Modbus Option Card Properties
Publication
PUB091-004-00_0918
23 of 64
Termination is passive (with no biasing resistors) on the single and dual channel MFU cards. The
biasing resistors are fitted to the single channel-and-repeater MFU card, so that the termination is
active when enabled.
When power is removed from the actuator, the termination network(s) is no longer connected
across the data highway. For this reason, we suggest a separate external active terminator is
used on the highway at one end or the other of the main trunk line.
4.5 Inter-connecting the Highway and Setting up the MFU
The three MFU variations (Single, Dual and Single Repeater) allow for various different highway
connections. The ‘single channel version’ can be used for simple highways. The ‘dual channel version’
is used for redundant highway applications, where the highway integrity is important
and the ‘single-
highway-with-
repeater version’ can be used for applications where distances are extended beyond the
values listed on page 20 or where a ring type topology is desired.
4.5.1
Single Highway
With the single channel option the actuator is connected to only one highway. The connection of the
actuator to the highway uses terminals 1A and 1B. The terminal numbers depend on the actuator type
and are indicated on the actuator wiring diagram. Take care to ensure that the correct polarity is
observed on the data highway connection.
Data line 1A is positive with respect to data line 1B when the MFU is transmitting a ‘1’.
There are no links or settings required on the MFU card when this version is being used.
4.5.2
Dual Highway
The actuator can be connected to two highways when redundant communication links are required.
The connection of the actuator to the highways uses terminals 1A and 1B for highway 1 and 2A and
2B for highway 2. The terminal numbers depend on the actuator type and are indicated on the
actuator wiring diagram. The last actuator on the highway should have the termination resistors
switched on. Take care to ensure that the correct polarity is observed on each data highway
Each data
line A is positive with respect to data line B when the MFU is transmitting a ‘1’.
The dual channel MFU uses a separate Modbus address for each channel, so it is possible to use the
same address or different address on each channel. It is possible to communicate with both channels
at the same time.
Note:
When using the single highway with internal repeater the termination should always be
fitted (or selected) on both sides of the repeater, since it will be the beginning and the
end of a highway segment. Each terminator on the repeater is biased on the card.
