Answer:

Introduction:
In data communication, data can be transmitted in two different formats: parallel transmission and serial transmission. The conversion between these two formats is necessary when interfacing different devices or systems. The interface circuits are responsible for converting data from parallel to serial and vice versa.

Explanation:
1. Parallel Transmission:
Parallel transmission involves the simultaneous transmission of multiple bits of data over multiple data lines. Each bit is transmitted over a separate wire, and all the bits are sent simultaneously. This type of transmission requires more wires and is commonly used for short-distance communication within a system, such as between the CPU and memory.

2. Serial Transmission:
Serial transmission involves the sequential transmission of bits over a single wire. The bits are sent one after another in a continuous stream. This type of transmission requires fewer wires and is commonly used for long-distance communication, such as over network cables or telephone lines.

3. Interface Circuits:
The interface circuits are responsible for converting data between parallel and serial formats. These circuits are commonly found in devices such as serial communication ports, network interface cards, and other communication devices.

Parallel to Serial Conversion:
When data needs to be transmitted in serial format, the interface circuits take the parallel data and convert it into a sequential stream of bits. This conversion involves taking each bit from the parallel data and sending it one after another in a specific order. The interface circuits synchronize the transmission of these bits and ensure that they are sent at the correct rate.

Serial to Parallel Conversion:
When data needs to be received in parallel format, the interface circuits take the serial data stream and convert it into parallel data. This conversion involves collecting a specific number of bits from the serial data stream and grouping them together to form a parallel data word. The interface circuits synchronize the reception of these bits and ensure that they are collected at the correct rate.

Conclusion:
In conclusion, the conversion between parallel and serial data transmission takes place inside the interface circuits. These circuits are responsible for converting data from parallel to serial and vice versa, ensuring proper synchronization and transmission/reception of the data.