RS-232 Signals
The number preceding each signal name correspond to the pin number
defined in the standard
- Protective
Ground
- Transmitted
Data
- Received
Data
- Request to
Send
- Clear To
Send
- Data Set
Ready
- Signal
Ground
- Received
Line Signal Detect (Carrier Detect)
- +P (for
testing only)
- -P (for
testing only)
- (unassigned)
- Secondary
Received Line Signal Detect
- Secondary
Clear To Send
- Secondary
Transmitted Data
- Transmission
signal element Timing
- Secondary
Received Data
- Receiver
Signal Element Timing
- (unassigned)
- Secondary
Request To Send
- Data
Terminal Ready
- Signal
Quality Detector
- Ring
Indicator
- Data Signal
Rate Selector
- Transmitter
Signal Element Timing
- (unassigned)
General: The first letter of the EIA signal name categorizes the signal into
one of five groups, each representing a different "circuit":
- A - Ground
- B - Data
- C - Control
- D - Timing
- S - Secondary channel
This pin is usually connected to the frame of
one of the devices, either the DCE or the DTE, which is properly grounded. The
sole purpose of this connection is to protect against accidental electric
shock and usually this pin should not be tied to Signal Ground.
This pin should connect the chassis (shields) of the two devices, but this
connection is made only when connection of chassis grounds is safe (see ground
loops below) and it is considered optional.
Ground loops are low impedance closed electric loops composed from ground
conductors. When two grounded devices are connected together, say by a RS-232
cable, the alternating current on the lines in the cable induces an electric
potential across the ends of the grounding line (either Protective Ground or
Signal
Ground), and an electric current will flow across this line and through
the ground.
Since the loops impedance is low, this current can be quite high and easily
burn out electric components. Electrical storms could also cause a burst of
destructive current across such a loop. Therefore, connection of the
Protective Ground pin is potentially hazardous. Furthermore, not all signal
grounds are necessarily isolated from the chassis ground, and using a RS-232
interface, especially across a long distance, is unreliable and could be
hazardous. 30 meters is considered the maximum distance at which the grounding
signals can be connected safely.
2 Transmit Data
- Name: BA
- Direction: DTE -> DCE
- CCITT: 103
Serial data (primary) is sent on this line from
the DTE to the DCE. The DTE holds this line at logic 1 when no data are being
transmitted. A "On" (logic 0) condition must be present on the following
signals, where implemented, before data can be transmitted on this line : CA,
CB,CC and CD (Request To
Send, Clear
To Send,Data
Set Ready, Data Terminal
Ready).
3 Receive Data
- Name: BB
- Direction: DTE <- DCE
- CCITT: 104
Serial data (primary) is sent on this line from
the DCE to the DTE. This pin is held at logic 1 (Mark) when no data are being
transmitted, and is held "Off" for a brief interval after an "On" to "Off"
transition on the Request To Send
line, in order to allow the transmission to complete.
4 Request To Send
- Name: CA
- Direction: DTE -> DCE
- CCITT: 105
Enables transmission circuits. The DTE uses this
signal when it wants to transmit to the DCE. This signal, in combination with
the Clear To Send signal, coordinates data transmission
between the DTE and the DCE.
A logic 0 on this line keeps the DCE in transmit mode. The DCE will receive
data from the DTE and transmit it on to the communication link.
The Request To Send and Clear To Send
signals relate to a half- duplex telephone line. A half duplex line is capable
of carrying signals on both directions but only one at a time. When the DTE
has data to send, it raises Request To Send, and then waits until the DCE
changes from receive to transmit mode. This "On" to "Off" transition instructs
the DCE to move to "transmit" mode, and when a transmission is possible, the
DCE sets Clear To Send and transmission can begin.
On a full duplex line, like a hard-wired connection, where transmission and
reception can occur simultaneously, the Clear To Send
and Request To Send signals are held to a constant "On" level.
A "On" to "Off" transition on this line instructs the DCE to complete the
transmission of data that is in progress, and to move to a "receive" (or "no
transmission") mode.
5 Clear To Send
- Name: CB
- Direction: DTE <- DCE
- CCITT: 106
An answer signal to the DTE. When this signal is
active, it tells the DTE that it can now start transmitting (on Transmitted
Data line). When this signal is "On" and the Request To
Send,Data
Set Ready, and Data Terminal
Ready are all "On", the DTE is assured that its data will be sent to the
communications link. When "Off", it is an indication to the DTE that the DCE
is not ready, and therefor data should not be sent.
When the Data
Set Ready and Data Terminal
Ready signals are not implemented, in a local connection which does not
involve the telephone network, the Clear To Send and Request To Send
signals are sufficient to control data transmission.
6 Data Set Ready
- Name: CC
- Direction: DTE <- DCE
- CCITT: 107
On this line the DCE tells the DTE that the
communication channel is available (i.e., in an automatic calling system, the
DCE (modem) is not in the dial,test or talk modes and therefor is available
for transmission and reception). It reflects the status of the local data set
,and does not indicate that an actual link has been established with any
remote data equipment.
7 Signal Ground
- Name: AB
- Direction: -
- CCITT: 102
This pin is the reference ground for all the
other signals, data and control.
8 Receive Line Signal Detect or Data Carrier Detect
- Name: CF
- Direction: DTE <- DCE
- CCITT: 109
The DCE uses this line to signal the DTE that a
good signal is being received (a "good signal" means a good analog carrier,
that can ensure demodulation of received data).
9 +P
This pin is held at +12 volts DC for test purposes.
10 -P
This pin is held at -12 volts DC for test purposes.
12 Secondary Receive Line Signal Detect
- Name: SCF
- Direction: DTE <- DCE
- CCITT: 122
This signal is active when the secondary
communication channel is receiving a good analog carrier (same function as the
Receive Line
Signal Detect signal).
13 Secondary Clear To Send
- Name: SCB
- Direction: DTE <- DCE
- CCITT: 121
An answer signal to the DTE. When this signal is
active, it tells the DTE that it can now start transmitting on the secondary
channel (on the Secondary
Transmitted Data line).
14 Secondary Transmitted Data
- Name: SBA
- Direction: DTE -> DCE
- CCITT: 118
Serial data (secondary channel) is sent on this
line from the DTE to the DCE. This signal is equivalent to the Transmitted Data
line except that it is used to transmit data on the secondary channel.
15 Transmission Signal Element Timing
- Name: DB
- Direction: DTE <- DCE
- CCITT: 114
The DCE sends the DTE a clock signal on this
line. This enables the DTE to clock its output circuitry which transmits
serial data on the Transmitted
Data line.
The clock signal frequency is the same as the bit rate of the Transmitted
Data line. A "On" to "Off" transition should mark the center of each
signal element (bit) on the Transmitted
Data line.
16 Secondary Receive Data
- Name: SBB
- Direction: DTE <- DCE
- CCITT: 119
Serial data (secondary channel) is received on
this line from the DCE to the DTE. When the secondary channel is being used
only for diagnostic purposes or to interrupt the flow of data in the primary
channel, this signal is normally not provided.
17 Receiver Signal Element Timing
- Name: DD
- Direction: DTE <- DCE
- CCITT: 115
The DCE sends the DTE a clock signal on this
line. This clocks the reception circuitry of the DTE which receives serial
data on the Received Data
line.
The clock signal frequency is the same as the bit rate of the Received Data
line (BB). The "On" to "Off" transition should indicate the center of each
signal element (bit) on the Received Data
line.
19 Secondary Request To Send
- Name: SCA
- Direction: DTE -> DCE
- CCITT: 120
The DTE uses this signal to request transmission
from the DCE on the secondary channel. It is equivalent to the Request To Send
signal.
When the secondary channel is only used for diagnostic purposes or to
interrupt the flow of data in the primary channel, this signal should turn
"On" the secondary channel un-modulated carrier.
20 Data Terminal Ready
- Name: CD
- Direction: DTE -> DCE
- CCITT: 108.2
When on, tells the DCE that the DTE is
available for receiving. This signal must be "On" before the DCE can turn Data Set Ready
"On", thereby indicating that it is connected to the communications link.
The Data Terminal Ready and Data Set Ready
signals deal with the readiness of the equipment, as opposed to the Clear To Send
and Request To
Send signals that deal with the readiness of the communication channel.
When "Off", it causes the DCE to finish any transmission in progress and to
be removed from the communication channel.
21 Signal Quality Detector
- Name: CG
- Direction: DTE <- DCE
- CCITT: 110
This line is used by the DCE to indicate whether
or not there is a high probability of an error in the received data. When
there is a high probability of an error, it is set to "Off", and is "On" at
all other times.
22 Ring Indicator
- Name: CE
- Direction: DTE <- DCE
- CCITT: 125
On this line the DCE signals the DTE that there
is an incoming call. This signal is maintained "Off" at all times except when
the DCE receives a ringing signal.
23 Data Signal Rate Selector
- Name: CH/CI
- Direction: DTE -> DCE
- CCITT: 111/112
The DTE uses this line to select the
transmission bit rate of the DCE. The selection is between two rates in the
case of a dual rate synchronous connection, or between two ranges of data
rates in the case of an asynchronous connection.
Typically, when this signal is "On", it tells the DCE (modem) that the
receive speed is greater than 600 baud.
24 Transmitter Signal Element Timing
- Name: DA
- Direction: DTE -> DCE
- CCITT: 113
The DTE sends the DCE a transmit clock on this
line. This is only when the master clock is in the DTE.
A "On" to "Off" transition should indicate the center of each signal
element (bit) on the Transmitted
Data line.
-
A note on signal travel direction
The pin names are the same for the DCE and DTE. The Transmit Data (pin
number 2) is a transmit line on the DTE and a receive line on the DCE, Data
Set Ready (pin number 6) is a receive line on the DTE and a transmit line on
the DCE, and so forth.
Electrical Signal Characteristics
-
Voltage levels defined in the standard
Data signals "0","Space" "1","Mark"
Driver (Required) 5 - 15 -5 - -15 Volts
Terminator (expected) 3 - 25 -3 - -25 Volts
Control signals "Off" "On"
Driver (Required) -5 - -15 5 - 15 Volts
Terminator (expected) -3 - -25 3 - 25 Volts
-
The Noise Margin Issue
Note that terminator (receiving end)
voltages are not the same as driver required voltages. This voltage level
definition compensates for voltage losses across the cable.
Signals traveling along the cable are attenuated and distorted as they
pass. Attenuation increases as the length of the cable increases. This effect
is largely due to the electrical capacitance of the cable.
The maximum load capacitance is specified as 2500pf (picofarad) by the
standard. The capacitance of one meter of cable is typically around 130pf,
thus the maximum cable length is limited to around 17 meters. However, This is
a nominal length defined by the standard, and it is possible to use longer
cables up to 30 meters, with low-capacitance cables, or with slow data rates
and a proper error correction mechanism.
-
Interface Mechanical Characteristics
The connection of the DCE
and the DTE is done with a pluggable connector. The female connector should be
associated with the DCE. The following table lists the pin assignments defined
by the standard. The type of connector to be used is not mentioned in the
standard, but the DB-25 (or on IBM-AT's, a minimal DB-9) connectors are almost
always used.
-
Pin designation for the 25-pin and 9-pin DB connector
includes equivalent CCITT V.24 identification, and signal direction
DB-25 DB-9 Common EIA
Pin # Pin # Name Name CCITT DTE-DCE Formal Name
----- ----- ----- ----- ----- ------- -------------------
1 FG AA 101 - Frame Ground
2 3 TD BA 103 ---> Transmitted Data, TxD
3 2 RD BB 104 <--- Received Data, RxD
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, GND
8 1 DCD CF 109 <--- Data Carrier Detect
9 -- -- - - +P
10 -- -- - - -P
11 -- -- - - unassigned
12 SDCD SCF 122 <--- Secondary Data Carrier Detect
13 SCTS SCB 121 <--- Secondary Clear To Send
14 STD SBA 118 ---> Secondary Transmitted Data
15 TC DB 114 <--- Transmission Signal Element Timing
16 SRD SBB 119 <--- Secondary Received Data
17 RC DD 115 ---> Receiver Signal Element Timing
18 -- -- - - unassigned
19 SRTS SCA 120 ---> Secondary Request To Send
20 4 DTR CD 108.2 ---> Data Terminal Ready
21 SQ CG 110 <--- Signal Quality Detector
22 9 RI CE 125 <--- Ring Indicator
23 -- CH/CI 111/112 ---> Data Signal Rate Selector
24 -- DA 113 <--- Transmitter Signal Element Timing
25 -- -- - - unassigned
-
Diagram of the DB-25 and DB-9 connectors
male connectors , front view
1 13 1 5
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\ . . . . . . . . . . . . / \ . . . . /
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14 25 6 9
DB-25 DB-9
