Selec VFD Series

62
User Manual
9. EXTERNAL COMMUNICATION
The drive features external communication on the form of MODBUS RTU over RS485 interface,
which is compliant with International Communication Standards. Centralized control can be
implemented by PC/PLC as well as superior controller to adapt specific requirements for applications,
including setting of control command, operating frequency, modification of parameters for relevant
function code, monitoring of operating status or malfunction.
9.1. Protocol Content
Modbus, also known as serial communication protocol, has defined its frames and format during the
asynchronous transmission, including the format of polling to the host, broadcast frames and
response fames to slave controller. The frames generated by the host included slave
address/broadcast address, executive command, data & error correction; and the response of the
slave controller has the similar frames as the host, including confirmation of instructions, data
feedback and error correction. If any error happens when the slave controller receives the frames, or
any instruction requested by the host can't be done, it will generate a malfunction frame and send
feedback to the host.
9.1.1. Application mode
The drive is compatible with control network equipped with RS485 bus which has single host &
multiple slave controllers.
9.1.2. Bus Structure
1. Hardware Interface
RS485 hardware interface
Serial, asynchronous, half-duplex communication
The range of slave address should be 1~127; the number 0 stands for broadcast communication
address. Every single slave address on the network is unique, which is the foundation of Modbus
serial communication.
3. System with single host and multiple slave controllers
2. Transmission mode
9.2. Protocol Instructions
according to the "query /command" from the host. The host herein mainly refers to personal
computer, industrial control facility or Programmable Logic Controller (PLC); and the slave herein
mainly refers to the drive or the other control facilities with similar/same communication protocol. The
host can not only communicate with a certain slave controller independently but also send broadcast
message to all the slave controllers. As to any independent access to the host's "query /command", a
message (called response) must be sent out from the slave controller; however, regarding the
broadcast message issued by the host, it's not necessary for the slave controller to send feedback to
the host.
The protocol of VFD-S Series Converter is a kind of serial & asynchronous communication protocol
with principal and subordinate Modbus; there is only one device(host) could set up protocol (named
"query /command"), and the other devices(slave) can only response to the host's "query /command"
by sending data, or issue commands/instructions