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.
RS232 Brief History
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.
Defining RS232
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: Electrical Specifications
Electrical specifications of the RS-232 are as follows:
Voltage Levels:
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.
Line Impedance:
The impedance of the cable is up to 3 to 7 ohms, and the maximum length of the cable is 15 meters.
Operating Voltage:
The maximum operating voltage of RS-232 is 250 Volts AC.
Current Rating:
The maximum current rating is 3 Amperes.
Slew Rate:
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 connectors connects a DCE and DTE by specifying the 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.
RS232 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. 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
No Handshaking
During no handshaking, the DCE reads the received data while DTE is transmitting the next data.
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.