Modbus MFU Option Card Installation Manual
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Publication PUB091-003-00_0119
4.2 Highway Topology
Two different connection topologies can be used for RS-485. The first system is to use a trunk line
with various spurs or drop lines from it. This system will require the location of the taps in junction
boxes near to the actuators. With the second the actuators can be directly connected to the trunk line
in a daisy chain. All the configurations require the cumulative total of the drop lines and stub lines to
be kept within the permitted maximum for the network speed to be used.
It is good practice to avoid spurs wherever possible.
Fig 12: RS-485 Data Highway Topology
The maximum Trunk length and that for each spur and the cumulative total are dependent on the
system speed in use. The table below suggests the maximum figures for cable type A, also defined
below. The segment length is the total of the trunk and all the spurs added together.
Data Rate (Baud)
Maximum
Segment Length
Total Spur
Length
300
24.0 km
2.0 km
600
12.0 km
1.0 km
1k2
12.0 km
1.0 km
2k4
6.0 km
500 metres
4k8
3.0 km
500 metres
9k6
1.5 km
500 metres
19k2
1.2 km
500 metres
38k4
1.0 km
300 metres
57k6
750 metres
200 metres
115k2
500 metres
100 metres
(Note that each actuator includes up to 0.6 metres of Drop length)
Actuator Type
IQ / IQT
SI / EH
ROMpak / Q
Internal Drop length (metres)
0.6
0.6
0.3
Within each actuator there is a short drop line or stub connection wire from the terminals to the MFU
itself. This length must be included in any calculation for total and individual drop lengths.
PLC
T
Tap
Tap
Tap
Tap
Tap
PLC
T
Spur
Trunk Cable
Trunk Cable
T
T
