How to connect RS485 devices?

1, Choose the appropriate cable
It is recommended to use shielded twisted pair cables for RS485 communication, which can reduce external electromagnetic interference and ensure the stability of signal transmission. Common cable types include RVVP, which can be selected in 3-core 0.3 or 3-core 0.5 specifications. The two internal wires in twisted pair cables (usually one red and one black) correspond to the 485+and 485- signal wires, respectively.
2, Power and ground wiring
Power wiring: It is crucial to provide a stable power supply for the RS485 communication system. DC power supply is usually used, with a voltage range of 9-36V. To ensure the stability and reliability of the system, it is recommended to use a linear regulated power supply or a switching regulated power supply.
Grounding: Grounding is an important measure to protect personal safety and also plays a role in shielding external electromagnetic interference in 485 communication systems. The ground wire should be connected to the metal casing of the equipment to prevent damage caused by electrostatic discharge or other reasons.
3, A and B terminal wiring
The two main interfaces of RS485 communication system are A end and B end, which are used for sending and receiving data respectively. When wiring, the following points should be noted:
Differential signal: The signals at terminals A and B are both differential signals, meaning that the voltage difference between the positive and negative signal lines represents the signed value of the data. When connecting, it is necessary to connect the positive pole of the data line to the logic 1 (or high level) pin and the negative pole to the logic 0 (or low level) pin.
Hand in hand connection: All devices should be connected in series through twisted pair cables to form a straight line (bus topology). Avoid creating "star" connections, all devices should be directly connected to the main communication line without branching.
Terminal resistor: Find the 485+and 485- endpoints on the two devices of the bus, and connect a 120 ohm resistor between these two points. This can prevent the signal from bouncing back at the end of the line, similar to the echo of sound in a valley. If the communication distance is short or there are few slave devices, you can choose not to connect the terminal resistor. However, as the communication distance increases or the number of slave devices increases, terminal resistors should be added to eliminate signal reflection.
4, Shielding layer treatment
The shielding layer of the cable is grounded at one end of the system, usually connected to the ground terminal of the main controller or power supply. This can avoid forming a loop and introducing unnecessary interference. Ensuring that the shielding layer of the cable is properly grounded is an important measure to protect signals from external electromagnetic fields.
5, Common grounding
To ensure signal stability, all devices should be connected to the same grounding point. If the cable has a third wire (usually a bare wire or colored marker), this is the ground wire, ensure that all devices have this ground wire connected to the same electrical location.
6, Wiring precautions
Do not pull the cable too tightly: to avoid damaging the internal wires.
Avoid poor contact: Ensure that all connections are secure.
Avoid parallel wiring: Do not wire cables and strong wires (such as power cords) side by side to avoid interference.
Avoid electromagnetic interference: Try to avoid routing cables in areas with high electromagnetic field strength, such as near large motors.
Wiring should be in a straight line as much as possible: The wiring of the bus should be in a straight line as much as possible to avoid unnecessary bends and loops.
7, Common problem troubleshooting
Check if the A and B wires are connected correctly: Ensure that all equipment's A and B wires are connected correctly and not reversed.
Check the bus structure: Confirm whether the bus is wired according to the bus type structure and does not form a ring or star structure.
Check the pull-up and pull-down resistors: The RS485-A and RS485-B interfaces of the slave device should not contain pull-up and pull-down resistors, otherwise it will reduce the number of accessible slave devices.
Check terminal resistance: Ensure that only terminal resistors are connected to the terminals, and intermediate devices should not be connected to terminal resistors.
Check the number of devices: When using the Modbus protocol, the theoretical maximum number of slave devices is 31, ensuring that this number is not exceeded.
Check cable type: When transmitting over long distances, twisted pair cables must be used.