Elgas EDT 101, User Manual

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RS 485
Host
DATA+
DATA-
Power
Suppl
+
-
Transducer Transducer Transducer
D
A
T
A+
D
A
T
A+
D
A
T
A+
D
A
T
A-
D
A
T
A-
D
A
T
A-
PW
R
PW
R
PW
R
G
N
D
G
N
D
G
N
D
....
Fig.3 Transducers connection into data network.
To achieve low power consumption, the transducer uses a special RS 485 driver with limited
slew rate. It is recommended to leave the RS 485 bus unterminated to maintain power
consumption as low as possible. The bus impedance must not be lower than 1.5 kΩ. The length
of the bus should not exceed 100 m. Otherwise, a signal distortion due to reflections can occur
and communication reliability can be deteriorated.
Any standard RS 485 driver / receiver can be used in the host, however for the low power
applications with unterminated bus it is recommended to use a driver with a reduced slew-
rate and a receiver with a high input impedance. Suitable types are MAX 3471 form Maxim
Integrated Products or ADM 483E from Analog Devices.
3.3 Cabling, grounding
The transducer is interconnected with the host using a shielded 4-wire. The cable should not
be laid along power cables, lightning conductors and long metal objects, it should not pass
through areas with strong electro-magnetic disturbances or discharges.
The cable shielding on the side of the host should be connected with ground and with the host
enclosure, if this is metallic. The cable shielding is not connected to the transducer stainless-
steel stem.
Host
Transducer
Fig. 4. Transducer grounding