Wiring specification and debugging method for 485 bus structure of access control system

Concept one: The communication distance of 485 bus can reach 1200 meters.
Actually, the 485 bus structure theoretically can only achieve a transmission distance of 1200 meters under the premise of an ideal environment. It generally refers to the high quality standards of communication wires, and the baud rate is 9600. Only one 485 device can make the communication distance reach 1200 meters, and Communication does not mean that each communication is normal. Therefore, the actual stable communication distance of the 485 bus is far less than 1200 meters. There are many loads of 485 equipment, the wire resistance is not up to standard, the wire diameter is too small, the quality of the converter is bad, the equipment Lightning protection, baud rate increase and other factors will reduce the communication distance.

Concept Two: The 485 bus can carry 128 devices for communication.
In fact, not all 485 converters can bring 128 devices. According to the model used in the 485 converter chip and the model used by the 485 device chip to determine. Who is low on who. General 485 chip load capacity has three levels 32 sets of 128 sets of 256 sets. The theoretical nominal value is often inaccessible. The longer the communication distance, the higher the baud rate, the finer the wire diameter, the worse the wire quality, and the poorer the converter quality. Inadequate power supply (passive converter), the stronger the lightning protection, these will greatly reduce the actual load.
Concept 3: The 485 bus is the simplest, most stable and mature industrial bus structure.
This concept is wrong. It should be: The 485 bus is an economical, traditional industrial bus method for networking devices. The communication quality is tested and debugged according to construction experience. The 485 bus is simple but must be strict Installation and construction specifications for wiring.
The second chapter strictly enforces several construction specifications:

485+ and 485- data lines must be mutually twisted. The wiring must be multi-stranded shielded twisted pairs. The multiple strands are for standby. The shielding is for debugging in special cases. The twisted pair is because the 485 communication adopts differential mode. Communication principle, the best anti-interference twisted pair. Not using twisted pair, is extremely wrong.

485 bus must be hand-in-hand bus structure, and resolutely eliminate the star connection and bifurcated connection.

The equipment-powered AC and chassis must be grounded and have good grounding. There are many places with triangular sockets on the surface. In fact, there is no grounding at all. Be careful. When the grounding is good, you can ensure that the equipment is shocked by lightning. The lightning protection design releases energy well. Protecting the 485 bus device and related chips from damage.

Avoid going with strong electricity to avoid interference by strong electricity