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Serial interfaces
CER International bv 25
Network topology and wiring
Multi-drop RS-485 implies that there are two main wires in a segment labeled DATA+
and DATA– or simple A and B. The wiring comes as a twisted pair. To reduce electrical
noise, it should be twisted as tightly as possible. It is advisable to follow the standard
colors used for the communication lines: Yellow for DATA+ (A), Green for DATA– (B).
For safe operation, a signal ground line should also be connected in the system. All
connected devices tap from these two lines with so called drop cables. The connections
are parallel so connecting or disconnecting of a node does not affect the network as a
whole.
The network configuration isn't defined in the RS-485 specification. The designer can use
a configuration that best fits the physical requirements of the system. In existing systems
one should obey the rules of that network, of course.
A: Backbone with stubs (workable) C: Star network (avoid)
D: Mixed, backbone & star (avoid)B: Daisy chain (best)
If the traffic on the network is slow and the cable lengths are short, topology is not an
issue. But as soon as transmission speeds are higher or distances longer, you should be
aware of the physical network layout. The daisy chain configuration is the best choice.
The backbone and star networks are not impossible but it is hard to control reflections
and harmful transmission-line effects.
Line termination
Termination resistors are used to match impedance of a node to the impedance of the
transmission line being used. When impedances are mismatched, the transmitted signal
is not completely absorbed by the load and a portion is reflected back into the
transmission line. If the source, transmission line and load impedances are equal, these
reflections are eliminated.
Commonly used RS-485 cabling, such as Category 5 (CAT5) cable, has an impedance of
100 Ohm. When cables run longer than about 10 meters, termination resistors of 100
Ohm should be installed at the both ends of the cable.