Time Between Messages; Operation Of The Cfw-08 In The Modbus-Rtu Network; Interface Description - WEG CFW08 Vector inverter User Manual

CFW-08 OPTIONS AND ACCESSORIES
8.20.1.2.5 Times between
Messages
8.20.2 Operation of the
CFW-08 in the
Modbus-RTU
Network

8.20.2.1 Interface Description

148
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.
Signal
Time
Figure 8.25 - Times required during the communication of a message
Comunicação
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-08 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-08 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-08:
T
T
3,5 x
between bytes
T
11 bits
Message
T
11 bits
1,146 ms
573 µs
285 µs
).
11bits
T
3,5 x
T
3,5x
4,010 ms
2,005 ms
1,003 ms
).
3,5x