A short note on RS485

• RS485 is a Differential interface
• Balanced transmission is used for RS485 communication
• RS485 is a Multi-point protocol. There is Single Master and multiple slaves
• RS485 supports Full Duplex as well as Half duplex option
• Data Rates up to 10Mb/s can be achieved
• Speed of RS485 is superior to RS422
• RS485 can be used up to distances of 1200m
• The maximum distance is generally decided by the capacitance over the line
• The maximum of devices supported are 32, can be more and it depends on transceiver characteristics. Study RS485 datasheets for more information
• Termination is used on both ends of the line
• There are biasing resistors as well as termination resistors required on the line
• By default, the RS485 transceiver will be set in Receiver mode. When the system wants to transmit data, Data enable (DE) will be pulled high
• Data Enable is Active HIGH and Receiver Enable is Active LOW
• Star and Ring topology cannot be used as the coenctivity becomes complex and due to impedance mismatch this may cause reflections
• RS485 transceivers are used between micro controller and physical side for the RS485 transmission, UART is used on the micro controller side for connecting to RS485 transceiver
• RS485 Transceivers in the market: 

1. SN65HVD485E from Texas Instruments, 
2. LTC2856-2 from Linear Technology, 
3. ISL32704E  from Intersil

• There are several other vendors as well
• While selecting the RS485 transceivers, designer consider the following options,

1. Operated supply
2. Over voltage protection 
3. ESD protection
4. Galvanic isolation
5. Fault tolerance options like fault voltage
6. Data Rate consideration
7. Hot plugging feature
8. Number of transceiver options within a chip
9. Cost criteria