VEGA VEGACOM 557 Modbus protocol - Manual (Page 36)

36 VEGACOM 557 Modbus
5.4 Data transmission with Modbus RTU
Already several years ago, the firm AEG developed a communication technique for its PLC,
called Modbus. This specification is used today by several companies for the coupling of IO-
components.
For the Modbus, we distinguish between two versions: Modbus RTU and Modbus ASCII. Both
have the same data content, but the scale and the coding of the characters are different.
RTU mode
With the RTU coding, the message starts with a break of 3.5 character lapses (start se-
quence). The character lapse is directly deduced from the baud rate. This is followed by the
transmission and can be made with a slip of 1.5 character lapses. The transmission is followed
by an end sequence consisting of 3.5 characters. The demonstration of character image com-
prises the complete range of a byte and is implemented in binary format. This ensures a con-
siderably higher data transfer rate than with the ASCII mode.
Transfer measured values as DCS values in the RTU mode
Independent of the connected VEGA signal conditioning instrument (VEGAMET or VEGA-
LOG), the transmission of the DCS values is made through the Modbus always acc. to the
same procedure via the function code 04 "Read Input Registers“.
Procedure with Modbus-Master Modicon or Freelance 2000
When using an AEG-Modicon or a Freelance 2000 from Hartmann & Braun as Modbus-Master,
these systems take over the communication to the Modbus. In this case, only the respective
function enquiry "Read Input Registers“ must be programmed in the PLC, and the required
parameters such as Slave address, start register and number of register must be stated. The
conversion of the measured value addresses in VEGALOG or VEGAMET to the input registers
of the PLC is stated in the table in supplement A.
Example for data from VEGAMET 515:
Two DCS values should be collected from VEGAMET #2 (DCS outputs 1 and 2). VEGACOM
557 operates in mode "Sort measured values acc. to VEGAMET addresses“. The Slave-ad-
dress of VEGACOM 557 is set to 1.
Parameter setting for function enquiry "Read Input Registers“:
- Slave-address = 1
- Register word address (Start) = 30.029 (or 028 on Freelance 2000)
- Number of register words = 4
Data transmission with Modbus RTU