485 bus single-point to multipoint problem

485 bus single point to a single point of use there is no problem, can point to multipoint after the problem is very much, 485 of the host is 120 ohms, and then there are 5 from the device, some 120 ohms from the device has no, and then it is not good to make, all of the 120 even resistors are removed, in addition to the main equipment not removed, Then in a line to connect a 2k resistor to the vcc,b wire connected to the 2K resistor to GND, and so on, a separate use from the device when there is not good, I was dizzy, in short, experienced people said is the driving problem.

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The following are some of the specifications or experiences you should follow when conducting 485 cabling:

① strictly follow the principle of bus cabling. The bus (double line) goes from head to tail. If the two places are very far apart, in the wiring can be used "around" the way, never allow a fork (star).
The connection distance of each device to the junction box (i.e. node-to-bus) should not exceed 50 cm, in principle as short as possible.

② Theoretically, the length of the bus <=1.2 km, the number of nodes on the bus <=32. But in the actual situation cannot reach, especially the distance, mainly is affected by the wiring quality, the node distribution and the surrounding environment and so on factors.

③ is available with shielded or unshielded lines. In general, shielding lines have a better effect.
Can adopt super five twisted pair, the advantage is strong anti-jamming ability, long transmission distance, the disadvantage is easy to break.
Or the use of 0.5mm-1mm coarse copper wire effect is also more ideal, the advantage is not easy to break, but less than the transmission distance twisted pair.

④ online and line joints, as far as possible to maintain the smoothness and resistance continuity.

⑤, such as computers or servers, can be distributed to one end of the 485 bus as much as possible, rather than in an intermediate position.

The closer the ⑥ node device is to the computer, the worse the signal decays. Therefore, it is possible to distribute more node devices at one end of the bus.

⑦ when the bus distance is very long (generally more than 300 meters), can be considered at the bus ends and a 100-120 ohm resistor, which can improve communication quality.
When the bus distance is very short (generally within 300 meters), do not need to be in the bus at both ends and resistance, and sometimes the resistance is not effective.
Note that some 485 network adapters or converters already have a resistor inside, so there is no need to connect the resistor at one end of the computer.
This must be grasped by practical experience.

⑧ there should be no large electromagnetic interference source in the area where the cable is passing. If using shielded wire, then the screen of the network cable at both ends to connect the earth, to ensure the reliability and stability of data transmission.

The following is the wiring diagram provided by another

485 bus single-point to multipoint problem