VEGA VEGASCAN 850 Modbus signal output - Manual (Page 7)

Modbus signal output VEGASCAN 850 7
5 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, there are 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 periods (start se-
quence). The character period is directly deduced from the baud rate. This is followed by the
transmission and can be made with a slip of 1.5 character periods. The transmission is fol-
lowed by an end sequence consisting of 3.5 characters. Character representation comprises
the complete range of a byte and is made in binary format. This ensures a considerably
higher data transfer rate as with the ASCII mode.
Measured values transferred as DCS values in RTU mode
The transmission of the DCS values via the Modbus is made 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 VEGASCAN to the input registers of the PLC
is stated in the table in chapter "1.7 Measured value image“.
Procedure with Modbus-Master-simulation (e.g. PC)
In case the data enquiry should be made via a Modbus-Master-simulation, this can be done,
for example, by writing your own communication driver using the "Function code 04 = Read
Input Registers“. The following demonstration should explain the communication processes
between Modbus-Master and VEGASCAN on the Modbus.
Example:
A DCS value should be collected (DCS output 15). The slave address of VEGASCAN is set to
1. Actual measured value on VEGASCAN DCS output 15 is: 3.30% (= 0x014A).
Data transmission with Modbus RTU