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Working principles of RS485

USB DriverWhen we talk about the RS485 we first need to know that is stands for the “Recommended Standard no. 485” and that it is the serial communication standard which was established in 1998. The standard is used in many industrial equipment as the signal can be sent far and can still be sent simultaneously at multiple points. This uses Half duplex communication meaning that it can only send information in one direction at a time.

Working principles of RS485

The principle works by voltage changes as can be seen below:

  • When V a – V b is less than -200 mV,  the digital signal is 1
  • When V a  – V b  has a voltage greater than +200 mV  is digital signal 0

In order to work many of these devices together you have to have a Master and Slave connection. The Master is connected to the Slaves and will call on each Slave device by its number. You can then connect this to your computer. The computer will use the RS232 chipset and convert this to the RS485 chipset with an RS232 to RS485 converter in order for the computer to understand the Slave devices. You can have 256 devices or using a Rpeater you can connect thousands of devices.

Characteristics of RS232, RS422, RS423 and RS485
RS232 RS423 RS422 RS485
Differential no no yes yes
Max number of drivers
Max number of receivers
Modes of operation half duplex
full duplex
half duplex half duplex half duplex
Network topology point-to-point multidrop multidrop multipoint
Max distance (acc. standard) 15 m 1200 m 1200 m 1200 m
Max speed at 12 m
Max speed at 1200 m
20 kbs
(1 kbs)
100 kbs
1 kbs
10 Mbs
100 kbs
35 Mbs
100 kbs
Max slew rate 30 V/μs adjustable n/a n/a
Receiver input resistance 3..7 kΩ ≧ 4 kΩ ≧ 4 kΩ ≧ 12 kΩ
Driver load impedance 3..7 kΩ ≧ 450 Ω 100 Ω 54 Ω
Receiver input sensitivity ±3 V ±200 mV ±200 mV ±200 mV
Receiver input range ±15 V ±12 V ±10 V –7..12 V
Max driver output voltage ±25 V ±6 V ±6 V –7..12 V
Min driver output voltage (with load) ±5 V ±3.6 V ±2.0 V ±1.5 V

Why mix them is that the RS485 can send signals up to 1,200 meters while the RS232, that can send signals only 15 meters. See also the article on the RS232 as well as adapters for these standards. 

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About Tracey Brown

I develop websites and content for websites related to high tech from around the world. See more pages and content about technology such as USB and other IT developments around the world. You can follow the other websites as well and search this website for more information on mobile phones and other USB components.