What Is The Difference Between RS485 And RS232?

The Difference Between Rs485 And Rs-232

1、Different transmission methods

RS-232 uses unbalanced transmission, so-called single-ended communication. RS-232 takes an unbalanced transmission method, which is called single-ended communication, while RS485 uses balanced transmission, i.e. differential transmission. The data signal at the receiving and transmitting ends is relative to the signal ground, e.g. the data from the DTE device is at the level of 2 pins relative to 7 pins (signal ground) when using the DB25 connector. 

2、Different transmission distance

RS-232 is suitable for communication between local devices and the transmission distance is generally no more than 20m, while RS-485 has a transmission distance of several tens to thousands of metres.

3、Different transmission units

RS-232 only allows one-to-one communication, while the RS-485 interface on the bus is allowed to connect up to 128 transceivers. A typical RS-232 signal oscillates between positive and negative levels. When sending data, the driver on the transmitter side outputs positive levels at +5 to +15V and negative levels at -5 to -15V levels.

RS-232-C is a serial physical interface standard developed by the Electronic Industry Association (EIA), RS is the abbreviation for "Recommended Standard", 232 is the identification number and C indicates the number of modifications. RS-232-C The RS-232-C bus standard has 25 signal lines, including a main channel and an auxiliary channel, in most cases mainly using the main channel, for general duplex communication, only a few signal lines are needed to achieve, such as a transmit line, a receive line and a ground line, The RS-232-C standard specifies that the driver is allowed to have a capacitive load of 2500pF and the communication distance will be limited by this capacitance, for example, when using a 150pF/m communication cable, the maximum communication distance is 15m; if the electrical capacity per metre of cable is reduced, the communication distance can be increased. Another reason for the short transmission distance is that RS-232 is a single-ended signal transmission, and there are problems such as common ground noise and inability to suppress common mode interference, so it is generally used for communication within 20m.

RS-485 is widely used when communication distances of several tens to thousands of metres are required, and RS-485 uses balanced transmission and differential reception, so it has the ability to suppress common mode interference. In addition to the high sensitivity of the bus transceiver, which can detect voltages down to 200mV, the transmitted signal can be recovered over kilometres. RS-485 works in half-duplex mode and only one point can be in the transmitting state at any one time, therefore the transmitting circuit must be controlled by an enabling signal. RS-485 can be networked to form a distributed system, allowing up to 32 drivers and 32 receivers to be connected in parallel.

In the past, PCs communicated with intelligent devices by means of RS232, RS485 and Ethernet, depending mainly on the interface specification of the device. However, RS232 and RS485 can only represent the physical media layer and link layer of communication. If you want to realise bidirectional access to data, you have to write your own communication applications, but most of such programs cannot comply with the ISO/OSI specifications and can only achieve a relatively single function for a single device type, and the programs are not universal. In a network of RS232 or RS485 devices, if the number of devices exceeds 2, RS485 must be used as the communication medium. Only one master device is allowed to exist in the network, and all the rest are from the (Slave) devices. The fieldbus technology is based on the ISO/OSI model and has a complete software support system that can solve the problems of bus control, conflict detection, link maintenance, etc...

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