What is Asynchronous Transmission?

A transmission system is classified as asynchronous if it enables the physical channel to remain idle for arbitrary periods between transmissions. Asynchronous communication is well-suited for applications that generate data randomly, such as a user typing on a keyboard or browsing web pages with unpredictable intervals.

The main challenge of asynchronous transmission stems from the lack of coordination between sender and receiver when the channel is idle. Since the receiver cannot predict when new data will arrive, additional synchronization mechanisms are required.

How Asynchronous Transmission Works

Asynchronous technologies typically require the sender to transmit extra bits before each data element to notify the receiver that transmission is beginning. These additional bits, known as preamble or start bits, enable the receiver's hardware to synchronize with the incoming signal.

RS-232 Asynchronous Character Transmission

The most widely used example of asynchronous communication is RS-232 (RS-232-C), standardized by the Electronic Industries Alliance (EIA). This standard is commonly used for character communication between computers and devices like keyboards.

RS-232 Specifications

• Physical connection − Maximum cable length of 50 feet

• Electrical specifications − Voltage ranges from -15V to +15V (negative = logical 1, positive = logical 0)

• Data format − Each data item represents one character (7-bit or 8-bit configurable)

RS-232 Frame Structure

RS-232 specifies that each character transmission includes:

• Start bit − A 0 bit transmitted before the character data

• Data bits − 7 or 8 bits representing the actual character

• Stop bit − The line remains idle for at least one bit duration, creating a logical 1 "phantom bit"

Advantages and Disadvantages

Conclusion

Asynchronous transmission provides a flexible communication method for applications with irregular data patterns by using start and stop bits to frame each data element. While less efficient than synchronous methods, it remains ideal for character-based communications and simple device interfaces.