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RS485 interface

RS485 interface Figure describes RS485 pin diagram for 9 pin connector. Tx+ and Tx- carry transmit data and Rx+ and Rx- carry receive data. Due to differential signals distance carried by signals are more.

The RS485 interface supports higher data rate and distance compare to RS232. It supports distance of 10 meters for 30-35 Mbps and 1200 meters for the data rate of about 100 kbps. It is a balanced differential interface used mainly for multi drop configuration. Figure-1 depicts pin configurations of RS485 interface in a 9 pin DB9 connector.

As shown in the pin diagram, RS485 has all the signals in differential configurations.
RxD+ and RxD- used for receiving data
TxD+ and TxD- used for transmitting data
CTS+ and CTS- as well as RTS+ and RTS- are used as handshake control signals

In a multi drop configuration about 32 devices can be connected with 1 master controlling device. One of the application is VSAT NMS (Network Management System) where in software application running on one PC is used to monitor & control many sub systems. These subsystems are Modems, MUXs, RF up converters, RF down converters and more. All the subsystems and PC housing NMS software should have RS485 connectors with cable wiring properly done to make this multi drop configuration work properly.

RS485 Technical Specifications

Following table-1 describes RS485 technical specifications.


Specifications RS485
Mode of operation Differential
number of drivers/receivers on one line 1 driver, 32 receiver
maximum cable length 4000 feet
maximum data rate 30 Mbps
max. driver output voltage -7V to +12V
Max. driver current in high impedance state (power off) +/-100 µA
receiver input voltage range -7V to +12V
Receiver input sensitivity +/-200mV
Driver load impedance 54 Ohms
Receiver Input Resistance >=12 K Ohm

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