Why Do Hard Disks use Serial Transmission?

A computer can use two methods to exchange data between storage devices and the motherboard. These methods are serial transmission and parallel transmission.

In serial transmission, data is transferred bit by bit. In parallel transmission, multiple bits are transmitted simultaneously. For example, the IDE cable we use to connect a hard disk to the motherboard transfers eight bits at a time. Parallel transmission is faster than serial transmission. However, manufacturers and vendors prefer serial transmission due to the following reasons.

  1. Processing overhead
  2. Cable size
  3. Cross talk

Let us understand these reasons in detail.

This tutorial is the second part of the tutorial series "Hard Disk Type, Data Transmission Technology and Interfaces Explained.". Other parts of this series are the following.

Differences between Hard disks and SSDs Explained

SCSI, PATA, SATA, and NVMe Explained

Processing overhead

Computers store and process data bit by bit. When you create and save a file on a hard disk, the hard disk saves the file bit by bit. Similarly, when you read a file from the hard disk, the head of the hard disk reads all bits of the file one by one and sends them to the application you use to read the file. The application merges all bits and uses them as a single file.

Since all devices on a computer read, write, and process data bit by bit, parallel transmission creates processing overhead.

Let us understand it through an example.

Suppose a hard disk is connected to the motherboard through the IDE cable. As mentioned earlier, an IDE cable uses parallel transmission. It transmits 8 bits or one byte at a time.

To send these bits to the disk, the motherboard has to line up all bits and load them on the IDE cable. Once the loading is completed, the IDE cable transports them to the hard disk. The hard disk reverses the process. It unloads all bits one by one and processes them.

Since the motherboard and hard disk process serial data and IDE cables transport parallel data, the data is converted at both ends. The sender device converts it from serial to parallel. The receiver device converts it from parallel to serial. It creates a lot of processing overhead because the transmission medium doesn't match the original input or desired output.

serial and parallel transmission

We can remove this overhead by using a serial cable. Since a serial cable transports data in the same medium the motherboard and hard disk use, it does not create any processing overhead.

Cable size

Parallel cables are thick and bulky. They consume a lot of space. They block the airflow and trap the heat generated by the CPU and other components inside the cabinet. Unlike parallel cables, serial cables are thin and compact. They consume a little space and do not block airflow inside the cabinet.

cable size

Cross talk

When signals travel over copper wires, they create an electromagnetic field that affects the signals traveling in adjacent wires. This disturbance is called crosstalk.

cross talk

On a parallel cable, multiple signals travel side by side at the same time. It creates a lot of crosstalk. On a serial cable, only one signal travels at a time. It significantly reduces the crosstalk.

Due to the processing overhead, cable size, and crosstalk, manufacturers and vendors prefer serial cables over parallel cables.

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