RS485 is a standard that defines the electrical characteristics of drivers and receivers in a balanced digital multi-drop system, and it has a very wide range of applications in the field of environmental monitoring. RS-485 wiring not only enables multi-point bidirectional communication over a pair of twisted-pair cables, but also possesses noise suppression capabilities, data transmission rates, cable lengths, and reliability that are unmatched by other standards. When communication is required in a noisy environment and over a certain distance, RS-485 wiring is an excellent choice.
The RS485 bus is widely used due to its low cost and simple design, and it is extensively applied in various fields such as security monitoring, intelligent transportation, intelligent buildings, computer room monitoring, and industrial automation. Although RS485 bus wiring is relatively simple, there are some corresponding items that must be noted, otherwise it is easy to cause communication failures, leading to a significant amount of maintenance work in the later stages. The following points should be noted in the 485 wiring specifications compiled by the editor:
RS485 wiring considerations:
Wire issues
The wire used for 485 bus wiring must be shielded twisted-pair cable, preferably with a wire diameter of 0.75 or 1.0. Many people, for convenience, directly use network cables as 485 cables during construction. Network cables have eight wires, while 485 cables only require two or four wires, wasting the other wires. Moreover, the current network cable diameters are relatively thin and cannot fully meet the requirements of 485 buses. For communication needs, it is recommended not to use network cables.
Parallel lines, coaxial cables, or twisted-pair cables without shielding cannot be used. Since 485 uses differential balanced transmission, using twisted-pair cables can effectively cancel out the effects of external interference. It is not allowed to use 485 buses. Similarly, parallel lines should also use shielded twisted-pair cables, as the shielding layer can block external interference.
Wiring issues
The wiring of the 485 bus should be kept away from interference sources as much as possible. During construction, many people, for convenience, route the 485 cable together with the power cable. This is unreasonable. The power cable can cause interference, leading to unstable 485 communication. 485 wiring must be kept away from strong voltage interference sources such as transformers and frequency converters.
Bus topology issue
Due to the complex on-site environment, 485 devices are widely dispersed. For the convenience of construction personnel, the wiring did not follow the 485 specification and adopt a hand-in-hand daisy chain bus topology layout, but instead randomly used star, tree, or even a mix of multiple topologies, leaving too many long branches.
In practical applications, simple star, tree, or even hybrid topologies can sometimes be used, but communication will definitely be unstable. If a star or tree topology must be deployed, it is recommended to use 485 hubs and 485 repeaters. For related applications, please refer to the 485 bus topology issues page.
Grounding issue
The 485 bus wiring specification emphasizes the need for a single point of reliable grounding. However, in practical construction, 485 bus grounding can sometimes backfire. Since 485 wiring requires a daisy chain connection, the shielded twisted pair used will inevitably be cut and connected to the 485 device, and most will use the outer shield as a grounding wire. If the shielding is not properly connected, grounding may cause instability in the 485 signal, so the grounding of the 485 line must be a single point of reliable grounding.
Strict wiring specifications for RS485:
The 485+ and 485- data lines must be twisted pairs.
The wiring must be multi-pair shielded twisted pairs. The multi-pair is for backup, the shielding is for debugging in special cases, and the twisted pair is because 485 communication uses differential mode communication principles, and twisted pairs have the best anti-interference properties. Not using twisted pairs is very wrong.
The 485 bus must be a hand-in-hand bus structure, and star connections and bifurcated connections must be strictly avoided.
The AC power supply and chassis of the device must be grounded and well grounded.
There are many places where it seems like the triangular socket is not grounded at all, so be careful. When well grounded, it can ensure that the device can better release energy when subjected to lightning surge and static electricity accumulation. Protecting 485 bus devices and related chips from damage.
Avoid running together with strong electricity to avoid interference.
Recommended debugging methods for RS485 bus:
The first thing to do is to ensure that the device wiring is correct and strictly conforms to specifications.
Common grounding method:
Connect the GND grounding of all 485 devices with a single wire or shielded wire to avoid potential differences between all devices affecting communication.
Terminal Resistance Method: Connect a 120 ohm terminal resistor in parallel to the 485+ and 485- of the last 485 device to improve communication quality.
Intermediate Break:
Check whether the device is overloaded or the communication distance is too long by disconnecting it in the middle. Reasons such as damage to a certain device affecting the entire communication line.
Single Pull Method:
Simply temporarily pull a wire to the device, so as to eliminate whether the communication failure is caused by wiring.
Converter Replacement:
Carry multiple converters with you to eliminate whether the quality of the converter affects the communication quality.
Laptop Debugging Method:
First ensure that the laptop you carry with you is a device with normal communication, and replace the client computer for communication. If possible, it indicates that the serial port of the client computer may be damaged or injured.
Frequently Asked Questions:
Can we run RS485 cables adjacent to power lines or in the same conduit?
No, the high voltage and current present on the power line may cause interference. If there are insulation faults, arcing, etc. on the power line, it may generate dangerous voltage to the low-voltage RS485 network cable.
Can I use Cat 5e-UTP instead of shielded cable?
No. Cat 5e-UTP (unshielded twisted pair) can work over shorter distances in less demanding applications and has a low EMI noise level. However, the small solid wires commonly used in Cat 5 cables are prone to breaking and rendering the system unmonitored.
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