Section 2 Transmission Technology MBP network limits
3BDS009029R5001 B 35
MBP network limits
The maximum of connected devices per single segment is limited to 32. This is a
theoretical value, because this number can be further determined by the protection
type selected and the bus power.
In intrinsically safe applications the power (maximum voltage and current) is
defined with strict limits. For example the current in hazardous area’s restricted to
100 mA and in non intrinsically safe networks to 400 mA. To determine the
maximum line length, is necessary to calculate the power consumption of the
connected devices and to specify a power supply and the line length for the selected
cable type (see also MBP power and line length calculation on page 36).
The required current is summarized from the power consumption of all connected
field devices plus a reserve of 9 mA per segment for the operating current of the
FDE (Fault Disconnection Electronics). The FDE prevents faulty devices
permanently blocking the bus.
The maximum line length of the network comprises the total of the main line (trunk)
and all spur lines.
A PROFIBUS PA field device requires a minimum input voltage of 9 V. With an
unfavorable distribution of stations, for example if all PROFIBUS PA stations are
widely removed from the Linking Device, it can happen that the voltage drop along
the line is so great that the voltage level at the end is insufficient. This results in a
shortening of the transmission line or the necessity of using cable with a larger
cross-section.
Table 5. Limits of MBP networks
intrinsically safe non intrinsically safe
Measurement voltage 12,6 V ... 13,4 V 24 V ... 26 V
Measurement current 100 mA 400 mA
max. PROFIBUS PA slaves 10 32
max. line length 1000 m 1900 m
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