VEGA VEGACOM 557 Modbus protocol - Manual (Page 43)

VEGACOM 557 Modbus 43
Data transmission with Modbus ASCII
5.5 Data transmission with Modbus ASCII
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.
ASCII mode
In the ASCII mode the transmission starts with the character ‘:’ and ends with CR LF. For cod-
ing, hexadecimal figures ‘0’ to ‘9’ and ‘A’ to ‘F’ are available. The coding is made by converting
any characteristics hexadecimally. Due to this conversion, twice the number of characters
must be transmitted via the interface.
For communication, it is necessary that a Slave participant continuously monitors the bus. If
the character ‘:’ is received, a new data frame begins. The addressed Slave is then deter-
mined by the next two characters.
Transfer measured values as DCS values in ASCII mode
Independent of the connected VEGA signal conditioning instrument (VEGAMET or VEGA-
LOG), the transmission of the DCS values is made via the Modbus always acc. to the same
procedure via 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 Status“ must be programmed in the PLC and the required pa-
rameters such as Slave address, start register and number of register must be stated. The
conversion of the output addresses in VEGALOG or VEGAMET to the input registers of the
PLC are given in the table in supplement A.
Example for data of VEGAMET 515:
Two DCS values of VEGAMET #2 (DCS outputs 1 and 2) should be collected. VEGACOM 557
operates in mode "Sort measured values acc. to VEGAMET addresses“. The Slave-address
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