS-232-C are not neces-

sarily 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 be-

tween the data terminal equipment and the data

circuit-terminating equipment. RS-232-C describes

the four categories

of

interchange circuits that ap-

ply generally to all systems. They are ground

or

common return, data, control, and timing circuits.

Twenty-two circuits are specified by possible func-

tions, 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.

Data

Circuits

Two types

of

data circuits are described. A

primary

channel is a data transmission channel that has the

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Data Networking

Electronic

Indu.trl

••

A

••

ociatlon

lElA)

RS-232-C

Interface

Standard

2740

Standards

3

Table

1.

EIA

RS·232·C

Interface

Connector

Assignments

Data Signal Control Signal Timing Signal

from from from CCITT

Pin Number Function Circuit

DCE

DTE

DCE

DTE

DCE

DTE Equivalent·

1 Protective ground AA

101

2 Transmitted data BA X

103

3 Received data BB X

104

4 Request to send CA X

105

5 Clear to send CB X

106

6 Data set ready

CC

X

107

7 Signal ground/common return AB

102

8 Received line signal detector

CF

X

109

9 Reserved for data set testing

10 Reserved for data set testing

11

Unassigned

12 Secondary received line signal detector

SCF

X

122

13 Secondary clear to send SCB X

121

14

Secondary transmitted data SBA X

118

15

Transmission signal element timing (DCE) DB X

114

16

Secondary received data SBB X

119

17 Receiver signal element timing (DCE)

DO

X

115

18

Unassigned

19

Secondary request to send SCA X

120

20

Data terminal ready

CD

X

108.2

21

Signal quality detector

CG

X

110

22

Ring indicator

CE

X

125

23 Data signal rate selector CH/CI X X 111/112

24

Transmit signal element timing (DTE)

DA

X

113

25 Secondary clear to send SCB X

DCE

= Data Communication Equipment

DTE

= Data Terminal Equipment

Pin Number Assignments

2 3 4 5 6

14 15 16 17 18

Equivalents are outlined in CCITT Recommendation V.24.

highest signaling rate

of

all channels sharing a com-

mon 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 inde-

pendent

of

the primary channel and is controlled

by the appropriate set

of

secondary control inter-

change 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

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19

7 8 9 10

11

12

20

21

22

23

24

25

subchannel

to

carry control responses. Signals usu-

ally flow in opposite directions to one another: pri-

mary in one direction at a higher speed, and

secondary in the reverse direction at a lower speed.

RS-232-C accommodates primary and sec-

ondary arrangements with separately defined sec-

ondary interchange circuits for Transmitted Data,

Received Data, Request

to

Send, Clear

to

Send,

and Received Line Signal Detector. Secondary cir-

cuits may be independent

of

other interchange cir-

cuits in terms

of

direction and speed. The

secondary interchange circuits, however, are addi-

tional circuits that function in the same manner as

their basic counterparts and follow the same condi-

tions 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

4

2740

Standards

be present

on

the following four circuits:

SCA

(Sec-

ondary Request to Send);

SCB

(Secondary Clear to

Send); CC (Data Set Ready); and CD (Data Termi-

nal 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 repre-

sented 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 pub-

lic switched network, the control sequence might

be as follows:

1.

All control circuits OFF.

2.

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.

3.

DCE-2 turns RI

ON

to DTE-2.

4.

DTE-2 turns

DTR

ON

to DCE-2.

5.

DCE-2 goes

"off

hook" and turns DSR ON to

DTE-2.

6.

DTE-2 turns RTS

ON

to DCE-2.

7.

DCE-2 turns CTS

ON

to DTE-2 (ENQ trans-

mitted), DTE/DCE-2 turns OFF RTS and

CTS.

8.

DCE-l turns CD

ON

to DTE-I.

9.

DTE-l decodes ENQ, turns

ON

RTS.

10.

DCE-1 turns ON CTS; DTE-1 transmits re-

sponse.

The four basic control signals relating

to

transmis-

sion are:

1.

Request to Send

(RTS)-Circuit

CA

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1991

Electronic Industllea

Auocilltlon

(EIA)

RS·232-C

Inte~

Standard

Data Networking

• Conditions the local DCE for data transmis-

sion.

• Controls the direction

of

transmission on half-

duplex channels, maintains transmit, and inhib-

its 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 re-

ceive

or

nontransmit mode, and turn CTS OFF.

2.

Clear

to

Send

(CTS)-Circuit

CB

• Indicates to the DTE that data can be transmit-

ted (CTS ON) or cannot be transmitted (CTS

OFF).

• CTS

ON

is delayed until the remote DCE is ini-

tialized.

• 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 (alter-

native voice/data), or dial mode. The ON con-

dition

of

this circuit should not be interpreted

as either the status

of

any remote station equip-

ment

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 dis-

creet answer tone, the duration

of

which is con-

trolled by the local data set.

• OFF, occurring during transmission, indicates

that the connection has been lost. A new trans-

mission must be originated.

4.

Data

Terminal Ready

(DTR)-Circuit

CD

• Used to control switching

ofthe

DCE to the

communications channel.

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(EIAI

RS·232·C

Interface

Standard

• The ON condition prepares the data set to con-

nect and maintain connection to the communi-

cations channel.

• With automatic answering equipment, connec-

tion 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

sig-

nal.

• In switched systems, Data Set Ready must be

OFF before Data Terminal Ready is turned ON

again.

Other control interchange circuits include the fol-

lowing.

The

Ring

Indicator, Circuit CE, indicates that

a ring (call) signal is being received on the commu-

nications 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 accept-

able for demodulation. An OFF condition indi-

cates 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, indi-

cates 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 retransmis-

sion.

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

Sig-

nal

Element

Timing.

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Standards

5

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 tran-

sition 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 provi-

sion is made for the addition

of

custom configura-

tions. Determining factors in the selection

of

an

interface type are whether the DTE transmits, re-

ceives,

or

does both; whether the mode

of

opera-

tion 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 Ta-

ble 2, where the direction

of

data transfer pertain-

ing to the interface is stated (function),

and

the use

of

Request to Send and Received Line Signal De-

tector 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 full-

duplex operation without using RTS. When inter-

face D is used in full-duplex operation, however,

R

TS is used with special significance.

MAY

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2740

Standards

Electronic Induatrt

..

__

elation lElA)

RS-232-C

Interface

Standard

Data Networking

Table

2.

Selected

Configuration

Interfaces

Interface

Type

A

B

C

o

F

G

H

J

K

L

M

z

Function

Transmits only

Receives only

Transmits

or

Receives (HOX)

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

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)

Circuits specified by suppliers

HDX-Half-duplex

channel.

FDX-Full-duplex channel.

RTS-Request

to Send signal.

RLSD-Received

Line Signal Detector.

Comment

RTS not used

RTS used

RLSO used

RTS used·; RLSO used

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

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

"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 synchroniza-

tion signals

as

required.

Interfaces E, F, G, and H define two-way

transmission, where both the primary and second-

ary directions are one-way only.

Interfaces J, K,

L,

and

M define less restric-

tive primary and secondary arrangements.

Interface Z simply allows a special arrange-

ment to be established.

The complete relationship

of

interchange cir-

cuits

to

standard interface types is depicted in Ta-

bles 3a, 3b, and

3c.

Summary

of

Electrical Characteristics

The EIA RS-232-C standard prescribes bipolar-

voltage serial data transmission between communi-

cating devices. Within the EIA RS-232-C standard,

transmitted data is represented by the mark condi-

tion for binary one

and

the space condition

for

bi-

nary zero. A data signal

on

an

interchange circuit is

MAY

1991

in the mark condition when the voltage at the in-

terface 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 be-

tween - 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 func-

tion is not defined for voltages in the transition

region between - 3 and + 3 volts.

Mandatory Interchange Circuit conditions

follow (see Figure

1):

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Association

lElA)

RS·232-C

Interface-

Standard

2740

Standards

7

Table

3a.

Required

Interchange

Circuits

for

Standard

Interface

Types

A,

B, C.

D.

and

E

Interchange Circuit Interface Type

A B C D E

AA

Protective Ground

AB

Signal Ground b b b b b

BA

Transmitted Data b b b b

BB

Received Data b b b

CA Request

to

Send b b

CB

Clear to Send b b b b

CC Data Set Ready b b b b b

CD Data Terminal Ready s s s s s

CE Ring Indicator s s s s s

CF

Received Line Signal Detector b b b

CG Signal Quality Detector

CH/CI Data Rate Selector

DA/DB Transmitter Timing

DO Receiver Timing

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

AA

Protective Ground

AB

Signal Ground

BA

Transmitted Data

BB

Received Data

SBA

Secondary Transmitted Data

SBB

Secondary Received Data

CA

Request

to

Send

SCA Secondary Request

to

Send

CB

Clear

to

Send

SCB

Secondary Clear

to

Send

CC Data Set Ready

CD Data Terminal Ready

CE Ring Indicator

CF Received Line Signal Detector

SCF Secondary RLSD

CG Signal Quality Detector

CH/CI Data Rate Selector

DA/DB Transmitter Timing

DO Receiver Timing

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 mag-

nitude when the terminator resistance (RL) is

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F

b

s

b

G

b

s

b

Interface Type

H

b

s

b

s

b

between 3000 and 7000 ohms, and the termina-

tor

open voltage (EL) is zero.

•

The

effective shunt capacitance (CL), associated

with the terminator, must not exceed 2500 pico-

farads at the interface point.

MAY

1991

8

2740

Standards

Electronic

Industries

AssocIation

(EIA)

RS·232-C

Interface

Standard

Data

Networking

Table

3c.

Required

Interchange

Circuits

for

Standard

Interface

Types

'""

K,

L,

andM

AA

AB

BA

BB

SBA

SBB

CA

SeA

CB

SCB

CC

CD

CE

CF

SCF

CG

CHICI

DA/DB

DO

Interchange Circuit

Protective Ground

Signal Ground

Transmitted Data

Received Data

Secondary Transmitted Data

Secondary Received Data

Request

to

Send

Secondary Request

to

Send

Clear

to

Send

Secondary Clear

to

Send

Data Set Ready

Data Terminal Ready

Ring Indicator

Received Line Signal Detector

Secondary RLSD

Signal Quality Detector

Data Rate Selector

Transmitter Timing

Receiver Timing

b-Baslc

Interchange circuits required.

s-Requlred

for switched network operation.

t-Required

for synchronous operation.

•

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

Figure

1.

Interchange Equivalent Circuit

T

o

V

e

MAY

1991

'b

-

Open

circu~

driver

Voltage.

AO

-

Driver

internal

DC

resistance.

Co

-

Total

effective

capackance

associated

with

the

driver.

V

1 -

Voltage

at

interface

point

C L -

Total

effective

espaounce

associated

with

the

terminator.

'\.

-

Terminator

load

DC

resistance.

EL

-

Open

circuk

terminator

voltage

(bias).

'\.

~

m

j

n

a

t

o

J

b

s

b

Interface Type

K L M

b b b

b b

b b b

b

b b

b b b

s s s

b b b

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):

• 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.

• The direction

of

voltage must not change while

in the transition region.

•

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 ex-

ceed 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

2740_EIA_RS 232 C 2740 EIA RS

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