slave response message (from to the master)
and CMD occupies
m
For example, read continuous 2 data content from0004H from
The command code is used to read the working step
Communication Protocol
132
In ladder logic, CKSM calculated the CRC value according to the frame with the table inquiry. The
method is advanced with easy program and quick calculation speed. But the ROM space the
program occupied is huge. So use it with caution according to the program required space.
9.4 RTU command code and communication data illustration
9.4.1 RTU mode
9.4.1.1 Command code: 03H
read N words (Word) (N
≤16)
Command code 03H means that if the master read data fro
, the reading number
depends on the "data number" in the command code. Max. continuous reading number is 16 and
the parameter address should be continuous. The byte length of every data is 2 (one word). The
following command format is illustrated by hex (a number with "H" means hex) and one hex
occupies one byte.
.
with the address of 01H
(read the content of data address of 0004H and 0005H), the frame structure is as below:
RTU master command message (from the master to the VFD)
START
T1-T2-T3-T4 (transmission time of 3.5 bytes)
ADDR
01H
CMD
03H
High bit of the start address
00H
Low bit of the start address
04H
High bit of data number
00H
Low bit of data number
02H
CRC low bit
85H
CRC high bit
CAH
END
T1-T2-T3-T4 (transmission time of 3.5 bytes)
T1-T2-T3-T4 between START and END is to provide at least the time of 3.5 bytes as the leisure
time and distinguish two messages for the avoidance of taking two messages as one message.
ADDR
= 01H means the command message is sent
with the
address of 01H and ADDR
occupies one byte
CMD
=03H means the command message is sent to read data from
one byte
"Start address"
means reading data form the address and it occupies 2 bytes with the fact that the
high bit is in the front and the low bit is in the behind.
"Data number"
means the reading data number with the unit of word. If the "start address" is
0004H and the "data number" is 0002H, the data of 0004H and 0005H will be read.
CRC
occupies 2 bytes with the fact that the high bit is in the front and the low bit is in the behind.
RTU
