Rice Lake SCT-10 - Manual (Page 39)

Operation 35
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 2.4 “Analog
Output” on page 21) 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. The functions supported relative to the MODBUS
standard are the READ HOLDING REGISTER and the PRESET MULTIPLE
REGISTERS.
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
Function 16: Preset Multiple Registers (Multiple Register Writing)
Tot. bytes = 7+2*No. registers+2
Tot. bytes = 8
No. REGS: Number of registers to write beginning from the address.
FUNCTIONS DESCRIPTION
03 (0x03) Read Holding Register (Programmable Register Reading)
16 (0x10) Preset Multiple Registers (Write Multiple DI Register)
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