手册应细细地读：
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 7-21), 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 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.