- 45 -
The data exchanged by the instrument are:
Output Data
from instrument (reading)
Input Data
to instrument (writing)
Exchange register R1 (H - L)
Exchange register W1 (H - L)
The data received and transmitted via MODBUS-RTU protocol have the following characteristics:
- 1 start bit
- 8 bit of data, least significant bit sent first
- Settable parity bit
- Settable stop bit
FUNCTIONS SUPPORTED IN MODBUS
Among the commands available in the MODBUS-RTU protocol, only the following are utilised for
management of communication with the instruments; other commands could be incorrectly
interpreted and generate errors or blocks of the system:
READ HOLDING REGISTER (READ PROGRAMMABLE REGISTERS)
PRESET MULTIPLE REGISTERS (WRITE MULTIPLE REGISTERS)
Interrogation frequency is linked to the communication speed set (the instrument stands by for at
least 3 bytes before starting calculations an eventual response to the interrogation query). The
parameter allows the instrument to respond with a further delay and this directly influences
the number of interrogations possible in the unit of time.
For additional information on this protocol refer to the general technical specifications
PI_MBUS_300.
In general queries and answers toward and from one slave instrument are composed as follows:
FUNCTION 3: Read holding registers (READ PROGRAMMABLE REGISTERS)
QUERY
Tot. byte = 8
To Next Page