Manual

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25
¾ RS485 Bus
¾ Introduction to RS485
RS485 are serial communication methods for computers and devices.
RS485 bus advances in simpler cabling,longer transmitting distance and higher dependability.
RS485 is the most versatile communication standard in the standard series defined by the EIA, as it
performs well on all four points.
¾ Differential signals with RS485
Longer distances and higher bit rates
As we know noise is easily picked up and limits both the maximum distance and communication speed.
With RS485 on the contrary there is no such thing as a common zero as a signal reference. Several volts
difference in the ground level of the RS485 transmitter and receiver does not cause any problems.
Noise in straight and twisted pair cables
In the picture above, noise is generated by magnetic fields from the environment. The picture shows the
magnetic field lines and the noise current in the RS485 data lines that is the result of that magnetic field.
In the straight cable, all noise current is flowing in the same direction, practically generating a looping
current just like in an ordinary transformer. When the cable is twisted, we see that in some parts of the
signal lines the direction of the noise current is the oposite from the current in other parts of the cable.
Because of this, the resulting noise current is many factors lower than with an ordinary straight cable.
Shielding—which is a common method to prevent noise in RS232 lines—tries to keep hostile magnetic
fields away from the signal lines. Twisted pairs in RS485 communication however adds immunity which
is a much better way to fight noise. The magnetic fields are allowed to pass, but do no harm. If high noise
immunity is needed, often a combination of twisting and shielding is used as for example in STP,
shielded twisted pair and FTP, foiled twisted pair networking cables. Differential signals and twisting
allows RS485 to communicate over much longer. With RS485 communication distances of 1200 m are
possible.
Differential signal lines also allow higher bit rates than possible with non-differential connections.
Currently RS485 drivers are produced that can achieve a bit rate of 35 mbps.