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.

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RS-232 Signals Functional Description

1 Protective Ground