When 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.
|Max number of drivers
Max number of receivers
|Modes of operation||half duplex
|half duplex||half duplex||half duplex|
|Max distance (acc. standard)||15 m||1200 m||1200 m||1200 m|
|Max speed at 12 m
Max speed at 1200 m
|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.