Modbus MFU Option Card Installation Manual
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Publication PUB091-004-00_0918
With RS-485 the data passes over a single 2-wire cable, but the Modbus protocol is such that there
are periods between messages when no devices are actively driving the lines. In order to ensure that
data continues to flow correctly after these periods, it is advisable to ensure the lines are biased to
suitable voltage levels during the time the line is idle. This is usually achieved by biasing at the PLC,
which forms the active terminator. The MFU does not contain biasing components and therefore
requires biasing to be made externally. If the PLC does not do the biasing, then an active terminator
must be fitted to the network highway and it will provide the necessary biasing.
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 spur 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 stub lines to be kept within the
permitted maximum for the network speed to be used. A combination of both topologies may be used.
Topologies, such as ring and star are non-standard and should not be used with standard Modbus
hardware. However, a ring network can be achieved using the single channel with repeater hardware.
It is good practice to avoid spur lines wherever possible.
PLC
T
T
SPUR
TAP
TRUNK CABLE
T
T
TAP
TAP
TAP
TAP
TAP
PLC
T
T
TRUNK CABLE
T
T
Fig 12:
RS-485 Data Highway Topology
The maximum trunk length, length of each spur and the cumulative total are dependent on the system
speed in use. The table below suggests the maximum figures for copper cable. The segment length is
the total of the trunk and all the spurs added together.
Data Rate (Baud)
Maximum Segment Length (m)
Total Spur Length (m)
9,600
1,500
500
19,200
1,200
500
38,400
1000
300
57,600
750
200
115,200
500
100
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.
