Siemens S7-300 Installation Manual page 75

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The following figure shows you where to connect the terminating resistors on the
MPI subnet  . In this example, the programming device (PG) is connected by
means of a stub cable only during commissioning or servicing  .
S7-300
PS PS ET 200 M
S7-300
PS PS ET 200 M
CPU CPU CPU
1
Installing Figure 5-19

Terminating resistor inserted

PG connected by means of a stub cable for maintenance purposes
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Disturbance of data traffic might occur on the bus.
A bus segment must always be terminated at both ends with the terminating
resistor. This, for example, is not the case if the last slave with bus connector is off
power.
Since the bus connector draws its power from the station, the terminating resistor
has no effect.
Please make sure that power is always supplied to stations on which the
terminating resistor is active.
As an alternative, you can also use the PROFIBUS Terminator as active bus
termination.
S7-300 Automation System, Hardware and Installation: CPU 312IFM - 318-2 DP
A5E00203919-01
PG
CPU CPU
CPU
1
S7-300
PS PS ET 200 M
CPU CPU CPU
OP 27
the terminating resistors in an MPI subnet
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1
RS 485-
Repeater
OP 27
2
PG
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S7-300
PS PS ET 200 M
CPU CPU CPU
1
5-49