Working principle of RS232 and RS485 communication

Last RS232

The RS232 standard is a full-duplex communication standard used for receiving and controlling data. It serves as both the hardware and control system, managing all three interface connections for the wiring.

• Tx (data transmission) is a straight line in the deep part only
• Rx (receive data) is a data receiving line, which is a straight line for receiving data only
• GND (Signal ground) is a ground wire, which is a reference or comparison wire again 0V

The picture is an example of RS232 connection of industrial measuring instruments to a computer to set the measuring instruments through software.

• Tx (meter) will be connected to Rx (computer) to send data from the meter to the computer receiver.
• Rx (meter) will be connected to Tx (computer) to receive data sent from the computer.
• GND (meter) will be connected to GND (computer) to compare the 0V voltage signal.
  
  

Example of RS232 usage

Connection between Xinje PLC and HMI Panel Master via RS232 communication using DB9 to 8 Pin cable.

The last principle of RS485

The RS485 standard is a standard that can receive/control in Half duplex format, that allows for efficient data transmission, handling one operation at a time. Its capabilities make it comparable to radio communication that broadcast live one at a time.

For RS485 digital data transmission/receiving, data is transmitted using only 2 wires, A and B, which are digital code indicators, using the voltage difference between terminals A and B as an indicator as follows:

• When Va - Vb gets a voltage less than -200 mV, the digital signal is 1.
• When Va - Vb gets a voltage greater than +200 mV, the digital signal is 0.

The standard RS485 can connect network data transmission with up to 32 devices in the network. In that network, there must be 1 device that is responsible for queuing communication in the network. We call this device the "Master" and the rest of the devices are called "Slaves". Each Slave has its own Address number. When the Master wants to command the Slave, the Master will send a set of commands with the Address number to each Slave device. When the Slave device receives the command and the command has the same Address number as itself, the Slave device will follow the Master's commands in sequence.

Maximum number of devices in RS485 network

This is another question that most new users wonder about in the world. According to the standard, the RS485 network can have up to 32 devices in the system when those devices have an internal electrical resistance of 12 k. But now the design of electronic devices has been developed to have a very high internal electrical resistance (in the M digit), which allows the RS485 network to have up to 256 devices in the system. In addition, the RS485 network can use a repeater to add thousands of devices in the network and cover a distance of several kilometers.

Example of RS485 use

Connection between PLC Haiwell and Power meter using RS485 communication, where PLC Haiwell acts as Master and Power meter acts as Slave by setting Power meter 1 to Address "1" and Power meter 2 to Address "2".