User Manual

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5. Appendix
5.1 RS485 bus introduction：
RS485 bus, RS is the abbreviation of English "recommended standard", 485 is the identification number.
The RS485 serial bus is widely used in applications where the communication distance ranges between
dozens of meters to 1km more. RS485 uses balanced transmitting and split receiving, so it has the ability
to reject common mode interference. In addition to the high sensitivity of the bus transceiver, it can detect
voltages as low as 200mV, so the transmitted signal can be recovered beyond the kilometers away. As
RS485 adopts half-duplex working mode, and only one point is allowed to be under sending status any
time, the transmitting circuit must be controlled by the enable signal. RS485 is very convenient for
multi-point interconnection, which help save many signal lines. RS485 can be used to make a distributed
system, which allows up to 128 drivers and 128 receivers to be connected in parallel, depending on the
chip used by the driver and receiver, and the bus drive capability is limited by the weakest one. However,
in practical applications we can extend it with the RS485 distributor
5.2 Transmission distance：
When a 0.56mm (24AWG) twisted pair cable is used as the communication cable,the theoretical value pf
the maximum transmission distance varies with different baud rate: 1800 meters can be transmitted
when the baud rate is 2400 bps, and 600 meters under 19200 bps. When using a thinner communication
cable, or using the product in an environment with strong electromagnetic interference, or when too many
devices are connected with the bus, the maximum transmission distance will be shortened accordingly,
or the maximum distance is longer.
5.3 Connection method and terminating resistor
The RS485 industrial bus standard requires daisy chain connection between devices. The two ends
must be connected with a 120Ω termination resistor. The two balance distances must be within 7m.