FIEDLER H1, Руководство пользователя

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Hydro Logger H1 User manual
34
electronics for ecology
FIEDLER
4.4.6. General rules for RS485 bus wiring
Smooth use of the RS485 serial link requires compliance with established rules
for this type of communication:
SHIELDED TWISTED DATA CABLE
Use a suitable shielded 4- core data cable with an impedance of 50 Ω , consisting
of 2 pairs of twisted wires. In a 4 core cable, one pair is used for data and the
other pair is used either to power the connected sensor or to connect the signal
grounds (see below). Suitable cable types include, for example:
PAAR-TRONIC-
Li-2YCY 2x2x0.34 QMM
SINGLE-SIDED SHIELDED CABLE CONNECTION
The cable shield is only connected on the side (usually on the master side) be-
cause no current must flow through the cable shield. In the case of a double-
sided grounded shield, differing ground potentials could cause this unwanted
current to flow through the cable shield. The current flowing through the shield
would be induced into the internal conductors of the cable and cause noise on
the data bus.
CONNECTING SIGNAL COUNTRIES
A 2-wire interface does not mean 2 wires. Although the RS485 bus is referred to
as a 2-wire interface, it requires the connection of signal grounds on both sides
of the interconnect cable for proper operation in order to shut off the current
flowing through the data wires back to the exciter.
Usually this requirement is fulfilled in a short line when the master and slave are
located close to each other or have a common supply voltage, i.e. their SGND
signal grounds are connected.
The lack of interconnection of signal lands also has a significant impact on the
reliability of data communication , sometimes working and sometimes "inexpli-
cably" dropping out. If the second twisted pair of the data cable is not used to
power the sensor (the sensor is powered by a separate power supply at its in-
stallation location), then it is advisable to connect the two loose cores of the
data cable and use them to connect the signal grounds of both devices (the
lower ohmic resistance of the line will reduce the differential potential of the
signal grounds).
Correctly connected interconnection cable without sensor power supply and with
power supply voltage
If no wire is used to connect the signal grounds of both communicating devices,
the ground buffer current will be closed via the protective diodes of the RS485
exciters, which have only limited current immunity.
E n h a n c e d
i n t e r c o n n e c t i o n o f
s i g n a l c o u n t r i e s
M i s s i n g s i g n a l
g r o u n d
PO2D
surge protection of
RS485 data and
power wires