rs485 bus wiring method and specifications

Infineon / Mitsubishi / Fuji / Semikron / Eupec / IXYS

rs485 bus wiring method and specifications

Posted Date: 2024-02-04

rs485 bus wiring method

There are usually two common wiring methods for RS-485 bus: point-to-point connection and multi-point connection.

1. Point-to-point connection:

- In a point-to-point connection, only two devices are directly connected on an RS-485 bus.

- The connection between two devices requires the use of two data lines: Line A (positive line) and Line B (negative line).

- Line A and Line B are a pair of balanced transmission lines for bidirectional differential signal transmission.

- In a point-to-point connection, each device is connected directly to the bus and terminating resistors are required to prevent signal reflections.

2. Multipoint connection:

- In multipoint connection, multiple devices can be connected on the same RS-485 bus to form a bus topology.

- In a multipoint connection, line A and line B are also required for data transmission.

- All devices must have the characteristics of slave devices, and a master device is required to control data transmission on the bus during communication.

- Each device needs to have a unique address, and the master device selects the device to communicate with by sending a specific address.

- In multipoint connections, taps or terminal resistors are generally used to provide connections to branch networks.

RS-485 uses balanced transmission and differential reception to achieve communication: the transmitting end converts the TTL level signal of the serial port into differential signals a and b and outputs them in two ways. After cable transmission, the receiving end restores the differential signal to TTL level. flat signal. The RS-485 bus network topology generally adopts a terminal-matched bus structure. That is, a bus is used to connect various nodes in series, and ring or star networks are not supported. As shown below, Figure 1.

The wiring method in actual application should be as shown in Figure 2. Among them, the slaves RS485-A and RS485-B should not have pull-up and drop-down resistors. If there are pull-up and drop-down resistors, the number of slave machines that can be connected will be reduced.

The following are some common RS-485 bus wiring specifications, which can improve the reliability and anti-interference ability of communication:

1. Cable selection:

- Use shielded twisted pair (STP) or unshielded twisted pair (UTP) as the transmission medium of the RS-485 bus.

- Choose the cable specification that meets the requirements, such as Cat5e or Cat6, etc.

2. Cable length restrictions:

- Control the maximum length of bus cables to maintain signal integrity.

- Determine the appropriate cable length limit based on specific application requirements and bus drive capabilities, usually between tens to hundreds of meters.

3. Cable connection:

- Use correct connection methods, such as using the correct plugs and sockets.

- Ensure good connection to avoid problems such as looseness and poor contact.

4. Terminal resistor:

- Set appropriate terminal resistors at both ends of the bus, generally 120 ohms.

- Terminal resistors are used to suppress signal reflections and improve signal quality.

5. Grounding:

- Keep the ground wires of all equipment well connected and ensure a common ground.

- Avoid interference from ground loops.

6. Anti-interference measures:

- Use shielded cables or take other anti-interference measures to reduce external electromagnetic interference.

- Isolate sources of interference from other electrical equipment.

7. Driving ability:

- Ensure that the output capability of the bus driver is strong enough to accommodate the required transmission distance and number of devices.

8. Address allocation:

- In the case of multipoint connections, ensure each device has a unique address and assign addresses as needed.

Review Editor: Huang Fei

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