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What is the RS-232 interface standard?

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1. What is the RS-232 interface standard?

RS 232 is a standard communication protocol mainly used for serial communication between two devices. It was first introduced by the EIA (Electronic Industries Association) in 1960 with the purpose of providing a roadmap for connecting one device to other peripherals for perfect digital communication.

The advent of USB has greatly reduced the need for the RS232 protocol, but we still cannot rule out its importance in some industrial applications where Programmable Logic Controllers and Computerized Numerical Control Equipments (CNC) are programmed individually using the RS 232 connector which is an interface between DTE (Data terminal equipment) and DCE (Data communication equipment).
RS 232 is a standard protocol proposed for serial data transfer between electronic devices. It mainly operates on three signals: Transmission line, Receive line and Ground.

2. Features of RS-232

The logic level of RS232 is defined differently from the TTL logic:

  • DCE: a high level (corresponding to logic 0) is a voltage of +5 to +15 V, while a low level (corresponding to logic 1) is a voltage of -5 to -15 V
  • DTE: High levels are defined as from +3 to +15 V (called space), and low levels are defined as from -3 to -15 V (called mark).

+ Line impedance: The wire resistance is up to 3 – 7 ohms and the maximum cable length is 15 meters, but within the capacitance limit per unit length.

+ Operating voltage: 250VAC max

+ Rated current: Maximum 3 Amps

+ Insulation: 1000 VAC

+ Slew rate: The rate of change of signal levels is called Slew rate. With variable rates up to 30V / ms and the maximum bit rate will be 20 kbps.

3. Connector Pinout

In fact, the RS232 port, also known as the COM port, has two types of connectors, DB-25 and DB-9. In DB-25, there are 25 pins available for many applications, but some applications do not use all 25 pins. Therefore, the 9-pin connector is made to facilitate the connection of devices.

4. Types of serial communication

There are two main types of serial communication: Half Duplex and Full Duplex

+ Half Duplex only transmits information in one direction. It comes with two lines in which one is a data line and the other is a signal ground. In this communication, the terminal has the ability to send or receive data, but not at the same time. This method is an old method and is no longer practiced.

+ Full Duplex can transmit and receive data in both directions, requiring three main paths: data transmission, data reception and signal ground.

5. Application of RS-232

+ Connect devices such as PLC, HMI, Inverter, Motor,… together easily

+ Many applications in measuring equipment, electronic scales

+ Used a lot in CNC, Bar code machines

+ Low cost, easy integration should be widely used in civil and industrial

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