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How to build a RS485 network

Also see Troubleshooting RS485 Issues →

General features of RS485 networks

• RS485 serial networks are in the form of a bus that can extend up to 1,200m from one end to the other.
• The bus should not branch, with the exception of very short branches (e.g. 1m) from a set of terminals to a device.
• The data logger ("master") device does not need to be at the end of the bus. It's fine for it to sit anywhere on the bus, so long as the bus as a whole does not branch.
• You should follow the device manufacturer's advice regarding the maximum number of devices on an RS485 bus, however in general a single RS485 network can support up to 32 devices (each with a unique "unit ID" or address).
  • Note: The physical order of the devices on the network is not important, so long as the unit IDs (addresses) are unique.

Selecting a suitable cable

RS485 cable should:

• be a shielded cable, ideally with a drain wire that can be earthed at one end only
• use twisted pair
• for long runs, use a cable with more than one pair. Some devices provide three RS485 terminals, including a "C" (zero volt / GND) terminal, which should be used if it is available.
• use multi strand cable, i.e. should not be CAT5 / CAT6 cable as strand breaks are likely.
  • The one exception to this rule is where the equipment you are connecting makes use of RJ45 plugs (ethernet plugs) for the RS485 terminations.

Installing the RS485 cable

Pointers to getting the installation right:

• Different manufacturers use alternative labels for the terminals on an RS485 network. Read their installation manual to clarify how they are labelling their terminals.
  • Some hardware manufacturers use "A" meaning D+ and "B" meaning D-
  • Other manufacturers may use "A" to label the D- terminal and "B" to label the D+ terminal.
• For long runs, it is beneficial to add a 120Ω resistor at both ends of the RS485 network, to reduce signal reflections that increase noise on the network.
  • The resistor should be connected between the D+ to the D- terminals on the devices at the two ends of the network.
  • Some devices have a built-in terminal resistor that can be enabled using (e.g.) a dip-switch in the device.
• use one pair for the D+ / D- connections of the network. Do not use separate pairs for each terminal! This is a common error we regularly see on PV installations.

Illustration of correct and incorrect RS485 terminations on a two-terminal RS485 bus. (common "C" and earth connections omitted for clarity)

Correct	Incorrect
The installer has used multi-strand RS485 cable, with a single twisted pair used for the D+ and D- connections.	This image illustrates two errors. First of all, a CAT5 single-stranded cables has been used. Secondly a separate pair has been used for the D+ and D- connections. The purpose of using a twisted pair is to cancel out noise from external devices. Using two separate pairs completely removes this benefit of using a twisted pair. Single-stranded copper as found in CAT5 and CAT6 cable can easily fracture where the strands are removed from the protective outer sheath.

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