I have a couple of questions regarding the field installation of a Profibus DP network:

1) Are active terminators required at both the source and destination ends of each segment, or at one end only? If needed at both ends, why?

2) We need to make some outdoor tee (T) type junctions so that we can remove/isolate field equipment without breaking the bus. Are there any special technical requirements for this tee or can it be done using a simple junction box and ordinary terminals? If special T's are required, do you know of any standard equipment and why is this required?

3) Are there any maximum take-off distance requirements between the T and the field devices?

Regards,
James Stokes

 

You must have terminators at the extremes of your network, doesn't matter how many segments you have, if that's the case that you use tree topology, you must have a terminator at each end, so the signal doesn't is reflected and transmission error occurs.

i hope this help...

you can use repeaters to make a tree topology, and i don't think you will have problems with the length if you follow the rules for rs485 electrical transmission

Regards,
Lalo

 

Hello James;

This is just my 2 cents worth; for more detailed answers to your configuration questions, please download the Profn2_e.pdf manual from the Siemens support webpage at:

http://www4.ad.siemens.de/WW/llisapi.dll?func=cslib.csinfo=en&objID=4000024

Briefly:
1) Yes, active termination is required at both ends of a segment; this is a requirement brought over from RS-485 technology. If I understand the process correctly, reflections are the most disruptive elements on a RS-485 network. You want the data to transit once over the bus, then disappear at the termination. Note that an active termination means a powered circuit; voltage is supplied through the connector to feed the termination circuit. If you loose power to the terminator (by removing power to the last PLC on the bus for example) the whole bus structure will collapse. If that situation is possible (for regular maintenance purposes...) then an active bus terminator is available to power up the terminating circuit independantly of the PLC.
2) Siemens calls your T-junction a tap line. They are not recommended, especially at high speeds (over 1,5 MBauds), and really not necessary. Almost all Profibus connectors come in 2 types: with or without a piggy-back connector. Those that have the piggy-back connector allow you (through a short length of Profibus cable and Profibus connectors, or through a pre-made Profibus cable or through an MPI cable)to hook up a new partner directly at the connectiopn point of the connector. I have even seen installations where there is a connection point set-up with a Profibus connector in a connection box. But there are more elegant ways of doing that. Look up www.profibus.com.
3)The connection points are directly installed on the Profibus connectors, so the installation distance of the tap lines is not a problem.

Hope this helps,
Daniel Chartier

 

Hi All

I have a great no of Km's of Profibus behind my back by now, and I have never used an active terminator, this is both good and bad, in some cases where I should have used an active terminator, there is a profibus plug hidden in the trunking with the terminator switched on and plugged into nothing. I suggest you read the Siemens manual on active terminators, downloadable from their web site, and then decide for yourself.

On August 4, 2003, James Stokes wrote:
>1) Are active terminators required at both the source and destination ends
>of each segment, or at one end only? If needed at both ends, why? <

The implementation of T connections in profibus is not recommended in the manuals I have read on the subject, however we have had to do this on some occasions for example when a connector does not have both "in" and "out"
terminals for connecting cables. There is a Siemens T connector in the Profibus PA range, I use this in preference to normal terminals as it automatically ensures continuity of the screen which is otherwise a messy
business.

>2) We need to make some outdoor tee (T) type junctions so that we can
>remove/isolate field equipment without breaking the bus. Are there any
>special technical requirements for this tee or can it be done using a
>simple junction box and ordinary terminals? If special T's are required, do
>you know of any standard equipment and why is this required? <

If you must make them, keep them short.

Donald Pittendrigh

 

Hi,

at first, Profibus is a bus not a net, so you can just have a line structure for your devices if you are not willing to spend mony for repeaters.

Becaus of physical/electrical reasons (reflections) you need to terminate both ends of the bus.

The maximum length of a segment depends of the baudrate of the bus (e.g. 100 metres with 12 Mbit/s or 1200 metres with 19.2 kBit/s).

Tees and longer distances are possible with using repeaters ether from siemens (www.siemens.com) or Systeme Helmholz (www.helmholz.com).
Connectors with terminators are also available.

Best regards,
Konni ([email protected])

 

Yes to the first Question, but why I don't know.
Question 2 you can buy Turk interlinkBt they all types of Profibus connectors.

 

James,
It sounds like you need some installation training why not attend the one-day Certified PROFIBUS Installer Course and learn about all the pitfalls before you make expensive mistakes? See www.profibus.com or email me at [email protected] for details.

> 1) Are active terminators required at both the source and destination ends of each segment, or at one end only? If needed at both ends, why?

Yes terminators are required at both ends of every segment. Why - to stop reflections from the end of the cable. Signals will reflect from any point on the cable where the characteristic impedance changes - particularly at each end.

> 2) We need to make some outdoor tee (T) type junctions so that we can remove/isolate field equipment without breaking the bus. Are there any special technical requirements for this tee or can it be done using a simple junction box and ordinary terminals? If special T's are required, do you know of any standard equipment and why is this required?

It is not a good idea to use Tee junctions on DP. The wiring should be a linear bus, daisy chained from device to device. Spurs are allowed at lower baud rates (1.5Mbit/s or less) but only up to a maximum length. The preference is never to use spurs if possible since they can cause reflections.

You should use special PROFIBUS connectors which allow you to remove devices without disrupting the bus. Most manufacturers will supply these but some are better than others.

> 3) Are there any maximum take-off distance requirements between the T and the field devices?

At 1.5Mbit/s about 6m total (i.e. adding all spur lengths together). More at lower baud rates.

Regards, Andy Verwer, PROFIBUS Competence Centre, Manchester Metropolitan University.

 

Lalo

The question was about ACTIVE TERMINATORS, when would you recommend their use?

Donald Pittendrigh

 

Hi Donald,

I hope for you, you have a good insurance because PB without terminations at the end is a mess.
You get errors (sometimes after days or weeks) and nobody knows why...
Lets say BMW or Mercedes stops a part of the production just for 60 minutes because you did not read the Profibus spec right - 1,000,000$ to pay is nothing!
I'm developing PB devices and making support for them - I never hered 'you don't need termination'.

Perhapes you mean the device 'active terminator' from Siemens (or other)? You just need this if you have a slave at the end of your segment which is powered down during other PB parts still running?
Reason: Each PB device (master or slave) provides 5 VDC for termination and each PB connector provides a switch for the termination. So PB is easily to terminate at both ends (it is an active termination!) if the PB device at the end is powered on. If it is not powered on all the time pleas use the black box called 'active terminator'.

BTW: using tees is also a mess. In fact the PB spec says by using 12 MBit/s stubs with a total length of 12 cm (around 5 inch)are allowed. Unfortunetely you have already more as 12 cm if you have a PB segment with 32 members, because each connector is physically a short stub.
With a baudrate of 187.5 kBit/s the sum of stubs can be 1.5 meters (around 60 inch).

Hope I could explain clearly what really matters.


Best regards,
Konni ([email protected])

 

Hi Konni

Yes in fact I do have extremely good insurance, Loyds of London if you must know, your reply to my mail shows that you have paid little attention to the thread, which was a question about ACTIVE TERMINATORS and their use as opposed to passive terminators, I did not and would not advocate the use of Profibus without terminators, or I could hardly claim, as I did in the same mail, to have several hundred kilometers or stations or something like that, running in plants I have worked on. Generally I am skeptical about the use of active terminators, and yet to see an intelligent reply regarding their use on this thread, yours included, I will be very pleased to hear someone say, I have used active terminators here, here and here, because nothing else worked, then we will be getting closer to a real answer to the question, and may all learn something constructive.

With regards to your advice about use of active terminators in regard to powered down slaves, I have made the statement, also in the same mail, that a Profibus plug also serves as a terminator in the case of devices which have the problem you have described, this type of terminator is obviously plugged into nothing and therefore cannot be regarded as an active terminator, however in my (considerable) practical experience, this has always worked to solve the problem, and has never given rise to sporadic problems. I will agree with anyone who says this is a shabby or unprofessional solution, but it is also a good deal cheaper than an active terminator, what have you to contribute on this subject, why does this solution work, does it only work for a particular baud rate, cable length, what useful contribution can you make in this regard?

Regards
Donald Pittendrigh

 

Most people probably know this, but the Profibus "terminator" is in fact both a termination and biasing device. The biasing part is what requires the 0-5V, and cleans up the edges on the signal to allow transmission at high speed over long distances. The termination part is just termination (i.e. to stop echoes). Thus you can get away with just a plug, because the more or less essential part (the termination resistor) is provided even though the biasing doesn't work. The biasing will improve matters but may not be absolutely necessary depending on the distances involved in the installation and so on.

Active terminators are good because they mean that the network is resilient to cases where the slaves at the extreme ends (onto which a 'passive' termination/biasing network via a switched 9 way dtype is fitted) are powered down. But they only need to be a 5V PSU and three resistors.

Tim Linnell (Eurotherm)