Installation 2-13
3300 ACM Installation and Operation Manual Power Measurement Ltd.
Calculating Overall Cable Length
When determining the overall length of an RS-485 communi-
cation straight-line or loop connection, it is important to
account for all cable segments. For example, when RS-485
connections to the device are made via an intermediate
terminal block (Figure 2.6.1), the lengths of cable between the
device and the terminal block must be added to the total cable
distance. This length is equal to 2 times distance X in the
diagram.
Connection Methods to Avoid
Any device connection that causes a branch in the main RS-
485 bus should be avoided. This includes star and tee (T)
methods. Refer to Figure 2.6.3 for examples. These wiring
methods will cause signal reflections that may cause interfer-
ence.
RULE OF THUMB
At any connection point on the RS-485
bus, no more than two (2) cables should be
connected. This includes connection
points on instruments, converters, and ter-
minal strips. Following this guideline en-
sures that star and tee connections are
avoided.
Recommended Topologies
Devices on an RS-485 bus are connected in a point-to-point
configuration, with the (+) and (-) terminals of each device
connected to the associated terminals on the next device. This
is illustrated in Figures 2.6.2.
While there are many topologies that can be used to connect
devices on an RS-485 communication bus, the two recom-
mended methods are the straight-line and loop topologies.
STRAIGHT-LINE TOPOLOGY
The straight-line wiring method is illustrated in Figure 2.6.2.
The converter can exist at any position on the RS-485 bus,
including an end point.
To reduce signal reflections that can corrupt data on the bus,
each end point of the straight-line bus must be terminated.
Termination resistors are connected between the (+) and (-)
terminals of the device at each end of the bus. These resistors
should have a rating of 1/4 Watt, and have a value which
matches the line impedance of the cable being used. For 22
AWG (0.4mm
2
) shielded twisted pair cable, values between
150 and 300 ohms are typical. Consult the cable
manufacturer’s documentation for the exact impedance of
your cable.
LOOP TOPOLOGY
The loop wiring method is illustrated in Figure 2.6.2 The
converter can exist at any position on the RS-485 bus.
One advantage of the loop topology is that a single open
circuit fault condition anywhere on the loop will not result in
the loss of communication between the computer station and
any of the remote devices.
The loop topology does not require termination resistors at
any point on the bus.
Courtesy of NationalSwitchgear.com
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