Lumel RE15, User Manual, Page: 8 / 28

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significant logic 1. The error code is placed on the data field of the
response frame.
Data field
The data field is constructed using sets of two hexadecimal digits, in
the range of 00 - FF hexadecimal.
These can be made from a pair of ASCII characters or from one
RTU character, according to the network’s serial transmission mode.
The function code range is 1-255. The data field of messages sent
from a master to slave devices contains additional information which
the slave must use to take the action defined by the function code.
This can include items like discrete and register addresses, the
quantity of items to be handled, and count of actual data bytes in the
field, a.s.o. The data field can be non-existent (of zero length) in
certain kinds of frames. That occurs always when the operation
defined by the code does not require parameters.
Error checking field
Two kinds of error-checking methods are used for standard
MODBUS networks. The error checking field contents depends upon
the applied transmission mode.
When
ASCII mode is used for character framing, the error
checking field contains two ASCII characters. The error check
characters are the result of a Longitudinal Redundancy Check (LRC)
calculation that is performed on the message contents
(exclusive of the beginning „colon” and terminating CRLF
characters). LRC characters are appended to the message as the
last frame field preceding the end markers (CR, LF).
When
RTU mode is used for character framing, the error checking
field contains a 16-bit value implemented as two 8-bit bytes. The
error check value is the result of a Cyclical Redundancy Check
Calculation (CRC) performed on a message contents. The CRC field
is appended to the message as the last field in the message. When
this is done, the low-order byte of the field is appended first,
followed by the high-order byte. The CRC high-order byte is the last
byte to be sent in the message.