RS-485 Modbus RTU

At the facility, there was a problem of poor communication between the automation cabinet, in which the touch programmable controller SPK110 OWEN was located, and the adjacent cabinet, where four frequency converters PCHV3 OWEN were located. When the installation was completed, commissioning began.
When power was supplied to the frequency drives (without turning on the electric motors), the connection with the SPK110 was interrupted, although the distance between the cabinets was no more than 15 meters. It was immediately difficult to understand what the problem was, therefore, being a member of a group of like-minded ACS, I asked a question in the telegram messenger (https://t.me/ru_ASUTP). Together they began to discuss possible options for errors when setting up data exchange via the RS-485 interface. This article summarizes tips with graphical explanations.

Perhaps you will find a more detailed explanation of the problem you are interested in.

 

1.    Dividing the problem into parts

There is a saying that "it is more convenient to eat an elephant in parts." Dividing the network into segments and sequential analysis allows you to identify exactly where the problem is.
If you disconnect the interface cable from SPK110 and connect the controller through an interface adapter, through the Modbus Slave program to simulate a SLAVE device, you can check the exchange with the controller. If in the Codesys 3.5 program the device is stable in the “Operation” state, highlighted in green, then it can be argued that the controller and port are working. The program has connection diagnostics and an exchange error counter.
A good option for analyzing communication between components is MasterOPC Universal Modbus Server. Excellent functionality, convenient logs. Plus, everything is supported in general - both the master and the slave, ASCII / RTU / TCP, any specific type of RTU over TCP.

 

 
 

Further, it is advisable to check the section of the network that goes towards the frequency drives, having previously supplied power to them. It is important to use exactly the wire that departs from the SPK110. So we will check the operability (see the answers in the telegram messenger) and the devices and the cable line laid from one cabinet to another. Instead of SPK110, in this case, the Modbus Pool program will act.

 

Можно прибегнуть к научному методу диагностики сети и воспользоваться осциллографом.

 

A graph with similar notches may indicate that the line is heavily noisy with interference and it will be difficult for the SPK110 to recognize in such telegrams where it is zero and where it is one.
On this oscillogram, the signal level at the first port is only 1.5 volts, and at the second port it is about 3 volts, while changes in the terminator do not affect the nature of the signal reception and signal levels.
Thus, just one faulty component of an RS-485 network can cause the entire network to fail to function properly. Therefore, when troubleshooting, it is desirable to divide the entire network into segments and connect to the master one at a time, analyzing the quality of the exchange.

2.    Power supply and ground

Regarding the need to ground the neutral conductor of the 24V power supply: it is necessary to combine the zero of the 24VDC power supply and the cabinet ground terminals. This will reduce the likelihood of parasitic noise in DC voltage circuits. If during operation the main power phase is superimposed on a low-current line, then grounding will save the power source itself and low-current equipment by turning off the circuit breaker.

Important:
- try not to use plastic cases of cabinets where there are interface control tasks and at the same time there are frequency drives;
- avoid installing open Meanwell power supplies, as the latter give a mixed voltage when a 50Hz sine wave is mixed with a constant voltage of 24V. This greatly harms the exchange via the RS485 interface, since the SPK110 does not have a filter at the power input and part of the AC voltage will disperse through the board, including the RS-485 ports;
- do not connect the shield of the interface cable with the protective earth of the frequency drive (FCD), as its own interference "merges" onto it.
Thus, in case of problems in the RS-485 exchange, the zero of the 24V power supply should be connected to the ground.

3.    Installation of interface communication lines

It is necessary to ensure that the interface cables involved in digital exchange are not near high-voltage wires, regardless of whether they have a shield or not. If contact with power cables cannot be avoided, then the places where these cables touch each other should be minimized. The larger the cable contact area, the more interference will inductively penetrate into the interface cable.
The cross section, conductor material and its own resistance are also important. If you do not have a special cable, then you can take an ordinary mounting copper wire and twist it into a pigtail with a screwdriver. Such a gasket will help out in cabinets where the noise from interference from frequency converters is especially high. Unused twisted-pair wires must be grounded.

 
In the photo, the twisted pair is stripped too much, which allows induced interference to penetrate the line.
Important:
- analyze how exactly the twisted pair enters the frequency drive itself. Installers usually find it difficult to tell the difference between cable types and run low current wires directly onto the vinyl of the power wires, causing mutual induction. Moreover, shielding in this case does not help much, it is advisable to lay interface cables at least 10 cm away from power cables.
- try to reduce the unshielded part at the connection points with the terminal block inside the devices.

Thus, the cable screen must be grounded and connected to the zero of the 24V power supply in the cabinet where the polling takes place. The screen must be connected in the devices through which it passes.

4.    Noise analysis of communication lines

The best solution for diagnosing line noise is an oscilloscope. It graphically shows interference in the form of bursts and is a better aid in finding the source of the interference.
If you do not have the opportunity to find an oscilloscope, then you can use an inductive dialer to determine the noise level. When bringing the response part of the inductive dialer to the RS485 interface communication line, a crackling sound will be heard, which means that interference in the line is possible. You hear exactly that sinusoid, which is the basis of the power supply of any cabinet. It parasitic seeps into the line, thereby making it noisy. In other words, the noise that is on the line prevents the useful signal from getting through.

Thus, if you hear noise in the line, this may be due to spurious interference, which can be both of an external nature (through the cable shield) and through the circuits of the connected devices themselves.

5.    Terminators

What is terminating and why are terminators needed?

Pull-up resistor circuit in SIEMENS

According to the SIEMENS standard for RS485 networks, noise is eliminated by connecting 390 ohm pull-up resistors from A to positive and from B to negative.

 This is how the terminator connection diagram for Schneider Electric devices looks like.

Particular attention should be paid to the 1nF capacitor. This element has a practical role and without it there will be no data exchange.
The line with power off devices and connected terminators should be approximately 50-60 ohms.
For short runs of lines, in most cases it is best to put 100 ohms at the ends of the lines. This is enough and will work better than 120 ohms - this is how the wave impedance of the cable is "guessed".
Sometimes it is advised to install ferrite rings to calm interference, with a permeability of at least 2000n.
If you reduce the terminal resistance, then the signal amplitude will also decrease, for RS-485 the standard value is 1.5V.

Jumpers on SPK1**, which are needed to terminate the bus, should be used with caution, since the use of these switches does not always lead to the desired result. For example, you need to install a terminator on the network. When switching the jumper, in some cases, you can see that the connection has become less stable. In this case, you should stop using them.

It looks like an adapter that allows you to connect terminators to three RS485 ports.
Thus, terminators are needed, but their use should be approached sensibly. On very short line segments, they can even get in the way.

6.    Baud rate on the RS-485 bus

The quality of the exchange on the bus depends on the selected speed. In some solutions, it is better to choose a speed of 9600 baud, while in others, despite the length of the lines, they choose a speed of 115200 baud.
For short local solutions (according to the experience of the author of the article), 38400 baud is best for ARIES equipment, and 9600 baud for long lines.
Thus, the exchange rate is selected individually each time.

7.    Shielded cable grounding

An important condition for the correct shielding of the interface communication line is the principle of its grounding in the cabinet from which the line is polled.

 
If a decision has already been made to use FTP, then it must be taken into account that the common tinned core goes under the polyethylene film and does not have contact with the screen. It is necessary to connect the tinned core with this foil.
You can find a type of FTP cable that has a braided shield. This cable is easier to use in the installation for connecting the ground.
 

На снимке выше экран не выведен. Выведен «спутник», луженая жила, которая не является экраном. В данном случае стоит скорректировать монтаж: вывести наружу экранирующую фольгу с двух кабелей и соединить их с этим спутником.

 This type of cable is more convenient and conditionally suitable for laying an RS485 network and connecting all network segments along the screen.
Important:
- the screen must be connected between all sections of the network;
- connect the screen to earth only on the side of the control panel;
- the cabinet itself must be grounded;
- the potentials between cabinets must be equalized. Unshielded cables and stripped, unshielded cable sections are very vulnerable to interference.
In modern realities, it has become very common to use twisted pair cable for any application. Iron - please, RS-485 - no problem. By default, this type of cable is intended for Ethernet networks, and not for the purpose of laying RS485 lines. Low cost and availability take precedence over the need to use the cable for its intended purpose.
Why is this solution bad?
Cheap types of cable have very thin solid conductors, sometimes copper alloys rather than copper, and have a weak curl, which is not suitable for an RS485 network. The cross section of the cores of such cables rarely reaches 0.5 mm.
Such a solution, of course, will work, but subject to the quality of the gasket and other factors.
For rRS485 networks, it is necessary to purchase cable products, where the specification says: “cable for industrial RS-485 interface”. Here is an example of such a cable of a domestic manufacturer: KIPEV 1x2x0.6 or specialized brands of cable MKESh.

 
Таким образом, от качественного заземления зависит работоспособность шины для передачи сигнал по RS485.

8.    Following the rules for installing frequency drives

The frequency drive itself, by virtue of its design, is a source of interference. It can affect both devices with which it does not directly interact, and the power supply circuits from which it takes power.
Frequency converters work on the principle of converting direct voltage into alternating voltage, which means that the higher the generation frequency, the stronger the cable will radiate in space. At the same time, it is important to understand that this background perfectly extinguishes the shielding of the cable, both power and interface.

There is a standard describing the rules of electromagnetic compatibility:
GOST R 50397-2011.https://drives.ru/stati/ems-preobrazovatelej-chastoty/drives.ru

Important:
- install chokes in places where power is distributed to frequency drives;
- install frequency drives inside metal cabinets;
- use shielded cables to supply power to electric motors;
- frequency converters have their limits on the lengths of motor cables;
- when switching an asynchronous motor with a frequency converter, it is recommended to pass the phase conductors through the ferrite rings near the boron.
Thus, systems where there are frequency drives or other sources of high-frequency interference (soft start, generator, etc.) require special attention in matters of laying and grounding the interface cable. Otherwise, you can spend a lot of time searching for a problem area.

9.    Estimating the state of the network from an oscillogram
 

Для оценки состояния обмена и зашумленности линии был использован двухканальный осциллограф от фирмы Мотор мастер, подключаемый к ноутбуку по USB. Подойдут для данных целей недорогие модели Hantek 6022, доступные на AliExpress.
На снимке отображены одновременно два канала, к потенциалу А один провод и ноль, к каналу В второй провод и ноль. Тогда сигналы должны зеркально отобразиться и должны совпасть по фронтам.
На графике, представленном выше, наблюдаем зашумленность интерфейсной линии, которая предположительно возникает из-за ряда причин: щит пластиковый (не экранирован), ошибки в экранировании и заземлении.

 
When examining the waveform, you can see that there is more noise on the measurement line A, and the fronts are more crowded on the measurement line B.
On the upper signal, a “beard” is visible after reaching the signal level. It looks like a reflection due to poor termination, and on the lower fronts are littered. The collapse of the fronts is usually a sign of a long line, although it is known that the line is no more than 15m.

Thus, analyzing the graph from an oscilloscope allows you to quickly understand the situation, and if you manipulate the network, observe changes in the curves, you can achieve a better and faster result.

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