RS485 Bus Wiring Process Errors Summary

- Mar 01, 2018-

Summary for Bus RS485 wiring process errors 

Bus RS485,because of its simple wiring,stable and reliable,then it is widely used in video surveillance,access intercom, building alarm and other field.While RS485 bus wiring process, due to a number of incomplete and accurate concept ,lead to many problems.Now following make a summary of the wrong ideas.

1.The 485 signal cable can be routed together with the strong power cord.In the actual construction,because the alignment

is to go through the pipeline,sometimes the construction side for convenience,directly tie the 485 signal cable and power cable

together,because of strong power have strong electromagnetic signal interference on weak power,then resulting in 485 signal

instability,leading to instability communications.

2.RS485 signal cable can use parallel cable as the wiring,can also use non-shielded cable as the wiring.Since 485 

signal use differential mode transmission,that is,the voltage difference of 485 + and 485-  as a signal transmission.If an external interference source to interfere with it,use twisted cable to transmit 485 signal,because of its twisted pair,the interference on the 485 +, 485 - interference effect is the same,then the voltage difference still is the same , the interference to 485 signal shrinks to the minimum.The same,if there is a shielding effect of shielded cable,the influence of interference from external sources can be reduced as much as possible.

3.Choose to use the ordinary super five shielded pair twisted pair cable.Due to raw material price rising,leading to the mixed

 wirewire market,there are some unscrupulous businessmen to use some kind of alloy to replace the copper wire to make network cable,copper plating outside to mingle customers.Specific differences: look at the cable cross-section,if it is copper,then is a copper wire,such as white,it is shoddy with alloy.Alloy is generally more brittle,easily broken,and  bad conductive than copper wire,it is easy to cause problems in the construction.Generally recommend to buy the standard 485 cable, which is shielded twisted pair,the transmission cable is not like a single strand of copper wire,but stranded copper wire twisted together to form a line,so that even a small copper wire broken,it will not affect the entire use.

4.485 wiring can be arbitrarily arranged into a star-shaped wiring and tree wiring.485 wiring specification is that you must hand in hand wiring,once without 485 hub and 485 repeaters directly arranged into a star connection and tree connection,it is easy to cause signal reflection and lead to bus instability.Many construction parties in the 485 wiring process,use star wiring and tree wiring,and sometimes the entire system is very stable,but sometimes there are always problems,it is difficult to find the reasons,usually caused by irregular wiring.If restriction of site,you must do star connection or tree connection,can use 485 hub and 485 repeaters to solve the problem.

5.485 bus must be grounded.In many technical documents,it is mentioned that the 485 bus must be grounded,but no detail

information about how to ground it.Strictly speaking,485 bus must be a single reliable grounding.A single point is the entire 485 bus can only be a point grounding,can not be two or more points grounding.Since it is grounded to keep the ground (usually shielded wire for the ground) voltage in the same,to prevent common mode interference.If more points ground is counterproductive.Reliable grounding the entire 485 line ground must have good contact,so as to ensure that the voltage is consistent.Because in the actual construction,in order to facilitate the wiring,cut the wire into multi-segments and reconnect,but do not make a good connection of the shield wire,making the ground divided into multi-segments,the voltage can not be consistent,leading to common mode interference.

Now Shenzhen ECON Technology RS485 ModBus stepper drive is on selling,and this article help users to solve problems that caused by wiring.

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