ATI Q46 Modbus Communications Manual
9
O&M Manual
Rev-D (2/16)
1.9 Drops
Often, a short length of cable is used at a junction box to form a branch, or “drop”, from the bus to
the device. These cables must very short as compared to the main trunk length of the bus so as
to avoid signal reflections and require termination that would load the bus excessively. A rule of
thumb is to not allow any single branch length to exceed 3% of the total trunk length. Again, only
the devices at each end of bus require termination resistors, intermediate connections along the
bus do not (bias resistors not shown for clarity).
Long branches requiring termination may be connected, however, a repeater must be used at a
short distance from the connection. Star topologies should be avoided, since terminating each
spoke will load the network excessively and reliable communications cannot be guaranteed.
1.10 Daisy Chaining
For devices not located at the ends of the bus, it may be possible to run the cable in and out of
the device, a practice referred to as “daisy-chaining”. Although this method eliminates the need
for a separate drop wire, it will require more connections inside the transmitter housing and
therefore consume more space.
Device
1
Device
n
Device
2
Termination resistor
not required at this
2. A(-)
Device
1
Device
n
Device
2
Termination resistor not
required at this device.
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