ABB AC 800M, User Manual

Page: 73 / 118

Section 3 Fieldbus Topologies Bus length and speed
3BDS009029R5001 B 73
Bus length and speed
A twisted two-wire, shielded copper cable (type A) must be used as the transfer
medium. It is also possible to exchange and extend devices during operation. The
fixed, uniform transfer rate is 31.25 kBit/s. At this rate, cable lengths per segment of
up to 1900 m are possible in non Ex(Haz.) case, or 1000 m in Ex(Haz.) cases.
If power is supplied to field devices via the PA bus, partial voltage drops on the bus
lines must also be taken into consideration. This depends on the type of cable, the
length of the line and the power consumption.
For non-intrinsically safe applications, a typical 24 V DC power supply is used, and
there are only very minimal limitations on the total length of the line.
For intrinsically safe applications, the voltage of the power supply is approx. 12.8 V
DC (LD 800P). The resulting permissible line lengths must be observed so that the
necessary minimum operating voltage to the devices is not exceeded. The optimum
design of a fieldbus segment requires detailed calculation of the partial voltage
drops. By selecting a cable with a larger diameter it will be compensated to a certain
extent (see also
MBP power and line length calculation on page 36)
The number of subscribers that can be connected to a segment is limited to 32. An
additional restriction in the Ex (Haz.) area results from the limited supply current
that is available, which is determined by the type of explosion protection selected. In
addition to the basic currents of the field devices, the modulation signal of 9 mA, the
maximum current in the case of error of the device with the greatest power
consumption and a reserve for dynamic power on procedures when the power is
restored to the DP/PA Linking Devices must all be taken into account for calculating
the required current. The assumption is also made here that the devices are equipped
with an electronics system for limiting the current (FDE = Failure Detection
Electronics).
Provided that maximum one FDE responds, the following condition must be
fulfilled:
Supply current
≥
sum of individual currents (per device
≥
10 mA)
+ Manchester current signal (9 mA)
+ FDE fault current (0 to 9 mA)