10K manual
Issue 1.0, 31st July 2019
Page 25
MODBUS COMMUNICATION
MODBUS IMPLEMENTATION BASICS
The Aqualeak 10K uses its EIA-485 port to communicate via Modbus. The 10K is configured to
act as a slave device on a common network and is a slave only device:
–
it will never initiate a
communications sequence.
Modes of Transmission
The Aqualeak 10K supports the Modbus RTU mode of transmission, with 8 data bits, no parity
and one stop bit. Every Modbus packet consists of four fields:
•
Slave Address Field.
•
Function Field.
•
Data Field.
•
Error Check Field (Checksum).
Slave Address Field
The slave address field is one byte in length and identifies the slave device involved in the
transaction. The valid Modbus slave address range is between 1 and 254. The Modbus address
was set on page 15 of this manual. Refer to Figure 1
3 “W2 DIP Switch Settings for
Communications Address on page 17 for more specific information.
Function Field
The function field is one byte in length and tells the Aqualeak 10K which function to perform.
Functions 03 (Read 4xxxx output registers) and 04 (Read 3xxxx input registers) are supported
by the Aqualeak 10K.
Data Field
The length of the data field varies depending on the function. The data fields for the
Aqualeak 10K are 16-bit registers, transmitted high order byte first (big-end first).
Error Check (Checksum) Field
The checksum field lets the receiving device determine if the packet has transmission errors.
On the Aqualeak 10K, RTU mode uses a 16-bit cyclic redundancy check (CRC-16).
Exception Responses
If a Modbus master sends an invalid command to the Aqualeak 10K, or attempts to read an
invalid register, an exception response is generated. The response packet will have the high
order bit of the function code set to one. The data field of the exception response contains
the exception error code.
