Instructions
ECL Comfort 210 / 310, communication description
9.4
Good behavior in designing a district heating Modbus network
In this chapter some basic design recommendations are listed. These recommendations are based on communication in heating
systems. This chapter is built as an example of a network design. The example can vary from a specific application. The typical
requirement in heating systems is to get access to a number of similar components and to be able to make a few adjustments.
The illustrated performance levels might decrease in real systems.
In general it can be said that the network master controls the performance of the network.
9.4.1
Considerations before implementing communication
It is very important to be realistic when network and performance are specified. Some considerations have to be made in order
to secure that important information is not blocked because of a frequent update of trivial information. Keep in mind that
heating systems typically have long time constants, and hence can be polled less frequent
.
9.4.2
Basic needs for information in SCADA systems
The ECL Comfort controller can support a network with some pieces of information concerning a heating system. It might be a
good idea to consider how to split up the traffic that these different information types generate.
Alarm handling: Values that are used to generate alarm conditions in the SCADA system.
Error handling: In all networks errors will occur, error means time out, check of sum error, retransmission and extra traffic
generated. The errors might be caused by EMC or other conditions, and it is important to reserve some bandwidth for error
handling.
Data logging: Logging of temperature etc. in a database is a function which typically is non-critical in a heating system. This
function must normally run all the time “in the background”. It is not recommended to include parameters such as set-points
and other parameters that require user interaction to change.
Online communication: This is direct communication with a single controller. When a controller is chosen (e.g. service picture
in a SCADA system) the traffic to this single controller is increased. Parameter values can be polled frequently in order to give
the user fast response. When the online communication is no longer needed (e.g. leaving the service picture in a SCADA
system), the traffic must be set back to the normal level.
Other devices: Do not forget to reserve bandwidth for devices from other manufacturers and future devices. Heat meters,
pressure sensors, and other devices have to share the network capacity.
The level for different kinds of communication types must be considered (an example is given in Figure 9.1).
Figure 9-1
9.4.3
Final number of nodes in the network
At start-up the network has to be designed with due consideration to the final number of nodes and the network traffic in the
network.
A network with few controllers connected might run without any bandwidth problems at all. When the network is increased,
however, bandwidth problems might occur in the network. To solve such problems, the amount of traffic has to be decreased in
all controllers, or extra bandwidth can be implemented.
Danfoss District Energy
VI.LG.V3.02
DEN-SMT/DK
33
