IDL 101
STRUCTURE OF THE BUS TOPOLOGY
HB_IDL101_E_V222.doc
27
Gantner Instruments Test & Measurement GmbH
4.5. Bus Cable
For setting up a bus topology a shielded twisted pair with at least two leads and the following electric characteristic
values must be used:
characteristic impedance : 100 ... 130
Ω
at f > 100 kHz
operating capacity : max. 60 pF/m
conductor cross section : min. 0.22 mm² (AWG 24)
attenuation : max. 6 dB referred to the overall length
4.6. Bus Plug
For installing the bus cable and the bus interface, 9-channel D-subminiature plugs and sockets are used. The pin
assignment for the RS485 connection according to PROFIBUS is given in table 4.2.
plug pin RS485 notation signal meaning
1
5
6
9
DB 9
1
2
3
4
5
6
7
8
9
-
-
B / B´
-
C / C´
-
-
A / A´
-
Shield
RP
RxD/TxD-P
CNTR-P
DGND
VP
RP
RxD/TxD-N
CNTR-N
Shield, Protective Ground
Reserved for Power
Receive/Transmit-Data-P
Control-P
Data Ground
Voltage Plus
Reserved for Power
Receive/Transmit-Data-N
Control-N
Table 4.2 Pin assignment D-subminiature plug according to PROFIBUS
The signal leads A and B (and Shield) are absolutely obligatory for a (shielded) connection. All others can be installed
together with these signal leads if required.
4.7. Bus Termination
In order to avoid signal reflections on the bus, each bus segment has to be blanked off at its physical beginning and at
its end with the characteristic impedance. A terminating resistor R
t
is installed between the bus leads A and B for this
purpose. In addition to that the bus lead A is connected via a pull-up resistor R
u
to potential (VP) and the bus lead B is
connected via a pull-down resistor R
d
to ground (DataGround). These resistors provide a defined quiescent potential in
case there is no data transmission on the bus. This quiescent potential is level
high
.
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