PROFIBUS. Network Case study

Transcription

1 PROFIBUS Network Case study

2 INDEX 1 OBJECTIVES NETWORKS TESTED PROFIBUS INFORMATION PROFIBUS SEGMENT TERMINATIONS BAUD RATE NETWORK DIAGRAMS RETRY LIMIT SYNCS CABLE LENGTHS MAXIMUM AMOUNT OF PROFIBUS DEVICES CONNECTORS AND CABLE TYPE STUB LINES EMC DUPLICATE ADDRESSES PIGGY BACK CONNECTORS CONNECTORS FIBER GATEWAYS AND REPEATERS RESULTS DEVICE SIGNAL STRENGTHS SUMMARY

3 1 OBJECTIVES The objectives of the audit were to determine the following: Determine the health of the network Identify faults and areas that can cause possible future problems Generate a Profitrace report Explain fundamental concepts of PROFIBUS Networks Tested The following networks have been tested: Network Profitrace Report Available Network Profitrace Report Available 500NJB301 Report 360RJB004 Report 500RJB004 Visual Inspection 360RJB002 Visual Inspection 500NJB036 Visual Inspection 360RJB003 Visual Inspection 500RJB005 Visual Inspection 360RJB006 Visual Inspection 500NJB020 Report 200NJB004 Report 900NJB01R Report 170NJB115 Visual Inspection 800RJB001 Report 200RJB001 Report 800NJB201 Report 170NJB095 Visual Inspection 800RJB002 Report 200NJB201 Report 800RJB003 Report 200RJB002 Report 900NJBT02F Report 200RJB003 Visual Inspection 400RJB001 Visual Inspection 200NJB301 Report 400RJB002 Report 200RJB004 Visual Inspection 400RJB003 Visual Inspection 200RJB008 Visual Inspection 400RJB004 Visual Inspection 200RJB006 Visual Inspection 400RJB005 Report 200RJB005 Report 800RJB001 Visual Inspection 200RJB007 Visual Inspection 800NJB201 Visual Inspection 200RJB009 Report 800RJB002 Report 150NJB003 Report 800RJB003 Report 150RJB001 Report 900NJBT01R Report 120NJB130 Not connected 400NJB011 Report 150RJB002 Visual Inspection 400NJB501 No piggy back plug 120NJB136 Visual Inspection 500RJB001 Report 150RJB003 Visual Inspection 500NJB302 Visual Inspection 150NJB401 Visual Inspection 500RJB003 Visual Inspection 170NJB057 Visual Inspection 500NJB301 Visual Inspection 150RJB004 Visual Inspection 500RJB004 Visual Inspection 170NJB060 Visual Inspection 500NJB036 Visual Inspection 150RJB005 Visual Inspection 500RJB005 Report 170NJB011 Visual Inspection 600NJB020 Report 150RJB006 Visual Inspection 900NJBT02F Report 150RJB007 Visual Inspection 300NJB005 Visual Inspection 170NJB011 Visual Inspection 300NJB006 Report 150RJB008 Report 300NJB007 Report 120NJB900 Visual Inspection 300RJB001 Report 900NJB101 Visual Inspection 3

4 300RJB005 Visual Inspection 150RJB009 Visual Inspection 300NJB501 Report 120NJB002 Report 300RJB002 Report 120RJB001 Report 300RJB004 Visual Inspection 120RJB002 Visual Inspection 300RJB003 Visual Inspection 120RJB003 Visual Inspection 360NJB008 Visual Inspection 120RJB004 Visual Inspection 360NJB009 Visual Inspection 120RJB005 Visual Inspection 360RJB001 Visual Inspection 120NJB003 Visual Inspection 360NJB501 Visual Inspection 120NJB025 Visual Inspection 640NJB005 Visual Inspection 120RJB006 No Fiber 360RJB005 Visual Inspection Table 1-1: List of networks tested 2 PROFIBUS Information Each of the sections seen below, explain important PROFIBUS concepts. The comment provided after each section can be categorised as follows: 2.1 PROFIBUS segment A segment is defined as PROFIBUS section of cable that is connected linearly and does not branch off at any point. I.E. it has one start and one end. If at any point a repeater or fiber gateway (OLM/OZD) joins another section of cable, this new section is considered a new PROFIBUS segment. Segment 1 Segment 2 Repeater 2.2 Terminations Each end of the PROFIBUS cable must be terminated. This termination can be switched on at the PROFIBUS connectors or by using active terminator. The termination consists of three resistors matched to the impedance of the PROFIBUS cable (150 ohm). These resistors absorb the energy of the signal and reduce it theoretically to zero. The PROFIBUS cable must be terminated at the beginning and end of each cable. The second requirement is that the termination must be powered, if the termination is switched on at the PROFIBUS connector, the termination will draw power from the device it is connected to. It is important to ensure that the last device on the network is always powered or else that PROFIBUS segment will not be properly terminated. Active terminators guarantee that the segment is always terminated since they do not rely on a device for power. 2.3 Baud rate 4

5 The baud rate of the network is the rate of data that can be transmitted per second. The higher the baud rate (faster communication) the faster each device has a chance to communicate (cycle time). Motion control usually requires very high baud rates (fast cycle times) and will run at a baud rate of 12 mbps. The higher the baud rate the more susceptible the PROFIBUS network is to interference and therefore the environment must be clean of high voltages and large electro-magnetic fields. If the environment contains high voltages and currents then the baud rate must be reduced. The recommendation is to run the network as slow as possible for industrial environments. 2.4 Network Diagrams PROFIBUS network diagrams are essential for effective fault finding on PROFIBUS networks. Network diagrams contain the following information: Device type, device manufacturer, device model, PROFIBUS address, points of termination, PROFIBUS cable routing and distances (optional extra) and baudrate. 2.5 Retry limit The PROFIBUS master communicates with each PROFIBUS slave/device one at a time, sending a request and getting a response message from each and every slave. The PROFIBUS master will cycle through each slave requesting information and sending information, when it has finished this cycle, the master will start the cycle again. When a message has been sent to a slave and the message has been corrupted or the slave does not respond within a set amount of time, the master will have a number of turns to send the message again. This is defined as the retry limit. The default is 1 and by increasing this retry limit it will give the network more chances to communicate before registering a fault. 2.6 Syncs Sync messages are sent when the PROFIBUS master is trying to communicate with a slave but is not getting a response. This happens if a device has failed, switched off, a cable has broken or the slave has been setup in the PLC but has not been connected to the PROFIBUS network. 2.7 Cable lengths The maximum cable length for a segment is dependent on the baud rate for that network. Multiple segments can be joined together using repeaters or fiber gateways. The table below lists the maximum segment length based on the baud rate: Baud rate Maximum segment length 9.6 Kbit/s 1 000m 19.2 Kbit/s 1 000m Kbit/s 1 000m Kbit/s 1 000m 5

6 187.5 Kbit/s 1 000m Kbit/s 400m 1.5 Mbit/s 200m 3.0 Mbit/s 100m 6.0 Mbit/s 100m 12.0 Mbit/s 100m Table 2-1 : Maximum segment lengths 2.8 Maximum amount of PROFIBUS devices The maximum amount devices that can be attached on a PROFIBUS segment is limited to 32 and is based on the RS 485 standard. Every device connected on to this segment whether it has an address or not needs to be included in this calculation. The components inside of each device negatively affect the PROFIBUS signal and therefore regardless if the devices have an address or not the signal will deteriorate as it travels down the cable. Fiber gateways (OLM/OZD) are examples of devices that don t have PROFIBUS addresses. 2.9 Connectors and cable type The type of connector that is used is usually dependant on the type of cable that is used. If the cable core is stranded then screw in connectors must be used. Fast connect connectors must not be used for stranded core cables because the vampire teeth that make contact with the cores will eventually cut through the strands of cable and cause intermittent communication losses. Fast connect and screw-in connectors can be used for solid core cable although fast connect connectors are the recommended choice Stub lines Stub lines are caused when the cable branches from a single cable into two cables. Stub lines add capacitance to the line which changes the cable impedance at that point. When the cable impedance is changed, it causes reflections to occur along the cable. These signal reflections cause the network to become unstable. 6

7 2.11 EMC EMC errors can be generated from when the PROFIBUS cable is run too close to high voltage cable or cables/devices that have large electro-magnetic fields. If this requirement is not met the EMC will inject noise onto the PROFIBUS cable and corrupt messages travelling past that point of interference. The result is devices seem to sporadically fail to communicate. It is important to understand that when a PROFIBUS message has been corrupted it is thrown away. If there is enough corrupted messages in a short amount of time this will result in the PROFIBUS master (PLC) flagging that a device as not present on the network, There are three ways to reduce the effect of EMC 1. Cable shield connected to earth. 2. Run cable in steel conduit connected to earth. 3. Increase separation distance between PROFIBUS cable and high voltage cable (see diagram below) 20cm Cable Category I 10cm Cable Category II 10cm Cable Category III 50cm 50cm 50cm Cable Category IV Figure 2-1: Cable categories Category I: Lan Cables (PROFIBUS, Ethernet) Other Communication Cables Category II: Shielded and Unshielded Cables for DC Voltages > 60V and <= 400 V Shielded and Unshielded Cables for AC Voltages > 25V and <= 400 V Category III: Shielded and Unshielded Cables for DC and AC Voltages > 400 V Category IV: Signal Cables of Categories I - III at Risk From Direct Lightning Strikes, e.g., Connections Between Components in Different Buildings 7

8 2.12 Duplicate addresses The PROFIBUS specification requires that each PROFIBUS slave is given a unique address in the range of through which it can communicate. If two PROFIBUS slaves have the same address the messages will cause collisions on the network Piggy back connectors Piggy back connectors allow for diagnostic equipment to be attached without disrupting a running network while it is running. To accurately determine the health of a network every segment must be physically connected to. The recommendation is that every segment has a point where a diagnostic device can be connected without having to disrupt the network by unplugging devices Connectors It is important that at the ends of the cable where the termination switch will be turned on that the PROFIBUS cable is wired to the incoming cable opening. When the termination switch for the connector is switched on, the outgoing line is disconnected Fiber gateways and repeaters Fiber gateways (OLM/OZD) and repeaters are mostly used to extend the PROFIBUS network or to isolate problem areas. It is important to understand that both PROFIBUS repeaters and fiber gateways are not intelligent devices, they simply listen to the message on one end and recreate the message on the other side. If there is a large amount of interference or reflections this may look like a message to the repeater or fiber gateway. The repeater/fiber gateway will transmit this noise to the rest of the network and may cause network errors in the process. 8

9 3 Results Some of the problems that have been rectified can be seen below. 3.1 Device Signal Strengths Healthy voltage levels are between volts, all of the voltages are within this range. Figure 3-1 : Segment voltage levels 9

10 The waveform seen below should look similar to a square waveform with no large spike or dips for it to be considered healthy. As can be seen this device is representative of a healthy waveform. The small spike seen on the waveform is generated by the test equipment. The report generated by the Profitrace contains the waveforms for all of the devices. Figure 3-2 : Device 3 voltage waveform 10

11 4 Summary The network has been designed exceptionally well, the star topology that is used in this installation is designed to reduce the chance of faults propagating through the entire network. The network has been wired according to the specification and extra measures have been taken to reduce the chance of interference (using armoured cable wherever the PROFIBUS cable leaves a panel). None of the networks tested has any illegal or broken messages and all of the signal voltages were healthy. Network diagrams need to be drafted. It is beneficial for those monitoring the PROFIBUS networks to go for training with the PROFIBUS Competence Centre in South Africa. This will help anyone who is maintaining this network to understand how to fault find and troubleshoot any problems that may arise in the future. All networks tested (Table 1-1: List of networks tested) meet the PROFIBUS specification and are certified. Gerrie Reynecke (PROFIBUS, PROFINET Engineer) Client: RMM 11