PAVONE DAT 500, Техническое руководство, Страница: 43 / 56

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MODBUS RTU PROTOCOL
The addresses listed in the tables below follow the standard address specified in the guidelines of
the Modicon PI-MBUS-300. Below please find an excerpt that helps the user to communicate with the
instrument.
“All data addresses in Modbus messages are referenced to zero. The first occurrence of a data item
is addressed as item number zero. For example:
The coil known as ‘coil 1’ in a programmable controller is addressed as coil 0000 in the data
address field of a Modbus message.
Coil 127 decimal is addressed as coil 007E hex (126 decimal).
Holding register 40001 is addressed as register 0000 in the data address field of the message. The
function code field already specifies a ‘holding register’ operation. Therefore the ‘4XXXX’ reference
is implicit.”
To confirm a new value in E2prom, run the function of MAKE – BACKUP. If this function is not performed
by switching off, the DAT will return to the value before the change.
If not specified otherwise, the numerical values (such as addresses, codes and data) are expressed as
decimal values .
For any hardware configuration of the instrument (FIELDBUS or Analog), the MODBUS RTU protocol
is always available on COM1 RS232; in the event of Fieldbus absence, the MODBUS RTU protocol is
also available on COM2 RS485.
INSTRUMENT RESPONSE TIMES
In order to respond to most requests, the instrument takes a maximum time of 20 msec.
Exceptions are:
• the e2prom Backup command (max time = 350mSec.)
• writing of the registers of the cells capacity, cells sensitivity, net weight, system calibration, filter
(max time = 550mSec).
COMMUNICATION ERRORS HANDLING
The communication strings are controlled by the CRC (Cyclic Redundancy Check). In the case of a
communication error, the slave does not respond with a string. The master must consider a timeout for
the receipt of the response. In case of no answer, a communication error has occourred.
RECEIVED DATA ERROR HANDLING
In the case of string received correctly, but that cannot be executed, the slave responds with an EXCEP-
TION RESPONSE according to the following table.
Code Description
1 ILLEGAL FUNCTION (The function is not valid or not supported)
2 ILLEGAL DATA ADDRESS (The address of the specified data is not available)
3 ILLEGAL DATA VALUE (The received data have invalid value)