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Modbus message timing:
Master request
Slave processing &
Slave response
Master
Start of
transaction
The MODBUS response timeout setting of the master should be set to a value larger than the calculated
max. response time.
Error checking
MODBUS RTU networks employ two methods of error checking:
1. Parity checking of each data character (even, odd, or no parity)
2. Frame checking within the message frame (Cyclical Redundancy Check)
Parity checking
A LINAK MODBUS device can be configured for even or odd parity, or for no parity checking. This determines how
the parity bit of the character’s data frame is set. If even or odd parity checking is selected, the number of 1 bits
in the data portion of each character frame is counted. Each character in RTU mode contains 8 bits. The parity
bit will then be set to a 0 or a 1, to result in an even (even parity), or odd (odd parity) total number of 1 bits.
The use of ‘no parity’ requires 2 stop bits.
Frame checking
RTU Mode message frames include an error checking method that is based on a Cyclical Redundancy Check
(CRC). The error-checking field of a message frame contains a 16-bit value (two 8-bit bytes) that contains the
result of a Cyclical Redundancy Check (CRC) calculation performed on the message contents.
Message format
Any MODBUS message consists of the basic fields shown below: Slave address (Addr), Function code (Function),
up to 252 data bytes (Data) and a calculated 16 bit checksum (CRC).
Byte 0
Byte 1
Byte 2 .. N
Byte N+1, N+2
Addr
Function
Data
CRC