Siemens S7-200 - Determining the Distances, Transmission Rate, and Cable

Connecting Multiple Devices on a Network
B.3 Determining the Distances, Transmission Rate, and Cable
Text Display (TD) User Manual
156 System Manual, 08/2008, A5E00765548-03
Tip
The display time slows as more TD devices are added to the network.
Network connectors are available from Siemens. Using these connectors allows you to
isolate the CPUs from one another (the 24 VDC), but still allows you to power the TD device
from the CPU. See the SIMATIC S7-200 Programmable Controller System Manual for more
information about using network connectors.
B.3 Determining the Distances, Transmission Rate, and Cable
Determining the Distances, Transmission Rate, and Cable
As shown in Table B-1, 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 B- 1 Maximum Length for a Network Cable
Baud Rate Non-Isolated CPU Port
1
CPU Port with Repeater or EM 277
9.6 kbaud to 187.5 kbaud 50 m 1,000 m
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 B-2), 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.