The AquaVent only uses MODBUS functions that operate on registers (16 bits) and transfers data in Big Endian. The AquaVent only supports holding registers, and consequently only MODBUS functions that deal specifically with holding registers.
The following are the supported MODBUS functions:
03: Read Holding Registers (code 0x03): this command reads one or more registers.
06: Write Holding Register (code 0x06): this command sets a single register.
10: Write Holding Registers (code 0x10): this command sets one or more registers.
16: Mask Write Register (code 0x16): this command will set and/or clear one or more bits in a single register (see Section 4.5.1 for example of where used).
It is recommended to refer to the document: MODBUS Application Protocol Specification V1.1b, December 28, 2006, found at modbus.org, for more information on the specific function codes.