[Network installation] I, U, max length...

Hi guys!
Have been reading Profibus PA network installation guide and cannot understand the following:
IEC organisation published in IEC 61158-2 maximum spur cable length by number of fieldbus devices connected to the segment:

Number of devices: Spur cable length:
1-12 120m
13-14 90m
15-18 60m
19-24 30m
25-32 1m

What is described as well by IEC is maximum current consumed by all devices (hence max devices limit) and minimum voltage available for them (after cable voltage drop) - hence maximum spur length.

Everything if fine, but what sort of calculations must have been done to create above table? Why for 24 nodes one can have 24 devices, but when wants too add only one extra device the max spur length is limited 1m?
Am I missing some important factors that link voltage drop with current capacity?

luccass said:

Hi guys!
Have been reading Profibus PA network installation guide and cannot understand the following:
IEC organisation published in IEC 61158-2 maximum spur cable length by number of fieldbus devices connected to the segment:

Number of devices: Spur cable length:
1-12 120m
13-14 90m
15-18 60m
19-24 30m
25-32 1m

Click to expand...

This is not the maximum cable length but the maximum length of the spur line depending on the number of spurs (not instruments). Also be aware that this is only valid for non Ex zones. For Ex zones it is 1m for 25-32 spurs and 30 m for less than 25 spurs. Just for the info, the maximum cable length is 1000m for zone 0 and 1900m for zones 1, 2 and the non Ex zones.
Basically you have a main trunk line from start to end with termination resistor (R+C for PA) on both ends. The spur lines do not have these terminators and cause reflections. The more spurs you have and the longer they are, more reflections will occur on the network. And off course more reflections means a bigger chance for faulty messages.

luccass said:

What is described as well by IEC is maximum current consumed by all devices (hence max devices limit) and minimum voltage available for them (after cable voltage drop) - hence maximum spur length.

Click to expand...

The maximum current is limited by the PA coupler you use. And since every instrument uses a certain amount of current this limits the number off devices you can use on a coupler. Every device also needs a minimum voltage to function properly. The coupler has a certain supply voltage and the cable has a resistance and there is a current flowing through the cable. So you can calculate the voltage on the end of the line and this has to be higher than the minimum voltage an instrument needs.

luccass said:

Everything if fine, but what sort of calculations must have been done to create above table? Why for 24 nodes one can have 24 devices, but when wants too add only one extra device the max spur length is limited 1m?

Click to expand...

I don’t think that there is one “all explaining formula” behind the table. There will be a fair amount of experience behind those numbers. Your system probably still work when you use 25 devices and 20 meter spur lines. But if you follow the rules and the tables you will be sure that everything will work fine.

I hope this answers some of your questions

Pepperl and Fuchs has a nice software tool for calculating a PROFIBUS PA network on their site.
www.segmentchecker.com

Jobbe said:

This is not the maximum cable length but the maximum length of the spur line depending on the number of spurs (not instruments). Also be aware that this is only valid for non Ex zones. For Ex zones it is 1m for 25-32 spurs and 30 m for less than 25 spurs. Just for the info, the maximum cable length is 1000m for zone 0 and 1900m for zones 1, 2 and the non Ex zones.

Click to expand...

Where did I mention about cable length? Even if, I still meant the spur cable length.
Anyway - I do know this restrictions, but still want to know it 'deeper'.

... the longer they are, more reflections will occur on the network.

Click to expand...

Can you explain above please?

I don’t think that there is one “all explaining formula” behind the table. There will be a fair amount of experience behind those numbers.

Click to expand...

I don't expect one formula, just factors and conditions taken into account when desinging 'the spur lenght table'.

Your system probably still work when you use 25 devices and 20 meter spur lines. But if you follow the rules and the tables you will be sure that everything will work fine.

Click to expand...

OK, but going from 24 to 25 devices might double the cost of the system in worst case. But from the other hand the system might work fine, that's why I'd like to know more, to be able to assess the risk.

I hope this answers some of your questions

Pepperl and Fuchs has a nice software tool for calculating a PROFIBUS PA network on their site.
www.segmentchecker.com

Click to expand...

Yes, I do know it, but it still uses IEC regulations and gives you only 'correct/wrong installation' answer.

Thanks for your answer

I would be careful to add to many devices to a PA network. It seems that this bus is quite vounerable regarding emc (have experienced problems myself, it was about 10 nodes, ca 50m bus length, short spurs (apx 2m)). I saw somewhere, can't remember where - some forum, that the FF is very restrictive with the max recomended device number pr. segment compared to PA.

Kalle