Serial & Parallel Data Transmission

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There are two ways to transfer data between computers: Serial Transmission and Parallel Transmission.

Serial Transmission

Data is sent bit by bit from one computer to another in two directions.  Each bit has a clock pulse rate.  Eight bits are transmitted at a time, with a start and stop bit known as a parity bit, which is 0 and 1, respectively.  Data cables are used when transmitting data over a longer distance.  The data cable has D-shaped 9 pin cable that connects the data in series.

Categories of Serial Transmission

Asynchronous transmission – an extra bit is added to each byte to alert the receiver to the arrival of new data.  0 is used as a start bit, while 1 used as a stop bit.

Synchronous transmission – no extra bit is added to each byte.  Data is transferred in batches, each of which contains multiple bytes.

Parallel Transmission

Several bits are transmitted simultaneously with one clock pulse rate.  It transmits quickly as it utilises several input and output lines for sending the data.

It uses a 25-pin port with 17 signal lines and 8 ground lines.  The 17 signal lines are divided as

  • 4 lines – initiate handshaking
  • 5 lines – communicate and notify errors
  • 8 lines – transfer data


Serial transmission occurs between two computers, or from a computer to an external device located far away.

Parallel transmission can take place within a computer system, through a computer bus, or to an external device located nearby.


One example of serial mode transmission is a connection established between a computer and a modem using the RS-232 protocol.  An RS-232 cable can accommodate 25 wires, but only two of these wires are for data transmission; the rest are designated for overhead control signaling.  The two data wires run using simple serial transmission in either direction.

In this example, a computer may be far from the modem, making parallel transmission very expensive.  With this in mind, speed of transmission is considered less important when compared to the economic advantage of serial transmission.

An example of parallel mode transmission is a connection established between a computer and a printer.  Most printers are within 6 meters (about 20 feet) from the transmitting computer, and the slight cost for extra wires is offset by the added speed gained through parallel transmission of data.

Comparison between Serial and Parallel Transmission

Basis for Comparison Serial Transmission Parallel Transmission
Definition Data flows in 2 directions, bit by bit Data flows in multiple directions, 8 bits (1 byte) at a time
Cost Economical Expensive
Number of bits transferred per clock pulse 1 bit 8 bits or 1 byte
Speed Slow Fast
Applications Used for long distance communication Used for short distance communication
Example Computer to computer Computer to printer

Differences between Serial and Parallel Transmission

  • Serial transmission requires a single line to send data. Parallel transmission requires multiple lines to send data.
  • There are fewer errors and less noise in serial transmission, since the transmission is done one bit at a time. There are more errors and noise in parallel transmission, since the transmission is done multiple bits at a time.
  • Serial transmission is slower since data flows through a single line; conversely, parallel transmission is faster since data flows through multiple lines.
  • Serial transmission is ‘full duplex’ since the sender can send and receive data at the same time. Parallel transmission is ‘half duplex’ since the data can be sent or received at any given time.
  • The cables used in serial transmission are thinner, longer, and more economical compared to the cables used in parallel transmission.
  • Serial transmission is reliable and straightforward. Parallel transmission is unreliable and complicated.

Both serial and parallel transmissions have advantages and disadvantages.  Parallel transmission is used for shorter distances and provides greater speed,  while serial transmission is reliable for transferring data over longer distances.  Both serial and parallel transmissions are individually essential for transferring data.