Detcon 880S-N4X - Manual (Page 37)

880-N4X
880-N4X Instruction Manual Rev. 1.0 Page 33 of 40
The characteristic impedance of the cable must be between 100 to 120 ohms. Twisted pair is used because if
the cable does run near a noise source both conductors will pick up the same amount of noise; therefore,
effectively canceling it out.
Incorrect Wiring Schemes
Among the biggest problems with an RS-485 bus is the use of incorrect wiring schemes mixed with improper
or no line termination.
Most existing installations do not have wire runs in a daisy-chain like fashion; instead, wire runs typically
originate from a central point and run to different devices in a star-like pattern. This is a very undesirable way
to wire an RS-485 bus. The chances of problem free operation over the life of the equipment decreases
dramatically the further you stray from a straight daisy-chain wiring scheme. The bus needs to be terminated
at each end of the cable run to operate ideally. Other wiring schemes introduce multiple “cable ends” making
it impossible to balance the lines. Wiring as in Figure 39 (A, B & C) would provide reliable data flow.
Figures D, E & F could possibly work, but would require many repeaters and an experience worker with RS-
485 communications and networks knowledge to achieve implementation.
Figure 39 Correct and incorrect wiring schemes
Figure 40 shows a typical RS-485 communication setup using 2-wire digital communication. Each RS-485
loop should have (1) beginning and (1) end, with a 120-ohm resistor at the end of each loop.