
EK280
Application Manual
127
8.2
Operating a Modbus Master
Modbus is a stateless single master protocol. Within the Modbus slave function of the EK280 (see Chapter
8.1) the master (retrieval system) can only retrieve the data from the terminal (slave) during an open time
window. Opened time windows are always linked to increased energy consumption as the time window is
usually open for a longer period of time than what is required for communication and also during this time
e.g. the modem must be ready for reception.
With the Modbus master operation, this energy consumption can be reduced to the required minimum where
e.g. the modem is only switched on for the short period required for transmitting data. In such cases the
retrieval system takes over the role of the slave.
The Modbus master operation serves to send (time and/or) event-driven Modbus data (e.g. register or
archive data) to a slave, or to retrieve e.g. new configuration data or parameters from there.
The EK280 can communicate as a Modbus master by means of the terminal device (external modem) and
the internal modem in any transmission mode (ASCII, RTU, RTU-TCP).
8.2.1
General parameterization
Three parameter groups must be parameterized for operating the Modbus master.
1
The Modbus master task list:
The Modbus master task list contains up to 70 tasks. Each task consists of a register start address, the
number of registers and the access mode to the respective register(s).
There is only a single Modbus master task list in the device which is used together for all connections.
In the Modbus master task list (AL) only register start addresses can be used which are present in the
standard Modbus table (see 8.1.1).
2
The Modbus master parameters (Modbus master info):
There are 8 Modbus master info instances. In each of the 8 possible connections, it is possible to refer
to another Modbus master info instance. However, an instance may also be used more than once. The
Modbus master info contains the index of the task list and the number of tasks to be processed from
this index.
3
A link:
A minimum of one link from a possible 8 must be parameterized for Modbus master operations.
Amongst other things, the (trigger) event is established in this link which should lead to the construction
of this link. The link contains a reference to the instance of the corresponding Modbus master info.
i
As only register start addresses which are present in the standard Modbus table can be used in
the Modbus master task list (see Chapter 8.1.1) then before setting up Modbus master
operations this standard Modbus table must always be correctly parameterized first.
Figure 18 gives an overview of the dependencies of parameterization.
Figure 18: Dependencies within Modbus master parameterization