Operation Of The Cfw-09 In The Modbus-Rtu Network; Interface Rs-232 And Rs-485 Description - WEG CFW-09 User Manual

CHAPTER 8 - CFW-09 OPTIONS AND ACCESSORIES
8.14.2 Operation of the
CFW-09 in the
Modbus-RTU
Network
8.14.2.1 Interface RS-232 and
RS-485 Description
316
Times between Messages:
In the RTU mode there is no specific character that indicates the beginning or
the end of a message. Thus the only indication for the beginning or the end of
a new message is the data transmission absence in the network by 3.5 times
the time required for transmission of one data word (11 bits). Thus if a message
is initiated after elapsing of the minimum time required without transmission,
the network elements assume that the received character represents the
beginning of a new message. In similar mode, after this time has elapsed, the
network elements will assume that the message has been ended.
If during the transmission of a message, the time between the bytes is longer
than this minimum required time, the message will be considered invalid,
since the inverter will discard the already received bytes and will mount a new
message with the bytes that are being transmitted.
The table below shows the time for three different communication rates.
T
Signal
3.5 x
Time
Figure 8.52 - Times required during the communication of a message
Communication Rate
9600 kbits/sec
19200 kbits/sec
38400 kbits/sec
T = Time to transmit one word of the message.
11 bits
T = Time between bytes (can not be longer than T
entre bytes
T = Minimum interval to indicate the begin and the end of the
3.5x
message (3.5 x T
The CFW-09 frequency inverters operate as slaves of the Modbus-RTU network.
The communication initiates with the master of the Modbus-RTU network
requesting a service for a network address. When the inverter is configured to
the corresponding address, it processes the question and answers to the
master as requested.
The CFW-09 frequency inverters use a serial interface for the communication
with the Modbus-RTU network. There are two ways to perform the connection
between the network master and the CFW-09:
T
between bytes
T
11 bits
Message
T
11 bits
1.146 ms
573 s
285 s
).
11bits
T
3.5 x
T
3.5x
4.010 ms
2.005 ms
1.003 ms
).
3.5x