1. What Is RS485 Bus
The RS485 bus is commonly used in industrial settings to collect multi-point data, whether analog signals or switch signals. RS-485 operates in a half-duplex mode and supports multipoint data communication. The RS-485 bus network topology generally adopts a terminal-matched bus structure, which connects various nodes in series along a single bus and does not support ring or star networks.
RS485 does not have a specific physical shape and is selected based on the actual engineering circumstances. RS485 uses differential signaling with negative logic, where +2V to +6V represents “0” and -6V to -2V represents “1”.
There are two wiring methods for RS485: two-wire and four-wire. The four-wire method can only achieve point-to-point communication and is rarely used now; the two-wire method is more commonly adopted. This wiring method allows a maximum of 32 nodes to be connected on the same bus.
The communication distance of the RS485 bus can reach up to 1200 meters.
According to the theoretical structure of the RS485 bus, under ideal conditions, the transmission distance can reach 1200 meters. The conditions for this are that the communication cable is of high quality, the baud rate is 9600, and only one RS485 device is loaded. Therefore, the actual stable communication distance of the RS485 bus often does not reach 1200 meters. If multiple RS485 devices are loaded, if the cable impedance does not meet standards, if the wire gauge is too thin, if the quality of the converter is poor, or if the lightning protection is complex, and if the baud rate increases, these factors will all reduce the communication distance.
2. RS485 Cable
In general situations, ordinary twisted pair cables can be used. In environments with higher requirements, shielded coaxial cables can be used. When using RS485 interfaces, the maximum cable length allowed for data signal transmission from the RS485 interface to the load is inversely proportional to the baud rate of the signal transmission. This length is mainly affected by signal distortion and noise.
Theoretically, the longest transmission distance for RS485 can reach 1200 meters, but in practical applications, the transmission distance is usually shorter, depending on the surrounding environment. During transmission, relay methods can be used to amplify the signal, allowing for a maximum of eight relays. This means that theoretically, the maximum transmission distance of RS485 can reach 9.6 kilometers. If long-distance transmission is really needed, optical fiber can be used as the transmission medium, with a photoelectric converter added at both ends. The transmission distance for multimode fiber is 5-10 kilometers, while single-mode fiber can reach up to 50 kilometers.
3. RS485 Wiring Installation Precautions
1. What type of communication cable should be used for the RS485 bus?How many devices can be connected on one bus?
RVSP shielded twisted pair cables must be used. The specifications of the shielded twisted pair cable used depend on the distance of the RS485 communication line and the number of devices connected, as shown in the table below. Using shielded twisted pair cables helps reduce and eliminate the distributed capacitance between the two RS485 communication lines and the common-mode interference from the surrounding communication lines.
Some say that the RS485 bus can communicate with 128 devices.
However, not all RS485 converters can support 128 devices; it depends on the model of the chip inside the RS485 converter and the chips of the RS485 devices. The load capacity can only be determined by the lower-rated chip. Generally, RS485 chip load capacities have three levels—32 devices, 128 devices, and 256 devices. Additionally, the theoretical ratings are often not achievable in practice; longer communication distances, higher baud rates, thinner wires, poorer cable quality, lower quality converters, and complex lightning protection can all reduce the actual load capacity.
Most engineers habitually use Cat 5 or Cat 5e cables as RS485 communication lines, which is incorrect because:
(1) Ordinary network cables do not have shielding and cannot prevent common-mode interference.
(2) Wires that are too thin cannot be used, as they will reduce transmission distance and the number of devices that can be connected, with at least a 0.4mm² gauge or standard network cable being necessary.
(3) Network cables are made of solid copper wire, which is more prone to breakage compared to multi-core wires.
2. Why is grounding necessary?
RS485 transceivers can only operate normally when the specified common-mode voltage is between -7V and +12V. If it exceeds this range, it may affect communication and, in severe cases, damage the communication interface. Common-mode interference can increase the common-mode voltage mentioned above. One effective way to eliminate common-mode interference is to use the shield of the RS485 communication line as a ground line, connecting devices like computers and machinery in the network together and grounding them reliably.
3. How should RS485 communication lines be routed?
Communication lines should be kept as far away as possible from high-voltage power lines, fluorescent lights, and other sources of interference. If the communication lines cannot avoid being near power lines, they should be perpendicular to the power lines, never parallel, and should not be bundled together, using high-quality twisted pair cables for routing.
4. Why should the RS485 bus use a daisy-chain structure instead of a star structure?
A star structure can produce reflected signals, which can affect RS485 communication. The branch line from the bus to each terminal device should be as short as possible, generally not exceeding 5 meters. If the branch line does not connect to a terminal, it will have reflected signals that strongly interfere with communication, so it should be removed. It is best to have 120Ω termination resistors connected at both ends of the RS485 devices.
Daisy-chained connection as shown:
Figure 1
Star connection as shown:
Figure 2
5. Can there be junctions between devices on the RS485 bus?
In the same network system, using the same type of cable, it is best to minimize junctions in the line. Ensure that connections are well soldered, tightly wrapped, and avoid looseness and oxidation. Ensure a single, continuous signal path as the bus.
6. What are common-mode and differential-mode interference? How to eliminate interference on communication lines?
The RS485 communication line consists of two twisted wires, which transmit signals through the voltage difference between the two communication lines, hence called differential voltage transmission. Differential-mode interference occurs between the two signal lines and is symmetric interference. To eliminate differential-mode interference, a bias resistor (matching resistor in the camera) can be added to the circuit, and twisted pair cables should be used; common-mode interference occurs between the signal line and ground and is asymmetric interference. Methods to eliminate common-mode interference include:
(1) Using shielded twisted pair cables and ensuring proper grounding.
(2) In areas with strong electric fields, consider using galvanized pipes for shielding.
(3) During wiring, keep away from high-voltage lines, and do not bundle high-voltage power lines and signal lines together.
(4) Use linear regulated power supplies or high-quality switching power supplies (with ripple interference less than 50mV).
7. Under what circumstances should termination resistors be added on the RS485 bus?
Generally, termination resistors are not needed unless the RS485 communication distance exceeds 300 meters. In this case, termination resistors should be added at the start and end of the RS485 communication. Especially when the number of devices on the RS485 bus is low. When there are many devices (e.g., more than 22), termination resistors are generally not needed, as they can reduce the load capacity of the RS485 bus. The connection method for the 120Ω matching resistor at the camera terminal is as follows: The 120Ω matching resistor can be connected by toggling the dip switch on the camera chassis, as shown in the figure below. By default, the 120Ω matching resistor is not connected; it can be connected by toggling the 10th position of the dip switch to ON. Conversely, if it is not connected, toggle the 10th position to OFF.
Figure 3
8. Problems in Practical Applications
In practical construction, users often adopt star connection methods. In this case, termination resistors must be connected to the two devices that are farthest apart in the line (as shown in Figure 4, devices 1# and 15#). However, since this connection method does not comply with RS485 industrial standards, it can easily cause signal reflection and reduced anti-interference capability when the distances between devices are large, leading to decreased reliability of control signals. The resulting phenomenon is that the camera is completely uncontrollable or operates autonomously without stopping.
Figure 4
For this situation, it is recommended to add an RS485 distributor. This product can effectively convert star connections into connections that comply with RS485 industrial standards, thus avoiding problems and improving communication reliability, as shown in Figure 5.
Figure 5
9. Recommended Wires for Maximum Transmission Distance Without Relay
(1) Ordinary twisted shielded cable STP-120Ω (for RS485 & CAN) one pair 20 AWG, with an outer diameter of about 7.7mm. Suitable for indoor, pipeline, and general industrial environments. When using, one end of the shield should be grounded!
(2) Ordinary twisted shielded cable STP-120Ω (for RS485 & CAN) one pair 18 AWG, with an outer diameter of about 8.2mm. Suitable for indoor, pipeline, and general industrial environments. When using, one end of the shield should be grounded!
(3) Armored twisted shielded cable ASTP-120Ω (for RS485 & CAN) one pair 18 AWG, with an outer diameter of about 12.3mm. Can be used in environments with severe interference, frequent rodent damage, and requirements for lightning protection and explosion-proof. When using, it is recommended to ground both ends of the armored layer and one end of the inner shield.
4. Common RS485 Faults and Solutions
1. How to prevent faults?
To reduce communication faults, the following suggestions are proposed.
1. It is recommended that users use and purchase RS485 converters provided by the manufacturer or brands recommended by the manufacturer.
2. Manufacturers conduct extensive testing on RS485 converters that are compatible with their products and require RS485 converter manufacturers to produce and quality test according to fixed performance parameters, ensuring good compatibility with access control devices. Do not be tempted to buy cheap, unbranded RS485 converters.
3. Strictly follow the construction specifications for RS485 bus installation and avoid any complacency.
4. For long lines with heavy loads, adopt scientific solutions with reserved plans.
5. If the communication distance is too long, such as exceeding 500 meters, it is recommended to use a repeater or RS485 HUB to resolve the issue.
6. If the load number is excessive, such as exceeding 30 devices on one bus, it is recommended to use RS485 HUB to solve the problem.
7. Always carry debugging equipment during on-site debugging. It is essential to bring several converters that can handle long distances and multiple loads, a commonly used laptop, a multimeter to test circuit breaks, and several 120-ohm termination resistors.
2. Common communication faults when using RS485 bus structure?
1. No communication, no response.
2. Data can be uploaded, but not downloaded.
3. The system prompts interference during communication, or the communication indicator keeps flashing even when not communicating.
4. Sometimes it communicates, sometimes it does not; some commands work while others do not.
3. What debugging methods are available when faults occur?
Before debugging, ensure that the device wiring is correct and that construction complies with specifications. The following debugging methods can be employed based on the encountered issues.
1. Common Ground Method:
Use one wire or shielded cable to connect the GND ground of all RS485 devices together to avoid potential differences affecting communication between devices.
2. Termination Resistor Method:
Connect a 120-ohm termination resistor across 485+ and 485- on the last RS485 device to improve communication quality.
3. Intermediate Segment Disconnection Method:
Disconnect from the middle to check if the device load is too high, if the communication distance is too long, or if a particular device is affecting the entire communication line.
4. Individual Wiring Method:
Individually run a simple wire to the device to check if wiring issues are causing communication faults.
5. Replace Converter Method:
Carry several converters to rule out whether converter quality issues are affecting communication quality.
6. Laptop Debugging Method:
Ensure that the laptop you carry is a device that communicates correctly, and use it to replace the customer’s computer for communication. If it works, it indicates that the customer’s computer’s serial port may be damaged.
【END 】–
Transmission Technology Innovation Advocacy E A (Source: Network)
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