User manual WTS Ver.1.16
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15 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
Check if the Master MODBUS-RTU in use (or the development tool) requires the disclosure
of registers based on 40001 or 0. In the first case the registers numbering corresponds to
the one in the table; in the second case the register must be determined as the value in the
table minus 40001. E.g.: the register 40028 shall be reported as 27 (= 40028- 40001).
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).
The
parameter present in the paragraph SERIAL COMMUNICATION SETTING 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
organised as follows:
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