Technical Bulletin

Modbus RTU On-Board Communications
PowerXL DG1 Series VFD MN040010EN—May 2014 www.eaton.com 9
The parameters of every device must be set before
connecting to the bus. Each parameter must be the same as
the master configuration.
Modbus Communication Standards
The Modbus protocol is an industrial communications and
distributed control system to integrate PLCs, computers,
terminals, and other monitoring, sensing and control devices.
Modbus is a Master-Slave communications protocol. The
Master controls all serial activity by selectively polling one or
more slave devices. The protocol provides for one master
device and up to 247 slave devices on a common line. Each
device is assigned an address to distinguish it from all other
connected devices.
The Modbus protocol uses the master-slave technique, in
which only one device (the master) can initiate a transaction.
The other devices (the slaves) respond by supplying the
requested data to the master, or by taking the action
requested in the query. The master can address individual
slaves or initiate a broadcast message to all slaves. Slaves
return a message (“response”) to queries that are addressed
to them individually. Responses are not returned to
broadcast queries from the master.
A transaction comprises a single query and single response
frame or a single broadcast frame. The transaction frames
are defined below.
Figure 10. The Basic Structure of a Modbus Frame
Valid slave device addresses are in the range of 0–247
decimal. The individual slave devices are assigned addresses
in the range of 1–247. A master addresses a slave by placing
the slave address in the address field of the message. When
the slave sends its response, it places its own address in this
address field of the response to let the master know which
slave is responding.
P20.2.6 Slave Busy 0 589 0 = Not Busy
1 = Busy
P20.2.7 Parity Error 0 590
P20.2.8 Slave Fault 0 591
P20.2.9 Last Fault Response 0 592
P20.2.10 Comm Timeout Modbus RTU ms 2000 593
Table 7. Modbus RTU/BACnet MS/TP—P20.2, continued
Code Parameter Min. Max. Unit Default ID Note
Master’s
Message
Slave’s
Response
Start
End
Address
Function
Data
CRC
Start
End
Address
Function
Data
CRC