Introduction To Modbus II

 *Continuing from the previous blog*

To connect more than 2 devices on the same line and/or having a distance of more than 50 ft, we use RS485, and RS 422. For master communications with multiple slave devices, RS485 is by far the most popular method use. This standard can support up to 32 nodes and over a range of 4000 ft, roughly 1200 meters without a repeater.

The speed that Modbus messages are sent is referred to as the Baud rate or bit per second (bps). All the devices on an RTU network must use the same Baud rate.

Different devices support different transmission speeds, but between 9600 and 19200 bps is a typical speed. and the possible speed as low as 300 and high as 100000+.

A Modbus serial network has a master device that issues commands to slave devices. The slaves will not transmit information unless they receive a command to do so from the Master. There can only be one master on a network and a maximum of 247 slaves, each with a unique slave ID from 1 to 247.RS485 cannot drive more than 32 nodes in a single segment.

So for the rare applications that need more than 32 nodes, a repeater is required. The master, can write data to the slaves and can read data from them.