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Chapter 5 - MODBUS-RTU Communication
78
Example) Command Message (Request) for reading one address from address ‘3000’
ENQ Drive No. CMD Address
The Number
of address to
read
SUM EOT
05h “01” “R” “3000” “1” “A7” 04h
1 byte 2 bytes 1 byte 4 bytes 1 byte 2 bytes 1 byte
SUM = ‘0’ + ‘1’ + ’R’ + ‘3’ + ‘0’ + ‘0’ + ‘0’ + ‘1’
= 30h + 31h + 52h + 33h + 30h + 30h + 30h + 31h
= 1A7
h
5.6.3 Detail Communication Protocol
Request for Read: Request for read ‘n’ numbers of WORD from address ‘XXXX’.
ENQ Drive No. CMD Address
The number
of address to
read
SUM EOT
05h
“01” ~
“1F”
“R” “XXXX”
“1” ~ “8” =
n
“XX” 04h
1 byte 2 bytes 1 byte 4 bytes 1 byte 2 bytes 1 byte
Total byte = 12 bytes
The quotation marks (“ ”) mean character.
Acknowledge Response
ACK Drive No. CMD Data SUM EOT
06h “01” ~ “1F” “R” “XXXX” “XX” 04h
1 byte 2 bytes 1byte N * 4 bytes 2 bytes 1 byte
Total byte = 7 * n * 4 = max. 39 bytes
Negative Acknowledge Response
NAK Drive No. CMD Error Code SUM EOT
15h “01” ~ “1F” “R” “**” “XX” 04h
1 byte 2 bytes 1 byte 2 bytes 2 bytes 1 byte
Total byte = 9 bytes