All serial cables transmit data one bit at a time. The two types most likely to be found in the PC environment come with the DB-25 or DB-9 connectors. With a maximum cable length of 50 feet they can be used for anything that needs two way communications with the PC, such as modems, plotters or other PC's. Below is a head on view of two serial cable connectors. The cable has the female connector while the port has the male pins.
 
      
 
Pinout DB-25 /DB-9 serial cable
 

DB-9 pin	DB-25 pin	signal	dir.
1	8	CD	IN
2	3	RD/RX	IN
3	2	TD/TX	OUT
4	20	DTR	OUT
5	7	SG/GND	-
6	6	DSR	IN
7	4	RTS	OUT
8	5	CTS	IN
9	22	RI	IN
-	1	FG	-

 
Loopback
 

connect these pins	TX	RX
connect these pins	RTS	CTS	CD
connect these pins	DTR	DSR	RI

 
signal names
 

CD	carrier detect
RD	receive data
TD	transmit data
DTR	data terminal ready
SG	signal ground
DSR	data set ready
RTS	ready to send
CTS	clear to send
RI	ring indicator

Null modem cables are used to connect two computers (DTE's) together through their serial ports. Although a Null Modem cable might look like a regular RS-232 serial cable, it has some modifications to its internal wiring. Data wires are crossed over so that the output of one computer connects to the input of the other. The control wires are crossed over in such a way that they spoof the handshaking sequence that occurs prior to any PC to Modem communications. A Null Modem cable effectively nullifies the need for a modem, hence its name.
 
Crossover pinout DB-25 serial cable
 

pin	signal	pin	signal
1	FG	1	FG
2	TD	3	RD
3	RD	2	TD
4	RTS	5	CTS
5	CTS	4	RTS
7	SG	7	SG
6	DSR	20	DTR
8	CD	20	DTR
20	DTR	8	CD
20	TDR	6	DSR

 
Crossover pinout DB-9 serial cable
 

pin	signal	pin	signal
1	CD	4	DTR
2	RD	3	TD
3	TD	2	RD
4	DTR	1	CD
4	DTR	6	DSR
5	SG	5	SG
6	DSR	4	DTR
7	RTS	8	CTS
8	CTS	7	RTS

The AUI cable (Attachment User Interface) uses a DB-15 type connector. AUI cables are used in 10base5 Ethernets to transmit digital data, control information and 12v power from a PC's network card to a transceiver. The transceiver, in turn, converts the digital data to line signal and sends it down a truck line that is made of either fiber or coaxial. The maximum length of office grade AUI cable is 12.5 meters. Better grades of AUI could be as long as 50m.
 
AUI drop cable DB-15 pinout
 

pin	signal
1	control shield - in
2	control A - in
3	data A - out
4	data shield - in
5	data A - in
6	Vcc
7	-
8	control shield - out
9	control B - in
10	data B - out
11	data shield - out
12	data B - in
13	V+
14	V shield
15	-
cover	frame gnd

As the name suggests UTP cables have either 2 or 4 pairs of wires wound tightly around each other. Unshielded wiring absorbs radio waves from the environment which adds to the voltage across the wires. This effect corrupts signals and data. Proper shielding inhibits the absorption, leaving the data signals clean but adds to the cost of the cable. To overcome signal corruption while keeping cost low, ethernets use "differential transmission" to send data, which provides a good signal quality without expensive shielding. The wire pairs carry complimentary signals, one positive the other negative. Any outside interference raises the line noise on both wires equally, but the difference in voltage remains the same. Ethernet interface cards use the difference in the line signals to encode bits rather than the actual voltage measurement.
 
10/100baseT CAT5 pinout
 

pin	signal	color
1	out +	white w/ orange stripe
2	out-	orange w/ white stripe
3	in+	white w/ green stripe
4	n/a	blue w/ white stripe
5	n/a	white w/ blue stripe
6	in-	green w/ white stripe
7	n/a	white w/ brown stripe
8	n/a	brown w/ white stripe

 
10/100baseT CAT5 crossover cable
 

pin	<--->	pin
1		3
2		6
3		1
4		4
5		5
6		2
7		7
8		8