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How are nodes networked in the RS-485 standard?
The RS-485 standards suggests that its nodes be networked in a daisy-chain, also known as party line or bus topology (see Figure 1. In this topology, the participating drivers, receivers, and transceivers connect to a main cable trunk via short network stubs.
What are the specs for a RS-485 bus?
Having more than one driver accessing the bus simultaneously leads to bus contention, which must be avoided at all times. RS-485 drivers must provide a differential output of a minimum of 1.5V across a 54 load, while RS-485 receivers must detect a differential input with a minimum of 200mV (Figure 2).
Why is a printed circuit board required for RS-485?
Beyond network cabling, it is mandatory that the printed circuit board layouts and the connector pin assignments of RS-485 equipment keep both signal lines close and equidistant to another to maintain the network’s electrical characteristics
Which is the physical layer of RS-485?
As an electrical-only standard, RS-485 is commonly referenced by higher level standards as their physical layer. Bus nodes are networked in a daisy-chain, or bus topology (Figure 1). That is, each node connects to the main cable trunk via short stubs.
How to check termination in a RS-485 network?
Termination causes more problems than it solves in the vast majority of RS-485 networks. To check which nodes are terminated, power down each one and disconnect it from the network. Use an ohmmeter to measure between the A and B or + and – lines of the receiver. Terminated nodes will usually read less than 200 Ohms.
What is the impedance of a RS-485 cable?
Industrial RS-485 cables are of the sheathed, unshielded, twisted-pair type, (UTP), with a characteristic impedance of 120 W and 22–24 AWG. Figure 4 shows the cross-section of a four-pair, UTP cable typically used for two full-duplex networks.
How to check biasing on a RS-485 network?
To check the biasing, measure the voltage from B to A with the network powered up and idle. At least 300 mV is required to keep well away from the undefined state shown in the graphic. If no termination resistors are installed, bias requirements are very forgiving.