Communication as defined in the RS-232C standard is an asynchronous serial communication method. The word serial means, that the information is sent one bit at a time. Asynchronous tells us that the information is not sent in predefined time slots. Data transfer can start at any given time and it is the task of the receiver to detect when a message starts end ends. Asynchronous communication has some advantages and disadvantages which are both discussed in the next paragraph.
Bit Streams
The RS-232C standard describes a communication method where information is sent bit by bit on a physical channel. The information must be broken up in data words. The length of a data word is variable. On PC's a length between 5 and 8 bits can be selected. This length is the netto information length of each word. For proper transfer additional bits are added for synchronisation and error checking purposes. It is important, that the transmitter and receiver use the same number of bits. Otherwise, the data word may be misinterpreted, or not recognized at all.
With synchronous communication, a clock or trigger signal must be present which indicates the beginning of each transfer. The absence of a clock signal makes an asynchronous communication channel cheaper to operate. Less lines are necessary in the cable. A disadvantage is, that the receiver can start at the wrong moment receiving the information. Resynchronization is then needed which costs time. All data received in the resynchronization period is lost. Another disadvantage is that extra bits are needed in the data stream to indicate the start and end of useful information. These extra bits take up bandwidth.
Data bits are sent with a predefined frequency, the baud rate. Both the transmitter and receiver must be programmed to use the same bit frequency. After the first bit is received, the receiver calculates at which moments the other data bits will be received. It will check the line voltage levels at those moments.
The line voltage level can have two states. The on state is also known as marking, the off state as spacing. No other line states are possible. When the line is idle, it is kept in the marking state.
Start Bit
RS-232C defines an asynchronous type of communication. This means, that sending of a data word can start on each moment. If starting at each moment is possible, this can pose some problems for the receiver to know which is the first bit to receive. To overcome this problem, each data word is started with an attention bit. This attention bit, also known as the start bit, is always identified by the space line level. Because the line is in marking state when idle, the start bit is easily recognized by the receiver.
Data Bits
Directly following the start bit, the data bits are sent. A bit value 1 causes the line to go in marking state, the bit value 0 is represented by a space. The least significant bit is always the first bit sent.
Parity Bit
For error detecting purposes, it is possible to add an extra bit to the data word automatically. The transmitter calculates the value of the bit depending on the information sent. The receiver performs the same calculation and checks if the actual parity bit value corresponds to the calculated value. This is further discussed in error detection.
Stop Bits
Suppose that the receiver has missed the start bit because of noise on the transmission line. It started on the first following data bit with a space value. This causes garbled date to reach the receiver. A mechanism must be present to resynchronize the communication. To do this, framing is introduced. Framing means, that all the data bits and parity bit are contained in a frame of start and stop bits. The period of time lying between the start and stop bits is a constant defined by the baud rate and number of data and parity bits. The start bit has always space value, the stop bit always marking value. If the receiver detects a value other than marking when the stop bit should be present on the line, it knows that there is a synchronization failure. This causes a framing error condition in the receiving UART. The device then tries to resynchronize on new incomming bits.
For resynchronizing, the receiver scans the incomming data for valid start and stop bit pairs. This works, as long as there is enough variation in the bit patterns of the data words. If data value zero is sent repeatedly, resynchronization is not possible for example.
The stop bit identifying the end of a data frame can have different lengths. Actually, it is not a real bit but a minimum period of time the line must be idle (marking state) at the end of each word. On PC's this period can have three lengths: the time equal to 1, 1.5 or 2 bits. 1.5 bits is only used with data words of 5 bits length and 2 only for longer words. A stop bit length of 1 bit is possible for all data word sizes.
Error detection
One way of detecting errors is already discussed. It is the frame detection mechanism which is used to test if the incomming bits were properly surrounded by a start and stop bit pair. For further error checking, a parity bit can be used.
Parity is a simple way to encode a data word to have a mechanism to detect an error in the information. The method used with serial communications adds one bit to each data word. The value of this bit depends on the value of the data word. It is necessary that both the transmitter and receiver use the same algorithm to calculate the value of the parity bit. Otherwise, the receiver may detect errors which are not present.
Even Parity
Basically, the parity bit can be calculated in two ways. When even parity is used, the number of information bits sent will always contain an even number of logical 1's. If the number of high data bits is odd, a high value parity bit is added, otherwise a low bit will be used.
Odd Parity
The odd parity system is quite similar to the even parity system, but in this situation, the number of high bits will always be odd.
RS232 Connector Pin Assignment
The RS232 connector was originaly developed to use 25 pins. In this pin-out provisions were made for a secondary communication channel. In practice, only one communication channel with accompanying handshaking is present. For that reason the smaller 9 pin version is more commonly used today. The diagrams show the signals common to both connector types in black. The signals only present on the larger connector are shown in red. Note, that the protective ground is assigned to a pin at the large connector where the connector outside is used for that purpose with the DB9 version.
The pin assignment is also shown for the DEC modified modulair jack. Although this interface is differential (the receive and transmit have their own floating ground level) it is possible to connect RS232 compatible devices with this interface.
RS232 DB 9 pin assignment
RS232 DB 25 pin assignment
RS232 DB9 to DB25 converter
RS232 loopback test plug for terminal emulation software
RS232 Null Modem Cables
Simple null modem without handshaking
RS232 Null Modem Cables
Simple null modem without handshaking
Null modem with loop back handshaking
Null modem with partial handshaking
Null modem with full handshaking
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