PRFOBUS connectors and termination resistors
The plug is used to connect the ROFIBUS cable to the PROFIBUS station (Figure 1).
Figure 1 USE OF PROFIBUS PLUGS
On the PROFIBUS plug, there is an inlet hole (In) and an outlet hole (Out) that connects to the previous station and the next station, respectively.
1. For plugs on the two terminal stations of each physical network segment, you need to connect the network cable to the inlet port "In", and set the termination resistance to "On" at the same time (as shown in Figure 2)
2. For the station located in the middle of the network segment, you need to connect the network cable to the inlet port "In" and the outlet port "Out" in turn, and set the termination resistance to (as shown in Figure 2)
3. In order to facilitate system diagnosis and maintenance, it is recommended that the plugs at least two terminal stations of each network segment use a programming port as much as possible (as shown in the left connector in Figure 1)
Figure 2 CONNECTION AND SETTING OF THE PROFIBUS PLUG
4. For terminal devices in the bus that are connected by wiring (non-PROFIBUS connectors), use the self-connected termination resistor shown in Figure 6, or use the active termination resistor in entry 6.
Figure 3 Composition of termination resistors
5. When the equipment in the terminal position is dropped or artificially closed, the resistance on the standard connector also fails. Therefore, the overall network will lose termination resistance at this terminal and may cause the failure of the overall network.
Siemens offers active termination resistors (6ES7 972-0DA00-0AA0) to ensure that the resistance at this end position remains valid.
Figure 4 Active termination resistor
Can the CPU be placed in the middle of the bus network?
OK. As shown in the figure, note that when the CPU is placed in the middle position, the termination resistor is dialed to OFF.
The Profibus cable is broken or not long enough, can it be extended?
Can be extended. It is necessary to connect the two cables, and the two copper cores cannot be simply screwed up, because this will destroy the characteristic impedance of the cable and will cause communication problems.
1. You can use the pair of connectors in the figure below to connect the two cables that need to be connected.
Order numbers: 6GK1905-0EA10 and 6GK1905-0EB10
2. Repeater connection is also available.
Profibus bus installation specifications
The precautions for PROFIBUS installation are introduced here, combined with some on-site examples.
Notes:
Each network can theoretically connect up to 127 physical sites, including masters, slaves, and relay devices;
Each network segment supports 32 physical devices (nodes), and more than this number requires the addition of 485 repeaters, up to a maximum of 9 repeaters per network.
The network supports multi-master, but it is not recommended to have more than 3 masters in the same network;
Generally, 0 is the address of PG, 1~2 is the address of the main station, 126 is the default address of some slaves, 127 is the broadcast address, so these addresses are generally no longer assigned to the slave, so the DP slave can be connected up to 124, and the station number is generally set to 3~125.
Rules for network cabling
CHOOSE A STANDARD PROFIBUS COMMUNICATION CABLE
The characteristic impedance of the standard PROFIBUS communication cable is 150 ohms, which is just matched with the terminal resistance value when the terminal resistance of the PB head is set to "ON", if you choose an ordinary cable, its characteristic impedance and the termination resistance are likely to not match, the communication performance will be affected;
Standard PROFIBUS cables tend to be double-shielded, and the shielding effect is better. In addition, the standard communication cable is twisted-pair, so it can also play a self-suppressing role in the interference generated by the signal when it is transmitted in the cable.
The shield is grounded at multiple points
When the PROFIBUS cable is wired in the plug, the shielding layer must be peeled off and pressed on the metal part inside the plug, which is connected to the metal part outside the Sub-D plug, and when the plug is inserted on the DP port of the CPU or ET200M and other equipment, it is connected to the installation base plate through the device, and the installation bottom plate is generally connected to the cabinet shell and grounded, so as to achieve the grounding of the shield.
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Since grounding is beneficial to protect PLC equipment and DP communication ports, grounding treatment is required for all PROFIBUS sites, that is, "multi-point grounding".
Routing Rules (Important)
a. Cable distribution duct routing of different voltage levels
High voltage, high current power cable, and small voltage and small current cable should be split trunking wiring, and the trunking should be covered with a cover plate and fully enclosed as much as possible; If the site cannot be routed separately, the two types of cables should be separated as far away as possible, and a metal partition should be added in the middle to isolate them, and the metal trunking should be grounded (Figure 6).
Figure 6 Cable duct rack and cable handling in the trunk
Figure 7 Field wiring
The connection between cable ducts should also be handled with a large area of metal connection parts, while paying attention to the "ground" connection
Figure 8 Connection between cable trays and grounding treatment
b. When the communication cable is routed outside the wire slot alone, the metal pipe can be used according to the situation, which can not only protect the communication cable from being damaged, but also benefit from preventing EMC interference, but pay attention to the external metal pipe needs to be grounded
Figure 9 Communication cable at the site
The cable communication in Figure 9 is directly exposed and can easily be crushed, and similar situations can be considered partial or full pipe penetration.
c. Communication cables and power cables should avoid long-distance parallel wiring
Since spatial capacitive coupling needs to be considered between two cables routed in parallel lines, parallel routing should be avoided to avoid mutual influence (Figure 10).
Figure 10 Communication cable in the wire channel and power cable flat running line
In Figure 10, the communication cable not only does not meet the requirements of a. or b. two principles, but parallel to the larger power cable, which will cause the cable to be more susceptible to interference from the power cable.
Can be cross-routed:
The two cross-routed cables do not interfere with each other due to capacitive coupling.
d. Keep the cable as close as possible to a large metal plate (Figure 11)
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Communication cables should be close to a large area of metal plates or "ground planes".
e. Do not form a ring when the communication cable is too long (Figure 12)
Figure 12 Communication cables form a ring
At this time, if a magnetic field line passes through the middle of the ring, according to the "right-hand law", it is easy to generate interference signals.
In Figure 12, although the backplane is a relatively large metal plate, because the project has been completed, there is no possibility of cable length change, so it is recommended that the user cut the excessively long cable short and put it into the cable duct in the cabinet.
f. The equipment connected by the communication line should be connected with equal potential
PROFIBUS connected stations may be widely distributed, and in order to ensure the quality of communication, it is generally required that all communication stations should be at the same voltage level, i.e. they should be "equipotential" (Figure 13).
Figure 13 "Equipotential" treatment should be done between communication sites
If the potential is not equal between the "ground" of the two stations, when the two devices are grounded separately, a potential difference will be generated between the two grounding points, and the current will flow through the shield of the communication cable, thus affecting the communication. Therefore, equipotential binding should be performed between the two devices.
The "ground" of the two devices can be connected with an isopotential line: copper 6mm2, aluminum 16mm2, steel 50mm2.
Of course, this is not to require that all sites need to add additional isopotential lines and increase costs, but it is recommended that when the potential of the grounding point is not equal, if it affects communication or may cause equipment damage, it should be improved.
If the communication is unstable due to the grounding point itself, such as a strong interference in the "ground" of a system, grounding the shield here will affect the PROFIBUS communication, so it should be considered to deal with the "ground" first, and then ground the PROFIBUS shield.
Providing a good "ground" for field devices and proper "grounding" are prerequisites for improving EMC characteristics (Figure 14).
Figure 14 Good grounding design and implementation of the system
g. Wiring of communication lines in the electrical cabinet
When the communication cable is routed in the electrical cabinet, it should also follow the previous principle, that is, away from the source of interference.
The wiring in the cabinet should be carefully designed, try to avoid routing in the same trunking with high-voltage, high-current cables (Figure 15), and at the same time, do not form a "loop" in the cabinet, especially to avoid surrounding interference sources such as frequency converters in the "ring".
Figure 15 Interference of communication cables and power cables in the electrical cabinet
The shielding of the communication cable is handled in the electrical cabinet
The first is the PROFIBUS plug, in addition to the previously introduced need to laminate the shield on the metal part of the plug, it is also necessary to pay attention to the shielding layer not to peel off too long, otherwise it will be exposed to space and become an easily interfered "antenna" (Figure 16).
Figure 16 The shield is exposed to space and is easy to receive interference
The shielding layer of the communication cable should be grounded when entering/exiting the electrical cabinet
The shielding should guarantee a large area of contact with the grounded copper bar (Figure 17).
Figure 17 Grounding of the shield
When the communication cable enters / exits the cabinet, the shielding layer of the cable should be grounded. This prevents external interference signals from entering the cabinet, and also prevents interference generated in the cabinet from affecting external equipment (Figure 18)
If the communication cable needs to be connected through terminals in the cabinet, it is best to connect the shield on both sides of the terminal block separately (Figure 19)
Figure 19 Shielding of communication cables when connected through terminals
The avoidance is to peel off the shield and twist it into a connection to the terminal (Figure 20), which is called "pig's tail" in the EMC field.
Figure 20 "Pig's tail effect" at shielded cable joints
In the field connection, if the shield is peeled off too long, the communication cable will not be "protected" by the shield for a long time, and the shield will form an antenna when twisted together, making it easier to introduce interference into the system (Figure 21).
Figure 21 "pig's tail" connection of shielded cable
Overvoltage protection
If there is a risk of overvoltage in the use of the situation, please use a direct buried cable outside the cabinet, and use an overvoltage protection device on the cable inside and outside the cabinet (Figure 22).
If there is a lightning strike, please refer to the lightning protection design standard for lightning protection design.
Figure 22 Overvoltage protection device
Reduce the impact of interference source equipment such as frequency converters on communication
In addition to the normal operation of equipment such as interference with power supply and space radiation interference, as the inverter and other equipment have the ability to communicate with PROFIBUS, the interference generated by these equipment may also directly enter the communication system, so the inverter should be EMC treatment.
The first is the installation of the frequency converter. In the electrical cabinet, try to replace the painted base plate with a galvanized base plate as a mounting backplane (Figure 23) to improve EMC characteristics.
Figure 23 Galvanized mounting base plate instead of painted base plate
The outlet of the inverter should be treated with corresponding EMC, such as using a shielded cable for grounding for communication cables, grounding with shielded cables for power cables or filtering with ferrite magnetic rings (Figure 24).
Figure 24 Standardized EMC treatment of the cable of the inverter