Rice Lake Signal Conditioning Transmitter/Indicators - Manual (Page 51)

Operation 47
Modbus Supported Functions
Among the commands available in the MODBUS-RTU protocol, only the following are used to manage
communication with the instruments. Other commands may not be interpreted correctly and could generate errors
or system shut-downs:
The interrogation frequency is linked with the preset communication rate (the instrument will stand by for at least 3
bytes before beginning to calculate a possible response to the query). The dELAY parameter (see Section 2.9 on
page 24) allows for a further delay in the instrument response, and this directly influences the number of possible
queries in the unit of time.
For additional information on this protocol, refer to the general technical specification PI_MBUS_300. In general,
the query and response to and from a slave instrument are organized as follows:
Function 3: Read Holding Registers (Programmable Register Reading)
Tot. bytes = 8
Tot. bytes = 3+2*No. registers+2
in which:
No. registers= number of Modbus register to be read, starting from the Address 1° register;
No. bytes = number of data bytes to follow;
Function 16: Preset Multiple Registers (Multiple Register Writing)
Tot. bytes = 7+2*No. registers+2
Tot. bytes = 8
in which:
No. registers= number of Modbus register to be read, starting from the Address 1° register;
No. bytes = number of data bytes to follow;
Val. reg. 1 = register contents beginning from the first.
The response contains the number of records changed starting from the Address 1° register.
FUNCTIONS DESCRIPTION
03 (0x03) Read Holding Register (Programmable Register Reading)
16 (0x10) Preset Multiple Registers (Multiple Register Writing)
QUERY
Address Function Add. Reg. 1 No. register 2 bytes
A 0x03 0x0000 0x0002 CRC
RESPONSE
Address Function No. bytes Register1 Register 2 2 bytes
A 0x03 0x04 0x0064 0x00C8 CRC
QUERY
Address Function Add. reg. 1 No. reg. No. bytes Val. reg.1 Val. reg.2 2 bytes
A 0x10 0x0000 0x0002 0x04 0x0000 0x0000 CRC
RESPONSE
Address Function Add. Reg. 1 No. register 2 bytes
A 0x10 0x0000 0x0002 CRC