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3 VPC3+/C Extension Board
Figure 3-9: Assignment of the RS485 PROFIBUS Connector
To prevent reflections and associated communication problems the
bus cable must always be terminated with its characteristic imped-
ance
3.6.2 Fiber Optic Interface
The fiber optic (FO) cable transfers signals by means of electromagnetic
waves at optical frequencies. Total reflection will occur because the refrac-
tive index of the coating of the fiber optic cable is lower than that of the ker-
nel. Due to the total reflection the ray of light cannot escape from the fiber
optic conductor and it will be conducted to the end of the fiber optic cable.
The fiber optic system employs pulses of monochromatic light at a wave-
length of 650nm. The fiber optic cable can be used in the same manner as
any normal cable and it is not susceptible to external electrical interference.
Fiber optic systems have a linear structure. Each device requires two lines,
a transmit and a receive line (two-core). It is not necessary to provide a
terminator at the last device.
The FO PPROFIBUS interface of the VPC3+/C extension board has four
sockets. These sockets are used to connect the PROFIBUS slave directly
to the PROFIBUS system. The PROFIBUS FO network supports a maxi-
mum of 126 devices (including the master). The maximum distance be-
tween two devices is limited to 50 m, the maximum baud rate is 12 Mbit/s.
Advantages of FO over copper cables:
20
Pin
1
2
1
3
6
2
4
7
5
3
8
6
4
9
7
5
8
9
wide bandwidth
low attenuation
no crosstalk between cores
immunity to external electrical interference
no potential difference
lightning protection
may be installed in explosive environments
Revision 2.00
Assignment
n.c.
M24V
RxD/TxD-P
CNTR-P
GND
5V
P24V
RxD/TxD-N
n.c.
Copyright © profichip GmbH, 2005
VPC3+ Evaluation Kit
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