7.2
Modbus Transactions
The slave’s response message is also constructed using Modbus protocol. It
contains fields confirming the action taken, any data to be returned, and an error–
checking field.
If an error occurred in receipt of the message, or if the slave is unable to perform
the requested action, the slave will construct an error message and send it as its
response.
Request
FUNCTION CODE + REQUEST DATA
Response, no error
FUNCTION CODE + RESPONSE DATA
Response, with error
EXCEPTION FUNCTION CODE + REQUEST DATA
where
EXCEPTION FUNCTION CODE = (FUNCTION CODE + 0x80)
Table 2: Modbus Transactions
7.3
Modbus Register Types
Each Modbus register in the XVG/eXVG is identified by its 4-digit number
address. With XVG/eXVG, the term “register” refers to both Modbus Holding
Registers (read-write) and Modbus Input registers (read-only). The types of
XVG/eXVG Modbus registers are listed in Table 3.
Register
Type
Size Data
Type
Address
Range
Description
U16 16
bit Integer
3000-
3999
Unsigned 16-bit integers ranging
from 0 to 65535.
U32 16
bit Integer
5000-
6999
Use in pairs to store unsigned 32-bit
integers ranging from 0 to
4294967295.
Float 16
bit
Floating-point
7000-
7999
Use in pairs to store 32-bit floating-
points values in single-precision
IEEE 754 format.
ASCII
16 bit
8-bit ASCII
3000-
3999
One or more consecutive registers.
Each register stores two 8-bit ASCII
characters (16 bits total).
Table 3: Modbus Transactions
6
