RS-232(7) BSD Reference Manual RS-232(7)NAMERS-232 - RS-232 signal names and connector configurations
DESCRIPTION
The signal names:
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DB-25 DB-9 Source
Pin Pin Name EIA CCITT DTE DCE Signal
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1 - PG AA 101 - - Frame Ground
2 3 TD BA 103 -> Transmit Data
3 2 RD BB 104 <- Receive Data
4 7 RTS CA 105 -> Request To Send
5 8 CTS CB 106 <- Clear To Send
6 6 DSR CC 107 <- Data Set Ready
7 5 SG AB 102 - - Signal Ground
8 1 DCD DF 109 <- Data Carrier Detect
9 - POS - - <- Pos. DC Test Voltage *
10 - NEG - - <- Neg. DC Test Voltage *
11 - - - - - - Unassigned
12 - SDCD SCF 122 <- Sec. Data Carrier Detect
13 - SCTS SCB 121 <- Secondary Clear To Send
14 - STD SBA 118 -> Secondary Transmit Data
15 - TC DB 114 <- Transmit Clock
16 - SRD SBB 119 <- Secondary Receive Data
17 - RC DD 115 <- Receive Clock
18 - LL LL 141 -> - Local Loopback
19 - SRTS SCA 120 -> Sec. Request To Send
20 4 DTR CD 108.2 -> Data Terminal Ready
21 - SQ CG 110 <- Signal Quality Detect **
22 9 RI CE 125 <- Ring Indicator
23 - - CH CI 111 112 -> <- Data Rate Selector
24 - SCTE DA 113 -> External Clock Transmit
25 - TM TM 142 <- Test Mode ***
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* - These signals are officially Reserved, but usually used for test
voltages.
** - This signal may be used as a local loopback signal in RS-232-D,
if it is not being used as SQ.
*** - This signal was Unassigned in RS-232-C, but commonly used as BUSY
indicator.
EXPLANATION OF SIGNAL USAGE
A brief explanation of common signal usage by UNIX systems, referenced by
RS-232-D Signal Name, follows. Note that all signals are discussed from
the DTE point of view.
PRIMARY SIGNALS
PG Protective Ground (Often referred to as Frame Ground) - should be
connected to the equipment frame, and is where the shield should
be connected if shielded cable is being used to reduce RFI noise.
This signal should only be connected at the DTE end.
SG Signal Ground - provides d.c. reference potential for all signals,
including data, timing, and control operations. Required for all
connections.
TD Transmit Data - data signals originating by the DTE, including
signals to be transmitted via a data channel for receiving sta-
tions, data to be passed to the DCE for testing and maintenance
purposes, and signals for programming or control of DCE equipment.
RD Receive Data - data signals from the DCE, including responses to
data channel signals, test signals, and programming or control
signals from the DTE equipment.
RTS Request To Send - Defined by RS-232-D as a control signal used to
prepare the DCE for transmission of data. Commonly used by UNIX
systems as Ready To Receive, i.e., the DTE is ready to receive da-
ta from the DCE when RTS is in the on state.
CTS Clear to Send - the DCE is ready to receive data or control trans-
missions from the DTE.
DCD Received Line Signal Detector - commonly called Data Carrier De-
tect. Signal from the DCE indicating its data channel signal is
within appropriate limits, e.g. when DCE is a modem, carrier sig-
nal is present and modem is ready to communicate with other modem.
DTR Data Terminal Ready - signal from DTE to place DCE in on-line or
off-line state.
DSR Data Set Ready - the DCE signals the DTE that it is powered up,
connected, and ready to receive signals from the DTE (i.e. is not
currently conducting a data transmission, test, or dial opera-
tion). Often used in conjunction with the DTR signal.
RI Ring Indicator - signal used by an auto-answer modem to inform the
DTE that a ring signal is present on the telephone line. Most mod-
ern modems indicate call-progress on the TD line, and RI is no
longer used by most RS-232 circuits.
SYNCHRONOUS COMMUNICATION SIGNALS
STD Secondary Transmit Data - Equivalent to TD, except it is used to
transmit data over the backward channel in synchronous circuits.
SRD Secondary Receive Data - Equivalent to RD, except it is used to
receive data over the backward channel in synchronous circuits.
SRTS Secondary Request To Send - Equivalent to RTS, except it is used
to control the backward channel transmit function in synchronous
circuits.
SCTS Secondary Clear To Send - Equivalent to CTS, except it is used to
control whether the DCE is conditioned to transmit data on the
backward channel in synchronous circuits.
SDCD Secondary Received Line Signal Detector - Equivalent to DCD, ex-
cept it is used to indicate whether the backward channel is in ap-
propriate limits in synchronous circuits.
TC Transmit Clock - timing signal from the DTE that should be ON and
OFF for approximately equal periods of time. The DTE should pro-
vide a data signal on TR while the TC signal is ON.
RC Receive Clock - timing signal from the DCE that should be ON and
OFF for approximately equal periods of time. A transition from ON
to OFF should coincide with the center of each data segment of the
RD circuit.
SCTE Serial Clock Transmit External - provides DCE with signal timing
information. This signal should be ON and OFF for approximately
equal periods of time, and the transition from ON to OFF condition
should coincide with the center of each data segment of the TD
circuit.
DRS Data Rate Selector - signal used by both the DTE and DCE to select
one of the two data signalling rates of a dual rate synchronous
DCE.
TEST SIGNALS
POS Positive DC Test Voltage - an unofficial use of pin 9 in a DB-25
RS-232 circuit, used to provide positive test voltage from the
DCE.
NEG Negative DC Test Voltage - an unofficial use of pin 10 in a DB-25
RS-232 circuit, used to provide negative test voltage from the
DCE.
LL Local Loopback - new in RS-232-D, initiates a local loopback test
condition in the DCE.
SQ Signal Quality Detect - indicates whether there is a probability
of error in the data received in the data channel. Used prior to
RS-232-D. If this signal is not being used for SQ, RS-232-D de-
fines its use as Remote Loopback.
TM Test Mode - new in RS-232-D. Signal indicates if a maintenance
condition exists in the DCE. Prior to RS-232-D, this signal was
undefined, but commonly used as a BUSY indicator by the DCE.
SEE ALSO
EIA Standard EIA-232-D
CCITT Fascicle VIII.1-V.24
AUTHOR
Stephen Diercouff (sgd@tfm.com)
4.4BSD March 18, 1993 3