Difference Between RS232 and RS485 Communication Protocol - Comparative study

RS232 and RS485 Comparative study

Serial interfaces can be used to provide standardized logic levels from transmitters to receivers, define the transmission medium and connectors, and specify timing and data rates. In some cases, they can perform serial-to-parallel and parallel-to-serial conversion or specify a basic data protocol.

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The definition of logic levels, medium, and connectors is part of the physical layer (PHY) or layer 1 of the Open Systems Interconnection (OSI) networking model. Any additional functions such as data handling is part of the media access control (MAC) layer or layer 2 of the OSI model.




RS-232 -One of the oldest serial interfaces is generically called RS-232. It was originally established in 1962 as a method of connecting data terminal equipment (DTE) such as electromechanical teletypewriters to data communications equipment (DCE). Over the years its use has included connections to video terminals, computers, and modems.
The first personal computers included an RS-232 called a serial port for connection to a printer or other peripheral device. Today, it is still widely used in embedded computer development systems, scientific instruments, and all sorts of industrial control equipment.

RS-485 - Also defined by the EIA/TIA standard, this interface is now called TIA-485. It defines not only a single device-to-device interface but also a communications bus that can be used to form simple networks of multiple devices. Its configuration and specifications also extend the range and data rate beyond the RS-232 interface capabilities.

The RS-485 standard specifies differential signaling on two lines rather than single-ended with a voltage referenced to ground. A logic 1 is a level greater than –200 mV, and a logic 0 is a level greater than +200 mV. Typical line voltage levels from the line drivers are a minimum of ±1.5 V to a maximum of about ±6 V. Receiver input sensitivity is ±200 mV. Noise in the range of ±200 mV is essentially blocked. The differential format produces effective common-mode noise cancellation.

The standard transmission medium is twisted-pair cable of either #22 or #24 AWG solid wire. Two lines are minimum but a third reference wire can be used. Four-wire cables can also be used if full-duplex operation is desired. The cables may be shielded or unshielded, with unshielded the most common. The cable is treated as a transmission line. The nominal characteristic impedance is 100 or 120 Ω. Terminating load resistors are required to ensure a matched line condition, which prevents reflections that introduce data errors.

The standard does not define specific connectors. Various connection methods have been used, including the RS-232 DE-9 connector. Simple screw terminal connections are common in some types of industrial control equipment.

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