Highway Topology; Fig 12: Rs-485 Data Highway Topology - rotork Modbus IQ Pro Technical Manual

Modbus MFU Option Card Installation Manual

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.
PLC
Spur
T
PLC
T

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)
(Note that each actuator includes up to 0.6 metres of Drop length)
Actuator Type
Internal Drop length (metres)
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.
18 of 76
Tap Tap
Trunk Cable
300
600
1k2
2k4
4k8
9k6
19k2
38k4
57k6
115k2
Publication PUB091-003-00_0119
Tap
Trunk Cable
Maximum
Segment Length
24.0 km
12.0 km
12.0 km
6.0 km
3.0 km
1.5 km
1.2 km
1.0 km
750 metres
500 metres
IQ / IQT
0.6
Tap Tap
Total Spur
Length
2.0 km
1.0 km
1.0 km
500 metres
500 metres
500 metres
500 metres
300 metres
200 metres
100 metres
SI / EH ROMpak / Q
0.6
T
T
0.3