REXROTH Fv Series, Instruction Manual

Page: 181 / 222

12 Communication Protocols
12.1 Brief Introduction
Fv converters provide standard RS485 communication port to realize the
communication between the master station and the slave station via ModBus
or PROFIBUS protocols. With the help of a PC, a PLC or an external comput‐
er a “single master/ multiple slaves” network control can be realized (setting
of frequency control command and running frequency, modification of func‐
tion code parameters, monitoring of frequency converter running status and
failure messages) to address the specific requirements of applications.
12.2 ModBus Protocol
12.2.1 Protocol Description
Brief Introduction
ModBus is a master/slave protocol. Only one device may send commands in
the network at a particular time.
The master station manages information exchange by polling the slave sta‐
tions. Unless being approved by the master station, no station may send in‐
formation.
In case of an error during data exchange, if no response is received, the
master station will query the slave stations absent from the polling.
If a slave station is not able to understand a message from the master sta‐
tion, it will send an exception response to the master station.
Slave stations can not communicate with each other but through the master’s
software, which reads data from one slave station and send them to another.
There are two types of dialogs between the master station and the slave sta‐
tions:
●
The master station sends a request to a slave station and waits for its
response.
● The master station sends a request to all slave stations and does not
wait for their response (broadcasting).
Transmission
The transmission is of RTU (remote terminal unit) mode with frames contain‐
ing no message header or end mark. A typical RTU frame format is shown in
fig. 12-1 "Typical RTU fram format" on page 179 .
Slave address
(1 byte) ModBus
function code
(1 byte) Data
(Multiple bytes) CRC16 check
information
(2 bytes)
Fig.12-1: Typical RTU fram format
Data are transmitted in binary codes.
If an interval is 3.5 characters or longer, it is taken as the end of the frame.
Therefore, all information in a frame must be transmitted in a continuous data
flow. If an interval of 3.5 characters or longer occurs before a complete frame
is sent out, the receiving device will consider the information has ended and
start processing it, and mistake following bytes for a new frame’s address.
DOK-RCON02-FV*********IB01-EN-P
Rexroth Frequency Converter Fv Bosch Rexroth AG 179/219
Communication Protocols