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RS-485 interface for connecting multiple devices

2017-12-22 12:38  
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RS-485 is updated version of RS-422 interface. It is designed to communicate between multiple devices connected to single data line. Standard allow connecting 32 transceivers to data line. This is achieved by using tri-stable drivers that are separate programmable. This allows only one device work at a given time.


Resistors used in circuit are so called termination resistors for matching impedances of transmitter and receiver to avoid reflections. Matching resistor depends on cable used, but is 120Ohm.

Usually RS485 chips have two control inputs Data Enable(DE) and Receiver Enable(RE):


On the IC you see that DE active is high level and RE is low level. This allows connecting control line to single MCU pin. This assures that node won’t be receiving while it is transmitting:


In many cases CAT-5 cable is used with impedance of 100Ohm and terminations of 120Ohm resistors at both ends of transmitter and receiver. CAT-5 cable has already twisted pairs and it is cheap enough to use for many applications.

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