Bpt mitho+, Техническое руководство

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55
Technical Manual
GB
Types of communications
Differently from parallel systems, communications between devices in a bus system take place through a
single channel. This leads to the sharing of the line by all of the devices, which can transmit only one at a
time.
There are different ways of dealing with this issue.
Since they are networks of devices, the field buses can be with joint elements (multi-master structure) or with
a master and at least one or more slaves.
Master-Slave
If the network is configured with a MASTER-SLAVE structure, it is the MASTER that queries each single slave to
give it commands and/or to read variables from it. When a slave has available data, it does not take initiatives
and does not call the master. The master, following precise rules defined by the process on which the field
bus is applies, queries each single slave.
Multi-Master
The joint network allows each element that comprises it to communicate specific data to the others. For
example, a level sensor that detects an “overflow” can advise the control module to the valve so that a gate
valve is closed.
Both of these solutions have advantages and disadvantage.
In the Master-Slave structure it is easy to see that the transmission channel (the physical channel) does not
run the risk of being overloaded since two elements can never access the physical means at the same time.
The master calls each single slave, one at a time. However, these networks may have long response times
if an event occurs. the network must be constantly monitored by the master so that it knows the status of
every single slave.
With a Multi - Master bus, each element can access the network at any time. This is unquestionably a faster
way of meeting real-time needs. If there are no events in progress, data does not travel through the physical
means. Problems arise when more than one element of the network attempt to access the physical means
at the same time. Therefore, conditions must be inserted to determine access to the physical network. This
can be done, for example, by assigning a priority to each node in the network. For nodes that have a vital
role in the system, the highest priority is assigned in access to the network; the others have gradually lower
priority.
The MITHO communication system:
Communications of the Mitho network is Multi - Master type because it is necessary for all of the devices in
the network to be able to start communications by sending a request for information or by requesting the
execution of functions performed by the user.
ManTecnico_MithoPlus IT_GB 2484555 55 ManTecnico_MithoPlus IT_GB 2484555 55 18-06-2008 11:49:35 18-06-2008 11:49:35