In-Situ Aqua TROLL 400 Modbus and SDI-12 Reference Guide - Manual (Page 10)

1.6 RS485 Network Guidelines
The manufacturer uses RS485 as its main digital communications link. RS485 is often used in
an industrial setting as a small device network. There are some installation guidelines to follow
when configuring an RS485 network with implications to use with the instrument.
RS485 Rule 1
RS485 is a bus network. It does not work when configured in a star network topology. This
means that a user can have a network that looks like 1 long wire (up to 4000 ft) with short stubs
hanging off the main branch with a device. Each stub must be less than 1 meter in length. See
picture below:
RS485 Rule 2
The terminating resistor at the end of the network should be only 120 ohms. The bus is
terminated on the long main bus wire at the opposite end from the Master.
RS485 Rule 3
This rule is not specific to RS485; rather it applies to any situation where you have long wires
running across the ground or in the ground connected back to a computer. Always add an
optical isolator to the link between the main bus wire and the Master device. This reduces the
chance that a nearby lightning strike will damage the Master device.
RS485 Rule 4
There can be only 32 devices per network, including the Master.
Implications to users of this instrument are as follows:
These devices are typically deployed on a cable of much greater length than the 1 meter stub
supported by RS485. The above documented Rule 1 requires that only two devices are on an
individual RS485 link, the PLC and the instrument. Many PLC’s support multiple RS485
networks which can be used to connect multiple instruments to a single PLC.
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