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DCD (Pino 1 - PC Input)
Data Carrier Detect. The on condition of this signal line, as sent by the DCE, informs the DTE that it is receiving a carrier signal from the remote DCE that meets its criteria.
This is a signal to indicate from the communications equipment (DCE) that the phone line is still "connected" and receiving a carrier signal from the modem at the other end. Presumably well-written software or serial equipment could detect from this logic state when the telephone has been "hung up" on the other end. Null-modems often tie DCD to DSR at each end since there is no carrier signal involved. This signal pin can be independently detected by software, and a "real" hacker could have some fun playing with this pin to add some sort of signal to increase bandwidth. Still, it isn't worth the effort, when there are much better ways to increase data throughput....
RxD (Pino 2 - PC Input)
Receive Data line. Serial data from DCE to DTE. When DCD is in the off condition, the RD signal must be in the MARK state.
TxD (Pino 3 - PC Output)
Transmit Data Line. Serial data from DTE to DCE.
The reverse of RX, this is where the terminal equipment (DTE) is transmitting serial data, using the same format and protocol that the receiver is expecting...usually. More on the exact protocol further below. Like RX, think along the lines of "Terminal Transmit" when designing equipment that will be using this pin.
DTR (Pino 4 - PC Output)
Data Terminal Ready. This signal, in conjunction with DSR, indicates equipment readiness.
Basically a signal from the DTE that says "Hello!, I'm ready if you are". This is a general indicator to the DCE that the terminal is ready to start sending and receiving data. If there is some initialization that needs to happen in the communications equipment, this is a way for the terminal equipment to "boot" the receiving equipment.
GND (Pino 5 - Ref. Zero Volts)
Ground. This is an interesting pin to look at. What it does is try to make a common "ground" reference between the equipment that is being connected to compare the voltages for the other signals. Normally this is a good thing, because sometimes different pieces of equipment have different power supplies and are some distance away. The not so pleasant thing about this wire is that it usually is a physical piece of copper that can conduct electricity that is not normally supposed to go down the wire, like a short-circuit or worse yet a bolt of lightning (it happens far more often that you would normally think for this sort of equipment). That can fry both the DCE as well as the DTE. Things like fiber converters and ground isolators can help prevent this from happening, but can still be something to worry about. Over short distances this is generally not a problem.
DSR (Pino 6 - PC Input)
Data Set Ready. A signal turned on by the DCE to indicate to the DTE that it is connected to the line.
This is the counterpart to DTR with the communications equipment (or computer peripheral on the serial line). When the DTR is sent as a signal, the communications equipment should change this signal to -15V to indicate that it is ready to communicate as well. If the DCE goes through a "boot" sequence when the DTR gets signaled, it should not signal DSR until it is complete. But many connectors "hard wire" this pin to be directly connected to the DTR pin at each end to reduce the number of wires needed in the cable. This can be useful for connecting devices using existing telephone wires, but prevents applications from using the DTR and DSR for handshaking.
RTS (Pino 7 - PC Output)
Request To Send. Indicates the DTE is ready to transmit data, and then the DCE must prepare to receive data. After some delays, the DCE turns on the CTS line to inform the DTE it is ready to receive data. Once communication is complete, the DTE turns off the RTS signal. After a brief delay to ensure that all transmitted data has been received, the DCE turns off CTS.
The signal (logical "1"), it indicates to the DCE that the terminal is ready to receive more data. When it has a +15V (logical "0") signal, it indicates that the DCE should stop sending data temporarily until the buffer can be cleared.
CTS (Pino 8 - PC Input)
Clear To Send. Turned on by the DCE to inform the DTE that it is ready to receive data.
This is the response signal from the DCE regarding if the terminal equipment should be transmitting any data. When this signal is -15V the terminal is "permitted" to transmit data. Like the DTR/DSR pins, this one can be directly connected to the RTS pin to reduce the number of wires needed, but this eliminates the possibility of hardware flow control. Some software ignores this pin and the RTS pin, so other flow control systems are also used. That will be explained when we get to actual software.
RI (Pino 9 - PC Input)
Ring Indicator. Turned on by the DCE while ringing is being received.
Again, thinking back to a telephone modem, this is a signal that indicates that the telephone is "ringing". Generally, even on a real telephone modem, this is only occasionally set to -15V for the signal. Basically, when you would normally be hearing a "ring" on your telephone, this pin would be signaled. On Null-modems, often this wire isn't even connected to anything. If you really are connected to a real modem, this does have some strong uses, although there are other ways to have the terminal equipment (like a PC connected to an external modem) be informed that there are ways to communicate this information through the data pins as well. This will be covered lightly in the software section.
