Rice Lake SCT-20, Техническое руководство

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Operation
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3.11 Modbus-RTU Protocol
The MODBUS-RTU protocol enables management of the reading and writing of the registers, listed in this section, according to
the specifications contained in the reference document for this standard Modicon PI-MBUS-300.
To select the communication with MODBUS-RTU, see
Section 2.4 on page 16 .
When specifically indicated, certain data will be written directly to EEPROM type memories. This memory has a limited number
of writing operations (100.000); therefore, unnecessary operations at said locations must be avoided. The instrument ensures
that no writing occurs if the value to be stored is equal to the stored value.
The numerical data listed below are expressed in decimal notation, or hexadecimal notation if preceded by 0x.
Modbus-RTU Data Format
The data received and transmitted via MODBUS-RTU protocol have the following characteristics:
• 1 start bit
• 8 data bits,
least significant bit sent first
• Instrument settable parity bit
• Instrument settable stop bit
Modbus Supported Functions
Among the commands available in MODBUS-RTU protocol, only the commands in Table 3-28 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 SCT20-MB will stand by for at least 3 bytes before
beginning to calculate a possible response to the query). The

? parameter ( Section 2.4 on page 16 ) 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 source instrument are organized as follows.
Function 3: Read Holding Registers (Programmable Register Reading)
Tot. bytes = 8
Tot. bytes = 3+2*No. re2
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 Description
03 (0x03) Read Holding Register (Programmable Register Reading)
16 (0x10) Preset Multiple Registers (Multiple Register Writing)
Table 3-28. Modbus-RTU Supported Functions
QUERY
Address Function Add. Reg. 1 No. register 2 bytes
A 0x03 0x0000 0x0002 CRC
Table 3-29. Programmable Register Reading (Modbus-RTU)
RESPONSE
Address Function No. bytes Register1 Register 2 2 bytes
A 0x03 0x04 0x0064 0x00C8 CRC
Table 3-30. Programmable Register Reading Response (Modbus-RTU)