9 Pin Serial Port Connector

Connector may be reversed depending on which side is viewed. All pins are numbered.

Pin No. Function
1 DCD (Data Carrier Detect)
2 RX (Receive Data)
3 TX (Transmit Data)
4 DTR (Data Terminal Ready)
5 GND (Signal Ground)
6 DSR (Data Set Ready)
7 RTS (Request To Send)
8 CTS (Clear To Send)
9 RI (Ring Indicator)
Voltage sent over the pins can be in one of two states, On or Off. On (binary value "1") means that the pin is transmitting a signal between -3 and -25 volts, while Off (binary value "0") means that it is transmitting a signal between +3 and +25 volts.
An important aspect of serial communications is the concept of flow control. This is the ability of one device to tell another device to stop sending data for a while. The commands Request to Send (RTS), Clear To Send (CTS), Data Terminal Ready (DTR) and Data Set Ready (DSR) are used to enable flow control.

Introduction to Serial Ports

Serial ports are a type of computer interface that complies with the RS-232 standard. They are 9-pin connectors that relay information, incoming or outgoing, one byte at a time. Each byte is broken up into a series of eight bits, hence the term serial port. Serial ports are one of the oldest types of interface standards.

In traditional computers, serial ports were configured as follows:
Serial Ports Interrupt Memory Address
COM 1 IRQ 4 0x3f8
COM 2 IRQ 3 0x2f8
COM 3 IRQ 4 0x3e8
COM 4 IRQ 3 0x2e8
Devices configured to use serial ports COM 1 and COM 3 could not be active at the same time, as they shared interrupt IRQ 4. The same was true of COM 2 and COM 4 port devices. The serial port is much more than just a connector. It converts the data from parallel to serial and changes the electrical representation of the data.
Serial flow is a stream of bits over a single wire (such as on the transmit or receive pin of the serial connector). For the serial port to create such a flow, it must convert data from parallel (inside the computer) to serial on the transmit pin (and conversely).
The advantage is that a serial port needs only one wire to transmit the 8 bits (while a parallel port needs 8). The disadvantage is that it takes 8 times longer to transmit the data than it would if there were 8 wires. Serial ports lower cable costs and make cables smaller. Serial ports, also called communication (COM) ports, are bi-directional. Bi-directional communication allows each device to receive data as well as transmit it.
Serial ports rely on a special controller chip, the Universal Asynchronous Receiver/Transmitter (UART), to function properly. The UART chip takes the parallel output of the computer's system bus and transforms it into serial form for transmission through the serial port. In order to function faster, most UART chips have a built-in buffer of anywhere from 16 to 64 kilobytes. This buffer allows the chip to cache data coming in from the system bus while it is processing data going out to the serial port.

Serial Data Transfer

In a serial link, the transmitter, or driver, sends bits one at a time, in sequence. One signal required by all serial links is a clock, or timing reference, to control the flow of data. The transmitter and receiver use a clock to decide when to send and read each bit. There are two types of serial-data formats :

- Synchronous Format : Data transfer method in which a continuous stream of data signals is accompanied by timing signals(generated by an electronic clock) to ensure that the transmitter and the receiver are in step (synchronized) with one another. The data is sent in blocks (called frames or packets) spaced by fixed time intervals. After the synchronized characters are received by the remote device, they are decoded and used to synchronize the connection. After the connection is correctly synchronized, data transmission may begin. The following is a list of characteristics specific to synchronous communication:
* There are no gaps between characters being transmitted.
* Timing is supplied by modems or other devices at each end of the connection.
* Special syn characters precede the data being transmitted.
* The syn characters are used between blocks of data for timing purposes.

- Asynchronous Format : The term asynchronous is used to describe the process where transmitted data is encoded with start and stop bits, specifying the beginning and end of each character. Asynchronous, or character-framed, transmission is used to transmit seven or eight-bit data, usually in ASCII character format. Each character has a specific start and end sequence, usually one start bit and one or two end (stop)bits.
When gaps appear between character transmissions, the asynchronous line is said to be in a mark state. A mark is a binary 1 (or negative voltage) that is sent during periods of inactivity on the line. When the mark state is interrupted by a positive voltage (a binary 0), the receiving system knows that data characters are going to follow. The following is a list of characteristics specific to asynchronous communication:
* Each character is preceded by a start bit and followed by one or more stop bits.
* Gaps or spaces between characters may exist.