Siemens SIMATIC S7-200 System Manual page 236

S7-200 Programmable Controller System Manual
Using Repeaters on the Network
An RS-485 repeater provides bias and termination for the network segment. You can use a
repeater for the following purposes:
To increase the length of a network: Adding a repeater to your network allows you to extend
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the network another 50 m. If you connect two repeaters with no other nodes in between (as
shown in Figure 7-21), you can extend the network to the maximum cable length for the
baud rate. You can use up to 9 repeaters in series on a network, but the total length of the
network must not exceed 9600 m.
To add devices to a network: Each segment can have a maximum of 32 devices connected
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up to 50 m at 9600 baud. Using a repeater allows you to add another segment (32 devices)
to the network.
To electrically isolate different network segments: Isolating the network improves the quality
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of the transmission by separating the network segments which might be at different ground
potentials.
A repeater on your network counts as one of the nodes on a segment, even though it is not
assigned a network address.
Segment
50 m
Figure 7-21 Sample Network with Repeaters
Selecting the Network Cable
S7-200 networks use the RS-485 standard on twisted pair cables. Table 7-6 lists the specifications
for the network cable. You can connect up to 32 devices on a network segment.
Table 7-6 General Specifications for Network Cable
Specifications
Cable type
Loop resistance
Effective capacitance
Nominal impedance
Attenuation
Cross-sectional core area
Cable diameter
222
Segment
RS-485
Repeater
Up to 1000 m
Description
Shielded, twisted pair
±115 Ω/km
30 pF/m
Approximately 135 Ω to 160 Ω (frequency =3 MHz to 20 MHz)
0.9 dB/100 m (frequency=200 kHz)
2 2
0.3 mm to 0.5 mm
8 mm ±0.5 mm
Segment
RS-485
Repeater
50 m