What is the best way to wire the RS485 network?

A RS485 network should always be wired strictly as a bus. The network should never be wired in a star configuration or as a ring.

Network stubs should be kept to less than 1m in length.

Should the RS485 network be grounded?


It is very common that the RS485 network should should be connected to earth at one point on the network only. This is normally at the Master end of the network.

Should I terminate a RS485 Network?


Any RS485 network should be terminated as the start and end with a 120Ohm resistor. Otherwise the electrical signals could reflect / echo back down the network.

If the network length is less than 50m then termination can be avoided but may still be required depending on network speed, number of slaves etc.

Is there a maximum network / cable length?

An RS485 network has a maximum permissable distance of 1000m without any repeaters. This will depend on other environmental variables such as cable type, adjacent cables, connected devices, network speed.

What cable should a RS485 (Modbus) network be wired with?

A Modbus network should be wired in a twisted pair data cable with an overall screen.

A Belden 8762 is an example of such a cable.

As a general rule of thumb the thicker the cable the better, especially over a longer network run.

A cable such as CAT5 should never be used.

What is the maximum number of devices allowed on a Modbus network?

32 Devices.

The Modbus protocol specifically defines 32 unit loads. It should be assumed that 1x Modbus Slave device = 1 x unit load.

RS485 repeaters can be used to increase the number of possible Slave devices.