Lähtömerkistöistä

Lähtömerkistöistä, known in English as "start bits" or "start characters," are a concept found in certain communication protocols, particularly in older serial communication systems. Their primary function is to signal the beginning of a data transmission and to allow the receiving device to synchronize its clock with the incoming data stream.

In asynchronous serial communication, such as RS-232, each character is typically preceded by a start bit. This start bit is usually represented by a transition from a high voltage level (representing a logical '1') to a low voltage level (representing a logical '0'). This noticeable change alerts the receiver that data is about to arrive. After the start bit, a sequence of data bits follows, representing the actual information being transmitted.

The purpose of the start bit is crucial for maintaining data integrity. Since asynchronous communication does not use a continuous clock signal shared between the sender and receiver, the receiver needs a way to know when each transmitted bit begins and ends. The start bit provides this synchronization point. Once the start bit is detected, the receiver can then sample the subsequent data bits at predetermined intervals, determined by the agreed-upon baud rate.

Following the data bits, a stop bit or stop bits are typically sent to indicate the end of the character and to provide a guaranteed idle time before the next start bit can be transmitted. The presence and number of start and stop bits, along with the data bits and parity bit (if used), define the "framing" of the serial communication. Understanding start bits is fundamental to comprehending how asynchronous serial data is reliably transferred between devices.