Modbus - General Information; Protocol Description; Data Coding - Fronius Ri Fb Inside/I Bedienungsanleitung

Modbus – General Information
Protocol De- The MODBUS ADU is constructed by the client that initiates the MODBUS tran-
scription saction. The function tells the server which action is to be performed. The MOD-
BUS application protocol defines the format of a client-initiated request.
The function code field of a MODBUS data unit is coded in one byte. Valid codes
are in the range of 1 ... 255 decimal (the range 128-255 is reserved for exception
responses). When the server receives a message from a client, the function code
field tells the server which action to perform.
If several actions are to be performed, subfunction codes are added to some
function codes. When messages are sent to servers by a client, the data field in
the message contains additional information that the server uses to perform the
action defined by the function code. This can include elements such as discrete
addresses, register addresses, the quantity to be handled, or the number of actu-
al data bytes contained within the field.
With certain types of request, there might not be a data field (length: zero). In
this case, the server does not require any additional information because the ac-
tion is specified by the function code alone.
If a MODBUS ADU is correctly received without any errors occurring in connec-
tion with the requested MODBUS function, the requested data will be included in
the data field when a server responds to a client. If an error does occur in con-
nection with the requested MODBUS function, the field will contain an exception
code that the server application can use to determine what action to perform
next.
For instance, a client can read the ON/OFF statuses of a group of discrete inputs
or outputs, or it can read/write the data contents of a group of registers.
When sending a response to the client, the server uses the function code field
either to indicate that the response is normal (free of errors) or that an error has
occurred (this kind of response is called an "exception response"). In the case of
a normal response, the server simply echoes the original function code.

Data Coding

For addresses and data elements, MODBUS uses a big-endian format. When a
number larger than a single byte is transmitted, this means that the most signifi-
cant byte is sent first.
Register Size
16 bits, 1234
66
Value
12 is sent as the first byte and then 34
hex hex hex