Serial communication

Serial communication is a popular means of transmitting data between a computer and a peripheral device such as a programmable instrument or even another computer. Serial communication uses a transmitter to send data, one bit at a time, over a single communication line to a receiver. You can use this method when data transfer rates are low or you must transfer data over long distances.

Serial communication requires that you specify the following four parameters:

The baud rate of the transmission

The number of data bits encoding a character

The sense of the optional parity bit

The number of stop bits

Each transmitted character is packaged in a character frame that consists of a single start bit followed by the data bits, the optional parity bit, and the stop bit or bits. figure below shows a typical character frame encoding the letter m.

Baud rate is a measure of how fast data are moving between instruments that use serial communication. RS-232 uses only two voltage states, called MARK and SPACE. In such a two-state coding scheme, the baud rate is identical to the maximum number of bits of information, including control bits, that are transmitted per second. MARK is a negative voltage, and SPACE is positive. A start bit signals the beginning of each character frame. It is a transition from negative (MARK) to positive (SPACE) voltage. Its duration in seconds is the reciprocal of the baud rate. If the instrument is transmitting at 9,600 baud, the duration of the start bit and each subsequent bit is about 0.104 ms. The entire character frame of eleven bits would be transmitted in about 1.146 ms. Data bits are transmitted upside down and backwards. That is, inverted logic is used, and the order of transmission is from least significant bit (LSB) to most significant bit (MSB). To interpret the data bits in a character frame, you must read from right to left and read 1 for negative voltage and 0 for positive voltage. This yields 1101101 (binary) or 6D (hex). An ASCII conversion table shows that this is the letter m. An optional parity bit follows the data bits in the character frame. The parity bit, if present, also follows inverted logic, 1 for negative voltage and 0 for positive voltage. This bit is included as a simple means of error handling. You specify ahead of time whether the parity of the transmission is to be even or odd. If the parity is chosen to be odd, the transmitter then sets the parity bit in such a way as to make an odd number of ones among the data bits and the parity bit. This transmission uses odd parity. There are five ones among the data bits, already an odd number, so the parity bit is set to 0. The last part of a character frame consists of 1, 1.5, or 2 stop bits. These bits are always represented by a negative voltage. If no further characters are transmitted, the line stays in the negative (MARK) condition. The transmission of the next character frame, if any, is heralded by a start bit of positive (SPACE) voltage.

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