5 spur lines with profibus dp, Profibus – Bronkhorst PROFIBUS-DP slave interface User Manual
Page 10
BRONKHORST
®
2.5
S
PUR LINES WITH
PROFIBUS
DP
(Also known as stubs or branches)
In table 5 the maximum spur capacitance is set out to the baud rate.
With the use of PROFIBUS cable type A this will result in the stated length.
Transmission rate
(kbit/s)
Total allowable spur capacitance
(nF)
Total Spur cable length* (ΣL
S
)
(m)
19.2
15
500
93.75
3.0
100
187.5
1.0
33
500
0.6
20
1500
0.2
6.7
> 1500
none
None
* Calculated for PROFIBUS cable type A at 30 pF/m
•
The maximal Spur line length is 0.25m
•
The total Spur line length of a segment represents the sum of all Spurs attached to that segment.
•
The minimal Trunk line length should be longer then the sum of all Spur lines
•
Spurs on DP segments should be avoided where possible.
•
Each PROFIBUS device will have a short Spur line carrying the bus signals between the external bus
connector and it’s RS 485 driver chip (for calculations use about 5cm per node).
•
Devices are tested for reflections as part of the certification process.
•
Uncertified devices can cause reflections.
Example:
We are installing a M12 system with 19 nodes and a spur length of 25 cm.
Additional 30 Sub-D instruments are installed.
Transmission rate at 1500 kbits/s.
Is this allowed?
(19 nodes x 25cm + 19 nodes x 5cm) + 32 nodes x 5cm = 7.3m
The maximum spur length at 1500 kbits/s is 6.7m so this is not allowed.
When spur lines are installed, additional termination at the end of a spur-line should not be used!
The rule that we must never exceed two terminations per segment, still applies even if using spurs.
References:
Spur-lines are not allowed when using higher baud rates (>1.5Mbit/s).
At baud rates lower than 1.5Mbit/s, spur-lines are allowed up to the maximum capacitance for the
baud rate.
Page 10
PROFIBUS DP interface
9.17.025