Common RS485 Communication Faults and Solutions

In electrical automation projects, RS485 communication is often encountered, and many beginners are not very familiar with it. Today, we will discuss the applications related to RS485, and you will find that there is indeed a lot of knowledge involved. We will choose some common issues that are often considered in engineering for your reference.

(1) What is RS485 Bus?

In industrial sites, it is often necessary to collect multi-point data, whether analog signals or switch signals, which generally use RS485 bus. RS-485 operates in half-duplex mode and supports multi-point data communication. The topology of RS-485 bus networks 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 the interface is adopted according to the actual situation of the project. RS485 uses differential signaling with negative logic, where +2V to +6V represents “0” and -6V to -2V represents “1”.

RS485 has two wiring methods: two-wire and four-wire. The four-wire method can only achieve point-to-point communication and is rarely used now; the two-wire wiring method is commonly used, which allows a maximum of 32 nodes to be connected on the same bus in a bus topology structure.

According to the theoretical structure of the 485 bus, under ideal conditions, the transmission distance can reach 1200 meters. This condition requires high-quality communication cables, a baud rate of 9600, and only one 485 device connected to achieve the communication distance of 1200 meters. Therefore, the actual stable communication distance of the 485 bus often does not reach 1200 meters. Factors such as multiple 485 devices connected, non-compliant cable impedance, thin wire diameter, poor quality converters, complex lightning protection, and increased baud rate can all reduce communication distance.

(2) RS485 Cable and Transmission Distance

In general situations, ordinary twisted pair cables can be used. In higher requirement environments, shielded coaxial cables can be used. When using the RS485 interface, the maximum cable length allowed for data signal transmission from the RS485 interface to the load is inversely proportional to the baud rate of signal transmission. This length is mainly affected by signal distortion and noise.

Theoretically, the maximum transmission distance of RS485 can reach 1200 meters, but in practical applications, the actual transmission distance is shorter than 1200 meters, depending on the surrounding environment. During transmission, relay methods can be used to amplify the signal, with a maximum of eight relays added, meaning that theoretically, the maximum transmission distance of RS485 can reach 9.6 kilometers. If long-distance transmission is required, optical fiber can be used as the transmission medium, with an optical-electrical converter added at both ends. The transmission distance of multimode optical fiber is 5-10 kilometers, while single-mode optical fiber can reach up to 50 kilometers.

(3) RS485 Wiring Installation Precautions

(1) What type of communication cable should be used for the 485 bus?

How many devices can be connected on one bus? It is necessary to use RVSP shielded twisted pair cable. The specifications of the shielded twisted pair cable used depend on the distance of the 485 communication line and the number of connected devices, as shown in the table below. Using shielded twisted pair cable helps to reduce and eliminate the distributed capacitance generated between the two 485 communication wires and the common-mode interference from the surrounding communication lines.

Common RS485 Communication Faults and Solutions

Some say that the 485 bus can connect 128 devices for communication. However, not all 485 converters can support 128 devices. It depends on the model of the chip inside the 485 converter and the chip model of the 485 device to determine the load capacity, which can only be determined by the lower-rated chip. Generally, the load capacity of 485 chips has three levels – 32 devices, 128 devices, and 256 devices. Moreover, the nominal theoretical value is often not achievable in practice. The longer the communication distance, the higher the baud rate, the thinner the wire diameter, the poorer the quality of the wire, the worse the quality of the converter, and the stronger the lightning protection, all these factors will reduce the actual load number. Most engineering companies tend to use Category 5 or Category 5e network cables as 485 communication lines, which is incorrect. This is because:

1. Ordinary network cables do not have a shielding layer, which cannot prevent common-mode interference.

2. Wires with too thin a diameter cannot be used, as this will lead to reduced transmission distance and fewer devices that can be connected, at least 0.4mm square or use standard network cables.

3. Network cables are made of solid copper wire, which is more prone to breakage compared to multi-core wires.

(2) Why should grounding be done?

The 485 transceiver can only operate normally when the specified common-mode voltage is between -7V and +12V. Exceeding this range will affect communication and may severely damage the communication interface. Common-mode interference can increase the aforementioned common-mode voltage. One effective means of eliminating common-mode interference is to use the shielding layer of the 485 communication line as a ground wire, connecting the grounding of devices such as machines and computers in the network together, and reliably connecting to the ground at one point.

(3) How should the 485 communication line be routed?

The communication line should be kept as far away as possible from high-voltage power lines, fluorescent lights, and other sources of interference. If the communication line cannot avoid interference sources such as power lines, it should be perpendicular to the power line and not parallel, and definitely not bundled together, using high-quality twisted pair cables for routing.

(4) Why should the 485 bus adopt a daisy chain structure rather than a star structure?

The star structure can generate reflected signals, thereby affecting 485 communication. The branch line from the bus to each terminal device should be kept as short as possible, generally not exceeding 5 meters. If the branch line is not terminated, there will be reflected signals that can create strong interference with communication, so it should be removed. It is best to connect a 120Ω termination resistor at both ends of the RS485 device.

Daisy chain connection as shown:

Figure 1

Star connection as shown:

Figure 2

(5) Can there be junctions between devices on the 485 bus?

In the same network system, using the same type of cable, the number of junctions in the line should be minimized. Ensure that the junctions are well soldered, tightly wrapped, and avoid loosening and oxidation. Ensure a single, continuous signal path as the bus.

(6) What are common-mode interference and differential-mode interference?

How to eliminate interference on communication lines? The 485 communication line consists of two twisted pairs of wires, and it transmits signals through the voltage difference between the two communication wires, thus referred to as differential voltage transmission. Differential-mode interference occurs between the two signal wires, belonging to symmetric interference. One way to eliminate differential-mode interference is to add a bias resistor (matching resistor in the camera) in the circuit and use twisted pair cables; common-mode interference occurs between the signal line and ground, belonging to asymmetric interference. Methods to eliminate common-mode interference include:

1. Use shielded twisted pair cables and ensure effective grounding.

2. In areas with strong electric fields, consider using galvanized pipe 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 (ripple interference less than 50mV).

(7) Under what circumstances should termination resistors be added to the 485 bus?

Generally, termination resistors are not needed unless the 485 communication distance exceeds 300 meters, in which case termination resistors should be added at both the start and end of the 485 communication. Especially when the number of devices on the 485 bus is small. When the number of devices is large (e.g., over 22), termination resistors are generally not needed, as termination resistors will reduce the load capacity of the 485 bus. The connection method for the 120Ω matching resistor at the camera terminal is as follows: the 120Ω matching resistor can be connected via a dip switch on the camera chassis as shown in the figure. From the factory, the 120Ω matching resistor is not connected by default, and it can be connected by turning the 10th position of the dip switch to ON. Conversely, if the 120Ω matching resistor is not to be connected, turn the 10th position to OFF.

(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 the farthest apart (e.g., devices 1# and 15#). However, since this connection method does not comply with the usage requirements of the RS485 industrial standard, it can easily lead to signal reflection, reduced anti-interference capability, and other issues, resulting in decreased reliability of control signals. The phenomena that may occur include the camera being completely uncontrollable or operating autonomously and unable to stop.

Figure 3

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 issues and improving communication reliability, as shown in Figure 4.

Figure 4

(9) Recommended Wires for Maximum Transmission Distance Without Relays

1. Ordinary twisted shielded cable STP-120Ω (for RS485 & CAN) one pair 20 AWG, with an outer diameter of approximately 7.7mm. Suitable for indoor, pipeline, and general industrial environments. When using, the shielding layer should be grounded at one end!

2. Ordinary twisted shielded cable STP-120Ω (for RS485 & CAN) one pair 18 AWG, with an outer diameter of approximately 8.2mm. Suitable for indoor, pipeline, and general industrial environments. When using, the shielding layer should be grounded at one end!

3. Armored twisted shielded cable ASTP-120Ω (for RS485 & CAN) one pair 18 AWG, with an outer diameter of approximately 12.3mm. Can be used in environments with severe interference, frequent rodent damage, and where lightning and explosion protection are required. When using, it is recommended to ground the armor at both ends and the innermost shielding at one end.

(4) Common RS485 Faults and Solutions

(1) How to Prevent Faults?

To reduce communication faults, the following suggestions are made:

● It is recommended that users use and purchase 485 converters provided by the manufacturer or brands recommended by the manufacturer;

● The manufacturer will conduct extensive testing on the 485 converters that match their products and will require the 485 converter manufacturers to produce and quality test according to their fixed performance parameters, so they are well compatible with access control devices. Never be tempted to buy cheap, unbranded 485 converters.

● Strictly follow the construction specifications of the 485 bus to avoid any luck-based mindset.

● For long lines with many loads on the 485 bus project, adopt scientific and reserved solutions.

● If the communication distance is too long, such as exceeding 500 meters, it is recommended to use a repeater or 485 HUB to solve the problem.

● If there are too many loads, such as exceeding 30 devices on one bus, it is recommended to use a 485 HUB to solve the issue.

● Bring all debugging equipment during on-site debugging. It is essential to carry several converters capable of long distances and multiple loads, a commonly used laptop, a multimeter for testing circuit interruptions, and several 120-ohm termination resistors.

(2) Common Communication Faults in 485 Bus Structure?

● No communication, no response.

● Can upload data, but cannot download data.

● During communication, the system prompts interference, or the communication indicator keeps flashing even when there is no communication.

● Sometimes communication works, sometimes it does not; some commands can go through while others cannot.

(3) What Debugging Methods Are Available When Faults Occur?

Before debugging, first ensure that the device wiring is correct and that construction complies with specifications. The following debugging methods can be used based on the encountered problems.

Common Ground Method:

Use one wire or shielded wire to connect all 485 devices’ GND grounds together, which can avoid potential differences affecting communication between all devices.

Termination Resistor Method:

Connect a 120-ohm termination resistor across the 485+ and 485- of the last 485 device to improve communication quality.

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