NOVUS RS422 Basics - Manual (Page 3)

RS485 & RS422 Basics
NOVUS PRODUTOS ELETRONICOS LTDA www.novusautomation.com 3/6
TOPOLOGY
If the signals on the network are slow, the bit edges are long, and the cable runs are short, topology is not an issue. As
soon as transmission-line effects begin to show up, there is only one simple topology for managing them. The following
figure shows several network topologies. To avoid signal reflections that cause communication errors, the Daisy Chain
topology is recommended.
This is not to say, for example, that it’s impossible to implement a star configuration with RS-485 devices. Keeping
reflections under control in a star topology is more art than science in a practical network. The best way to ensure a
robust and reliable RS-485 network is to build it around a daisy-chain configuration.
When a backbone with stubs topology is used, the stub length should be much shorter than the main bus length.
CABLE LENGTH AND DATA RATE
RS422 and RS485 specified maximum cable length is 1200 meters. The maximum working data rate (in bits per second
– bps) depends on network devices characteristics, cable capacitance and installed termination resistors. The longer the
communication cables, the lower the maximum data rate. As a rule of thumb, no problems are expected when the cable
length (in meters) multiplied by the data rate (in bps) is lower than 10
8 (100,000,000), for a well designed and installed
system.
The following figure illustrates the length x rate trade-off. System performance will change depending on the cable type,
termination, network topology, environment electrical noise and quality of line drivers and receivers in each device.