Building Your Network - Siemens Simatic S7-200 System Manual

S7-200 Programmable Controller 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.
7
Determining the Distances, Transmission Rate, and Cable for Your Network
As shown in Table 7-4, 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 7-4
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.
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 the
network another 50 m. If you connect two repeaters with no other nodes in between (as shown in
Figure 7-18), 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 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 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.
218
Maximum Length for a Network Cable
Non-Isolated CPU Port
1
CPU Port with Repeater or EM 277
50 m
Not supported
Not supported
Not supported
1,000 m
400 m
200 m
100 m