Astraada DRV-24 frequency inverters Communication protocol
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7 Communication Protocol
7.1 Brief instruction to Modbus protocol
Modbus protocol is a software protocol and common language which is applied in the electrical
controller. With this protocol, the controller can communicate with other devices via network (the
channel of signal transmission or the physical layer, such as RS485). And with this industrial standard,
the controlling devices of different manufacturers can be connected to an industrial network for the
convenient of being monitored.
There are two transmission modes for Modbus protocol: ASCII mode and RTU (Remote Terminal
Units) mode. On one Modbus network, all devices should select same transmission mode and their
basic parameters, such as baud rate, digital bit, check bit, and stopping bit should have no
difference.
Modbus network is a controlling network with single-master and multiple slaves, which means that
there is only one device performs as the master and the others are the slaves on one Modbus
network. The master means the device which has active talking right to sent message to Modbus
network for the controlling and inquiring to other devices. The slave means the passive device
which sends data message to the Modbus network only after receiving the controlling or inquiring
message (command) form the master (response). After the master sends message, there is a period
of time left for the controlled or inquired slaves to response, which ensure there is only one slave
sends message to the master at a time for the avoidance of singles impact.
Generally, the user can set PC, PLC, IPC and HMI as the masters to realize central control. Setting
certain device as the master is a promise other than setting by a bottom or a switch or the device
has a special message format. For example, when the upper monitor is running, if the operator
clicks sending command bottom, the upper monitor can send command message actively even it
can not receive the message from other devices. In this case, the upper monitor is the master. And if
the designer makes the inverter send the data only after receiving the command, then the inverter
is the slave.
The master can communicate with any single slave or with all slaves. For the single-visiting
command, the slave should feedback a response message; for the broadcasting message from the
master, the slave does not need to feedback the response message.
7.2 Application of the inverter
The Modbus protocol of the inverter is RTU mode and the physical layer is 2-wire RS485.
7.2.1 2-wire RS485
The interface of 2-wire RS485 works on semiduplex and its data signal applies differential
transmission which is called balance transmission, too. It uses twisted pairs, one of which is defined
as A (+) and the other is defined as B (-). Generally, if the positive electrical level between sending
drive A and B is among +2~+6V, it is logic“1”, if the electrical level is among -2V~-6V; it is logic“0”.
485+ on the terminal board corresponds to A and 485- to B.
Communication baud rate means the binary bit number in one second. The unit is bit/s (bps). The
higher the baud rate is, the quicker the transmission speed is and the weaker the anti-interference
is. If the twisted pairs of 0.56mm (24AWG) is applied as the communication cables, the Max.
Transmission distance is as below:
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