Building Your Network - Siemens SIMATIC S7-200 System Manual

Building Your Network

General Guidelines
Always install appropriate surge suppression devices for any wiring that could be subject to
lightning surges.
Avoid placing low-voltage signal wires and communications cables in the same wire tray with AC
wires and high-energy, rapidly switched DC wires. Always route wires in pairs, with the neutral or
common wire paired with the hot or signal-carrying wire.
The communications port of the S7-200 CPU is not isolated. Consider using an RS-485 repeater
or an EM 277 module to provide isolation for your network.
Caution
Interconnecting equipment with different reference potentials can cause unwanted currents to
flow through the interconnecting cable.
These unwanted currents can cause communications errors or can damage equipment.
Be sure all equipment that you are about to connect with a communications cable either shares
a common circuit reference or is isolated to prevent unwanted current flows. See the information
about grounding and circuit reference points for using isolated circuits in Chapter 3.
Determining the Distances, Transmission Rate, and Cable for Your
Network
As shown in Table 7-5, the maximum length of a network segment is determined by two factors:
isolation (using an RS-485 repeater) and baud rate.
Isolation is required when you connect devices at different ground potentials. Different ground
potentials can exist when grounds are physically separated by a long distance. Even over short
distances, load currents of heavy machinery can cause a difference in ground potential.
Table 7-5 Maximum Length for a Network Cable
Baud Rate
9.6 kbaud to 187.5 kbaud
500 kbaud
1 Mbaud to 1.5 Mbaud
3 Mbaud to 12 Mbaud
1
The maximum distance allowed without using an isolator or repeater is 50 m. You measure this distance from
the first node to the last node in the segment.
Communicating over a Network
1
Non-Isolated CPU Port
50 m
Not supported
Not supported
Not supported
CPU Port with Repeater or EM 277
1,000 m
400 m
200 m
100 m
Chapter 7
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