The device then tries to re-synchronize on new incoming bits.įor re-synchronizing, the receiver scans the incoming data for valid start and stop bit pairs. This causes a framing error condition in the receiving UART. If the receiver detects a value other than mark when the stop bit should be present on the line, it knows that there is a synchronization failure. The start bit has always space value, the stop bit always mark value. 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. Framing means, that all the data bits and parity bit are contained in a frame of start and stop bits. A mechanism must be present to re-synchronize the communication. This causes garbled date to reach the receiver. It started on the first following data bit with a space value. Suppose that the receiver has missed the start bit because of noise on the transmission line. This is further discussed in another paragraph. The receiver performs the same calculation and checks if the actual parity bit value corresponds to the calculated value. The transmitter calculates the value of the bit depending on the information sent. Parity bitįor error detecting purposes, it is possible to add an extra bit to the data word automatically. The least significant bit is always the first bit sent. A bit value 1 causes the line to go in mark state, the bit value 0 is represented by a space. Data bitsĭirectly following the start bit, the data bits are sent. Because the line is in mark state when idle, the start bit is easily recognized by the receiver. This attention bit, also known as the start bit, is always identified by the space line level. To overcome this problem, each data word is started with an attention bit. If starting at each moment is possible, this can pose some problems for the receiver to know which is the first bit to receive. This means, that sending of a data word can start on each moment. RS232 defines an asynchronous type of communication. When the line is idle, it is kept in the mark state. The on state is also known as mark, the off state as space.
With RS232, the line voltage level can have two states. It will check the line voltage levels at those moments. After the first bit is received, the receiver calculates at which moments the other data bits will be received. Both the transmitter and receiver must be programmed to use the same bit frequency. These extra bits take up bandwidth.ĭata bits are sent with a predefined frequency, the baud rate. Another disadvantage is that extra bits are needed in the data stream to indicate the start and end of useful information. All data received in the re-synchronization period is lost. Re-synchronization is then needed which costs time. A disadvantage is, that the receiver can start at the wrong moment receiving the information. The absence of a clock signal makes an asynchronous communication channel cheaper to operate. With synchronous communication, a clock or trigger signal must be present which indicates the beginning of each transfer. Otherwise, the data word may be misinterpreted, or not recognized at all. It is important, that the transmitter and receiver use the same number of bits. For proper transfer additional bits are added for synchronization and error checking purposes. This length is the net information length of each word. On PC’s a length between 5 and 8 bits can be selected. The information must be broken up in data words.
The RS232 standard describes a communication method where information is sent bit by bit on a physical channel.
Asynchronous communication has some advantages and disadvantages which are both discussed in the next paragraph. Data transfer can start at any given time and it is the task of the receiver to detect when a message starts and ends. Asynchronous tells us that the information is not sent in predefined time slots. The word serial means, that the information is sent one bit at a time. Communication as defined in the RS232 standard is an asynchronous serial communication method.
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