From analog communication over the telephone wires to the typical USB cables, data exchange has come a long way. One key factor to the advancements made was RS232.
Brief History of RS232
RS232 has been one of the very first standards of communication during the journey. It has been an industry standard for serial communication for years. It has been a standard for electromechanical typewriters and digital data exchange in modems. The standard has made analog data exchange more reliable and efficient. It defined voltage levels that made the communication immune to noise disturbance and has reduced data exchange errors.
Although technological advancements in the communication industry have paved the way for even faster communication methods, RS232 is still one of the most popular modes of communication in the industry.
RS-232 or Recommended Standard 232 is a standard communication interface approved by the Electronic Industries Association (EIA) to connect serial devices. Simply put, RS-232 is a communication standard that describes the protocol and physical interface for low-speed serial communication between a computer and other related devices.
It is the interface that the computer uses to communicate and exchange data with modems and other serial devices. The serial port on computers uses a subcategory of RS-232 communication standard. This communication protocol is used widely over DB9 protocol, commonly known as a serial port. Earlier, DB9 was used over the DB25 port, commonly known as a parallel port.
RS-232 is a serial communication type used for transmitting data over a medium distance. Introduced in the late 1960s, this communication protocol has been widely used in several applications such as factory automation devices, computer printers, and other peripheral applications. Communication through RS-232 cable supports transmission like asynchronous and synchronous.
RS-232 communication is used for recognizing the difference between the two signal levels, Logic 0 and Logic 1. These signal levels are represented with +12 Volts and -12 Volts. RS-232 cable can operate at different baud rates like 2400 bits/sec, 4800 bits/sec and 9600 bits/sec, etc.
This serial communication standard can be used for transmitting data up to 50 feet at the rate of 1.492 kbps. According to EIA, the RS-232 is used to connect Data Communication Equipment (DCE) and Data Transmission Equipment (DTE).
RS-232 serial communication uses Universal Asynchronous Data Receiver and Transmitter (UART) for transmitting data between a computer and peripheral devices like printers. As the microcontrollers are not efficient for handling such voltage levels, connectors are attached between RS-232 signals. These connectors are DB-9 connectors and are of two types: Male connectors or DTE and Female connectors or DCE.
RS-232: Electrical Specifications
Electrical specifications of the RS-232 are as follows:
RS-232 uses two voltage levels: ground and 5 Volts. Logic 0 works with a voltage of +5 Volts to +15 Volts. This voltage level is called ‘Spacing’ or ‘ON,’ whereas Logic 1 works with a voltage of -5 Volts to -15 Volts DC. Logic 1 is also called ‘Marking’ (low-level voltage) or ‘OFF.’
Voltage Level of Received Signal:
Logic 0 on received signal voltage is +3 Volts to +13 Volts DC, and Logic 1 is a voltage of -3 Volts to -13 Volts DC.
The impedance of the cable is up to 3 to 7 ohms, and the maximum length of the cable is 15 meters.
The maximum operating voltage of RS-232 is 250 Volts AC.
The maximum current rating is 3 Amperes.
The slew rate is the rate of change of the signal level, and it is up to 30 Volts per microsecond, and the maximum bitrate is 20 kb/s.
Direct Withstanding Voltage:
The direct withstanding voltage is a minimum of 1000 Volts AC.
RS232: Mechanical Characteristics
The pin configuration of RS-232 is available in 9 pins and 25 pins, with each pin having its function. However, most devices using RS-232 communication don’t use all the pins, so the cabling requirements for a device may vary. Initially, a 25-pin configuration was used for RS-232 serial communication. The 25-pin connector specified the minimum connector size that can house all the signals.
In electrical communication, a connector is always available in pairs. One is a male connector, while the other is called a female connector. In RS-232, DTE has a male connector, while DCE has a female connector. A combination of SUB D 25-pin Male and Sub D 25-pin Female connector is used for connecting a DCE and DTE by specifying common signal and voltage level, a minimum control signal, and a common pin wire configuration.
Nowadays, most commonly used serial communication devices, such as modems, use a 9-pin connection. A 9-pin connector is sufficient in most applications as several pins in the RS-232 25-pin connector are not used. The 9-pin connector can provide all the connectivity and transfer for almost all the applications, allowing them to send and receive signals according to the requirements.
Pin Description for DB-9 Connector
What is Handshaking?
Handshaking helps the transmitter to transmit and receiver to receive data successfully. Handshaking is the process of transferring the signal from Data Transmission Equipment (DTE) to Data Communication Equipment (DCE) for making the connections before the actual data transfer. The communication between the transmitter and the receiver is completed by handshaking. There are three types of handshaking processes in RS-232 communication:
1. No Handshaking
2. Hardware handshaking
3. Software Handshaking
During no handshaking, the DCE reads the received data while DTE is transmitting the next data. The received data is stored in the memory location called the receiver’s buffer. This buffer can only store one bit, and the receiver needs to read the buffer memory before the next bit could arrive. If the receiver doesn’t read the stored bit before the next bit arrives, the stored bit will be lost.
It uses specific serial ports to control the flow of data like CTS and RTS.
In this handshaking process, the transmitter asks the receiver if it is ready for receiving the data, and the receiver checks if the memory buffer is empty. If it is empty, the receiver will signal to the transmitter to transmit the data.
Software handshaking is done in two forms: X-ON and X-OFF, where X means transmitter. In X-ON, the transmitter resumes the data transmission, while in X-OFF, it pauses data transmission. These two forms control the flow of data and prevent data loss during transmission.
RS-232 is the most popular and common mode of communication. It is still used in old computers to connect peripheral devices like modem, printers, mouse, etc. However, it is being replaced with an advanced USB now. RS-232 is widely used in industrial applications like CNC machines, PLC machines, and servo controllers.