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|Правила монтажу і прокладки кабелів Profbus DP
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|Автор:||san [ 06 лютого 2015, 21:55 ]|
|Тема повідомлення:||Правила монтажу і прокладки кабелів Profbus DP|
What is the story about pin-5 of the M12 connector (Shield)? Is it used or not?
The current situation is that pin 5 shall NOT be used anymore as Shield pin! The thread of the M12 connectors has to carry the shield. In the past pin 5 was used. But, this did not protect the PROFIBUS signals well enough due to pig-tails and ferrules. The shield should be covering the wires and not be buried in the wires (A-B line). The current difficulty is that still lots of connection materials out there are based on pin 5. The user has to verify this, because connection materials are not certified.
Can PROFIBUS repeaters be used in PROFIsafe and MPI networks?
ES, MPI is using PROFIBUS FDL telegrams with its own user data implementation and PROFIsafe is concentrated in the data area of PROFIBUS DP telegrams. Every standard PROFIBUS repeater and fibre optic coupler can interpret and forward these telegrams. Also the cabling rules are the same for PROFIsafe and MPI.
I am using a Busterminal with non-Siemens devices, but it does not work. What am I doing wrong?
A Busterminal is a very simple repeater that consumes +90 mA power supply from the DB9 connector of the attached device (5V from pin 5 and 6). Lots of devices cannot exceed 10 mA, because the power supply on the DB9 is initially implemented to solely provide power to an external termination (even with certified devices). Please check the manual of the Busterminal. It should contain a list of devices that can be used with this product. Of course, these are only the devices from this manufacturer.
Will the PROFIBUS plugs with diagnostic LEDs have influence on the busload?
The input resistance of the diagnostic plugs is approx. 1,5..2,0 MOhm (analog comparator input). This is 100 times higher as the RS 485 receiver input impedance (e.g. 75ALS176 has min.12-30 kOhm). In theory this should not cause any problems which have not been detected so far.
How many PROFIBUS DP repeaters can be installed in series?
There are lots of stories out there concerning this subject. The actual answer lies in the electronic components of the repeater. These determine the actual "maximum number?. The digitalization from and to the opto-couplers in the repeater will make bits smaller or wider. If you put too many repeaters in series, the bits differentiate too much and messages are not recognized anymore. There are also repeaters available with no limit in serial placement.
Check the manual of the repeater! It should state the maximum amount. For example: the Siemens repeater is 9, the ProfiHub is unlimited.
The second step is changing the busparameters. All repeaters have a delay time. If you put a large quantity of repeaters in series you might have to change the Tslot busparameter to a higher value.
Why is termination important?
Termination prevents reflections that can disturb the data communication. The higher the baudrate and the longer the cable, the more important termination becomes. Termination should be activated/placed at both ends of every bus segment. With PROFIBUS DP the termination is powered to provide an idle level when nobody is sending data. With PROFIBUS PA the termination is a combination of a resistor and a capacitor.
Should there be distance between PROFIBUS and power cables?
YES, even PROFIBUS can get disturbed by interference caused by parallel high voltage power lines. Always try to have at least 10 cm air space between the cables (depends on the level of the voltage). Most users mix all cables in one cable tray and that often leads to unreliable behaviour. Cables that cross each other will not cause problems. Creating distance is not the only requirement. Correct shielding is also a must!
Can you give some basic tips for installing a PROFIBUS DP network?
• Always use PROFIBUS cable and connectors.
• Do not exceed 32 devices per segment (including repeaters, OLMs and couplers).
• Make sure the segment length is in contrast with the baudrate.
• Make sure every segment has powered termination on both ends.
• Avoid spur lines.
• Avoid swapping the wires (A=green, B=red).
• Mark how long the cables really are and update the drawings.
After installation you should test your work:
• Are the addresses correctly set?
• No short circuit or break in the cable?
• Can you communicate with the devices?
Can I install PROFIBUS DP cable in a PA network?
Lots of users have the impression this will never cause a problem because of the high speed signal characteristics of DP cable (even if the impedance is not correct), but there are other factors that could make it very unstable:
• There will be too much voltage loss at the end of the cable because of the higher loop resistance of standard DP cable (110 Ohm/km). PA cable has a loop resistance of 44 Ohm/km. When it is an Ex coupler with only 12.xV output, there is a change the last devices "get lost" all the time".
• The DP cable gets over-powered because of its "thin" 0.64 mm diameter wires. PA cable wires have a diameter between 0.8 to 1.5 mm.
• PA Coupling components cannot handle the "thin" wires of the DP cable.
• When it is an Ex installation, the DP cable is probably not blue and not suitable for the FISCO Ex specs.
Our advice is to use only cable that complies with the PA specifications!
What is the minimum distance between 2 devices on PROFIBUS DP?
When the transmission speed is 1,5 Mbps or higher, it is highly recommended to have at least 1 m of cable between 2 devices. De input capacitance of both devices will be compensated by the cable in order to preserve the common impedance. When the devices would be very close together, there is a big chance the input capacitance causes reflections in the data communication (small short circuits). The effect is much less at transmission speeds lower than 1,5 Mbps.
Does PROFIBUS DP cable always have a violet colour?
NO, because of certain application, protection and isolation classes, PROFIBUS DP cable can have another colour. Example: ground cable and shipboard cable are black, robust cable is green. The standard cable is always violet!
Which pins on the DB9 connector are for communication?
• Pin 3 = B line (red wire)
• Pin 8 = A line (green wire)
The metal casing of the connector is used for the shielding.
What are the advantages of fibre optic against copper wire?
With fibre optic:
• Cable lengths can be longer.
• Complete electrical isolation.
• Elimination of possible ground loops.
But, fibre optic is more expensive than copper (cabling, installation and the required couplers)!
How do you terminate devices that have an M12 connector?
There are 3 ways to terminate devices that have an M12 connector:
- When it has two onboard M12 connectors, put an M12 terminator on the second connector.
- When it has one M12 connector, apply a t-connector that holds the termination and the last connector.
- When the device has onboard termination, switch it on.
Can 2 Profibus cables be installed inside the same metal conduit?
Yes, multiple (parallel) Profibus cables can be installed in a metal conduit. It gives good behaviour when EMC is involved.
Why is there only 1 terminating resistor in an RS 485-IS plug?
The PROFIBUS DP - RS 485-IS standard (for Ex environments) has defined that the pull-up and pull-down resistors have to integrated in the devices. This way they are always charged and can never cause a problem when somebody is moving the termination switch on a plug and it is more difficult to make a direct short circuit on the connector pins of the device (explosion protection). The resistor values are also different: 510 - 200 - 510.
It looks very confusing compared to regular PROFIBUS DP where the plugs contain 3 resistors. But it is correct!
Check this link to see a schematic of the RS 485-IS terminating resistors in a device with a DB9 connector.
When you have a device or repeater with on-board termination and is equipped with a DB9 connector, what is the best place to activate the termination (when it is the last device)?
The plug is the best place! When you remove the device from the bus you still have the termination in the plug which is still attached to the bus cable. Maybe the 5V power is gone (which is always required), but better this than nothing at all.
Plug is ON, device is OFF.
|Автор:||san [ 06 лютого 2015, 22:49 ]|
|Тема повідомлення:||Re: Правила монтажу і прокладки кабелів Profbus DP|
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