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RS232 pinout diagram

RS232 Information and Pinout

Unraveling RS232

RS232, or Recommended Standard 232, is a long-standing serial communication protocol used for decades to facilitate communication between computers and peripherals. Although newer standards have emerged, RS232 remains integral to the technological landscape, particularly for legacy systems. This tech blog post will delve into the fundamentals of RS232, covering its history, components, and how it works. We will also explore its limitations and discuss some popular alternatives.

History of RS232

RS232 was introduced in 1960 by the Electronic Industries Association (EIA) as a means of standardizing communication between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE). Initially used for teleprinters, RS232 gained widespread adoption and quickly became the de facto standard for serial communication. It has since undergone several revisions, the most recent being the EIA-232-F standard released in 1997.

Components of RS232

  1. Connectors and Pinouts – RS232 primarily uses DE-9 (DB-9) and DB-25 connectors, with DE-9 being the most common. Both connectors have different pin assignments, but the primary pins used for communication are the same: Transmit Data (TD), Receive Data (RD), and Signal Ground (SG).
  2. Voltage Levels – RS232 uses voltage levels to represent binary data. A voltage range of +3 to +15 volts represents a logical ‘0’ (space), while -3 to -15 volts represents a logical ‘1’ (mark). Voltages between -3 and +3 volts are undefined and may lead to communication errors.
  3. Baud Rate – The baud rate is the speed at which data is transmitted over the RS232 interface. Standard baud rates include 9600, 19200, 38400, and 57600 bits per second (bps). The transmitting and receiving devices must agree on the baud rate to ensure accurate communication.

How RS232 Works

RS232 Data Packet

RS232 is an asynchronous, full-duplex communication protocol, meaning that data can be transmitted and received simultaneously without a separate clock signal. A start bit, data bits, optional parity bit, and stop bits form the basic structure of an RS232 data packet.

  1. Start Bit – The start bit signals the beginning of a data packet. It is always a logical ‘0’ (space).
  2. Data Bits – The data bits represent the actual information being transmitted. RS232 can use 5 to 9 data bits per packet, with eight being the most common.
  3. Parity Bit – The optional parity bit is used for error detection. It can be set to even, odd, or none.
  4. Stop Bits – The stop bits signal the end of a data packet. They are always logical ‘1’ (mark) and can be either one or two bits long.

RS232 Pinout

The pinout of RS232 depends on the type of connector being used. There are two standard connectors for RS232 communication: DE-9 (also known as DB-9) and DB-25. Here, we’ll provide the pinout for the most commonly used DE-9 connector.

DE-9 (DB-9) Connector Pinout:

RS232 pinout diagram

Pin 1 – DCD – Data Carrier Detect

Pin 2 – RD – Receive Data (RxD)

Pin 3 – TD – Transmit Data (TxD)

Pin 4 – DTR – Data Terminal Ready

Pin 5 – SG – Signal Ground

Pin 6 – DSR – Data Set Ready

Pin 7 – RTS – Request To Send

Pin 8 – CTS – Clear To Send

Pin 9 – RI – Ring Indicator


It is important to note that not all pins are utilized in every implementation. The essential pins for communication include Transmit Data (TD), Receive Data (RD), and Signal Ground (SG). The remaining pins are typically employed for handshaking and flow control, but their usage depends on the specific setup. The majority of RS232 communications can be achieved by linking these three primary pins. Don’t forget to cross-connect the Tx and Rx pins.

Pin 2 – Rx Data

Pin 3 – Tx Data

Pin 5 – GND

RS232 cross connection details

Limitations of RS232

  1. Distance – RS232 is limited to relatively short distances, typically 15 meters (50 feet) or less, due to signal degradation.
  2. Speed – The maximum data rate for RS232 is 20 kbps, significantly slower than modern communication standards.
  3. Noise Susceptibility – RS232 is susceptible to electromagnetic interference, which can lead to communication errors.

Alternatives to RS232

  1. RS422– RS422 is a balanced serial communication standard offering longer distances and faster data rates than RS232.
  2. RS485 – RS485 is a multi-drop communication standard that allows multiple devices to share a single communication line.
  3. USB – Universal Serial Bus (USB) is a widely-used standard for connecting peripherals to computers, offering faster data rates and greater flexibility than RS232.
  1. Ethernet – Ethernet is a popular networking technology that supports high-speed data transfer, making it suitable for applications requiring large amounts of data or real-time communication.
  2. I2C and SPI – Inter-Integrated Circuit (I2C) and Serial Peripheral Interface (SPI) are communication protocols used in embedded systems and microcontroller-based applications, offering lower power consumption and more straightforward connections than RS232.

Despite its limitations, RS232 remains a relevant communication standard, particularly in legacy systems and industrial applications. Its simple and robust design has stood the test of time, making it a reliable option for specific use cases. However, as technology advances and requirements evolve, alternatives like RS422, RS485, USB, Ethernet, I2C, and SPI are often more suitable for modern applications. Understanding the strengths and weaknesses of RS232 and its alternatives is essential for selecting the right communication protocol for your specific needs.

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