KROHNE ALTOSONIC V12 EN - Manual (Page 114)

10
MODBUS PROTOCOL DESCRIPTION AND SET-UP
114
ALTOSONIC V12
www.krohne.com 04/2013 - 4002643502 - MA ALTOSONIC V12 R02 en
10.4.1 Address Field (Device Address)
The address field of a message frame contains:
Valid slave addresses are 1...247.
Address 0 is used for a broadcast to address all slaves.
10.4.2 Function Field
The function field of a message frame contains:
Valid function codes are 1...127.
The function code tells the slave which kind of action to perform.
For information on the supported functions, refer to
Supported functions on page 115.
A slave response always contains the function code of the request. If a function is not applicable,
the slave sends an exception response. An exception is indicated by a returned function code
with bit 8 (most significant bit) set.
10.4.3 Data Field
The data field contains 8 bit values (bytes) in the range of 0 to FF hexadecimal.
In ASCII mode each 8 bit value is represented by 2 ASCII characters.
The data field of messages contains information which both master and slave use to perform an
action. This includes the register address, quantity of registers, and the necessary data.
10.4.4 Error checking methods
Two error checking methods are defined for the Modbus protocol:
• Optional: an additional bit (parity bit) is appended to each character (or byte) for detecting
errors during the transmission of individual characters (or bytes).
• Obligatory: two bytes (or characters) are appended to the message for detecting errors
during the transmission of the message.
As an even number of bit errors in one character (or byte) will not be detected using a parity bit,
the second method is used to check the contents of the entire message.
Both character check and message check are generated in the transmitting device and
appended to the message before transmission. The slave checks each character and the entire
message frame during receipt.
ASCII mode 2 characters
RTU mode 8 bits
ASCII mode 2 characters
RTU mode 8 bits