RS485 wire and cable

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RS485 industrial bus cableThe standard is a serial communication protocol published by the Electronic Industries Association (EIA) in the United States, which uses a balanced transmission and differential reception data transceiver to drive the bus. It has a longer signal transmission distance, faster speed, and better anti-interference performance than the RS-232 standard, and is widely used in various industries, buildings, energy, and other fields.

becauseRS485 industrial bus cableThe characteristics of long distance, multiple nodes (32), and low transmission line cost make EIA RS-485 the standard for data transmission in industrial applications.

Specific specification requirements:

●The input resistance RIN of the receiver is ≥ 12k Ω;

●The driver can output a common mode voltage of ± 7V;

●The capacitance at the input end is ≤ 50pF;

●With 32 nodes and a terminal resistor of 120 Ω, the driver can still output a voltage of at least 1.5V;

●The input sensitivity of the receiver is 200mV (i.e. (V+) - (V -) ≥ 0.2V, indicating a signal of "0"; （V+）-（V-）≤-0.2V， Signal "1" is represented.

RS485 industrial bus cables are often used in industrial sites to collect multi-point data, analog signals or switch signals. Generally, RS485 bus uses a communication method with one master and multiple slaves. This method is easy to connect and only requires two shielded cables. The communication distance can be as long as 1500m, and adding a repeater can also extend the communication distance. The differential signal method has good resistance to electromagnetic interference. However, the communication speed of this method cannot be too fast. Generally, a master-slave summoning method is used to collect data from each subunit, that is, the master unit summons each subunit in turn, and which unit to summon and which unit to upload data to. The use of the bus is completely allocated by the master unit, and each subunit cannot occupy the bus without authorization. If there are multiple units in the system, the main unit takes a long time to collect data in a cycle of one week, and the sub unit information cannot be sent to the main unit in a timely manner when it changes, resulting in a slow response and processing speed of the system to sudden events. This article solves the problem through bus status detection and active transmission from the slave.

RS485 industrial bus cablestructure

RS485 industrial bus cableSupports half duplex or full duplex modes, and the network topology generally adopts a bus type structure matched with terminals, and does not support ring or star networks. Use a bus to connect each node in series, and the length of the lead wires from the bus to each node should be as short as possible to minimize the impact of reflected signals in the lead wires on the bus signal. If a star structure is required, a 485 repeater or 485 repeater must be used. As shown in the following figure:

RS-48The industrial bus standard requires the use of daisy chain connection between devices, with 120 Ω terminal resistors at both ends (as shown in Figure 1). Simplified connections can be made using Figure 2, but the distance between the "D" segment must not exceed7 meters.

RS-485The maximum communication distance is approximately1219M, the maximum transmission rate is 10Mb/S, and the transmission rate is inversely proportional to the transmission distance. Only at a transmission rate of 100Kb/S can the maximum communication distance be achieved. If longer distances need to be transmitted, a 485 repeater needs to be added.

The RS-485 bus generally supports up to 32 nodes, but if a specially designed 485 chip is used, it can reach 128 or 256 nodes, and the largest can support up to 400 nodes.

RS485 industrial bus cableTransmission medium

The 485 signal line should be twisted pair, with a shielding layer added as a ground wire to connect 485 devices in the 485 network, and reliably grounded at one point.

Priority should be given to using independent power supply solutions for each system, and a large power supply should not be used in parallel to supply power to the system. At the same time, the power line (AC/DC) cannot share the same multi-core cable with the RS-485 signal line.

RS-485 signal line should be selected with a cross-sectional area of 0.75mm²The above twisted pair cables are not straight lines. For each small capacity DC power supply, choosing a linear power supply is more suitable than choosing a switching power supply.

As the transmission distance of the 485 bus increases, it will generate echo reflection signals. If the transmission distance of the 485 bus exceeds 100 meters, it is recommended to use a 120 ohm terminal resistor at the beginning and end of the 485 communication during construction.

If there are many 485 devices on the 485 bus, it is recommended to use a 485 converter with a power supply. Passive 485 converters have insufficient power supply and load capacity due to stealing electricity from the serial port.

When used0.56mm²When using (24AWG) twisted pair cables as communication cables, the theoretical values for the maximum transmission distance vary depending on the baud rate, as shown in the table below:

RS485 industrial bus cableWiring construction

The 485 signal line cannot be wired together with the high-voltage power line;

Parallel or unshielded wires cannot be used as wiring for 485 signal lines. Shielded twisted pair wiring should be used;

The 485 signal line cannot use ordinary Category 5e shielded twisted pair cables;

485 wiring cannot be arbitrarily arranged as star or tree wiring. The 485 wiring specification requires hand in hand wiring. Once the 485 hub and 485 repeater are directly set up into star and tree connections, it is easy to cause signal reflection and bus instability;

The 485 bus must be grounded. In many technical documents, it is mentioned that the 485 bus must be grounded, but there is no detailed explanation on how to ground it. Strictly speaking, the 485 bus must be reliably grounded at a single point. Single point means that only one point on the entire 485 bus can be grounded, and multiple points cannot be grounded, because grounding it is necessary to maintain the voltage on the ground wire (usually a shielded wire as the ground wire) to prevent common mode interference. If multiple points are grounded, it will have the opposite effect.

RS485 industrial bus cableTroubleshooting methods

RS-485 is a low-cost and easy to operate communication system, but it has weak stability and strong mutual restraint. Usually, a node failure can cause overall or partial system paralysis, and it is difficult to determine. Here are several commonly used troubleshooting methods:

1. If the system is completely paralyzed, it is mostly due to the breakdown of the VA and VB of a certain node chip by the power supply. Using a multimeter to measure the differential mode voltage between VA and VB is zero, and the common mode voltage to ground is greater than 3V. At this time, the common mode voltage can be checked by measuring the magnitude of the common mode voltage;

2. The bus cannot function properly for several consecutive nodes. It is usually caused by a failure of one of the nodes. A node failure can cause neighboring 2-3 nodes (usually subsequent nodes) to be unable to communicate, so they should be disconnected from the bus one by one. If the bus can be restored to normal after a node is disconnected, it indicates that the node is faulty;

3. The RS-485 system with centralized power supply often experiences some abnormal nodes when powered on, but each time it is not completely the same. This is due to the unreasonable design of the RS-485 transmission and reception control terminal TC, which causes confusion in the node transmission and reception status when the system is powered on, resulting in bus congestion. The improvement method is to install power switches on each system and then power them on separately, or adopt power isolation measures.

4. The system is functioning normally, but occasional communication failures may occur. Generally, it is due to unreasonable network construction that the system reliability is in a critical state, requiring changes in routing or the addition of relay modules. One of the emergency methods is to replace the failed nodes with chips with better performance. Or increase the use of 485 repeaters.

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