12. MODBUS-RTU PROTOCOL
The MODBUS-RTU protocol enables to manage the reading and writing of the registers
listed here below according to the specifications contained in the reference document for
this standard Modicon PI-MBUS-300.
To select the communication with MODBUS-RTU, refer to paragraph
SERIAL COMMUNICATION SETTING
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, in any case,
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 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
FUNCTIONS DESCRIPTION
03 (0x03)
READ HOLDING REGISTER (PROGRAMMABLE REGISTER
READING)
16 (0x10)
PRESET MULTIPLE REGISTERS (MULTIPLE REGISTER
WRITING)
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).
80
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