1 Protective Ground
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
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
4 Request To Send
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
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
7 Signal Ground
8 Receive Line Signal Detect or Data Carrier Detect
9 +PThis pin is held at +12 volts DC for test purposes.
10 -PThis pin is held at -12 volts DC for test purposes.
12 Secondary Receive Line Signal Detect
13 Secondary Clear To Send
14 Secondary Transmitted Data
15 Transmission Signal Element Timing
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
17 Receiver Signal Element Timing
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
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
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
22 Ring Indicator
23 Data Signal Rate Selector
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
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.