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saia-pcd.com
t
1
t
3
t
2
t
1
t
2
t
3
t
1
t
2
t
3
t
4
XOB
XOB
XOB
XOB
XOB
BACnet®
BACnet®
BACnet®
BACnet®
BACnet®
BACnet®
Automation stations – the basics
5
4
3
2
1
Cabinet
components
Dedicated
room controller
HMI Visualization
and operating
Automation
stations
Consumption data
acquisition
The user program consists of one or more organization blocks that are executed by the PCD Interpreter. Each user program has at least
one cyclical COB organization block, COB0.
The PCDs are mono-processor systems. Saia PCD® 1, 2, 3 control and regulation devices have a main processor that processes all the
tasks. The user program has a specific role here and is treated as a core task. In addition to the user program, any communication tasks
and server functions (web, FTP) still to be done are processed. The capacity of the CPU is divided accordingly. The cycle time for the user
program not only depends on the length of the program itself but also on the additional load at the same time.
Execution of user program
S
Cycle time without any further communication
S
Cycle time with BACnet® communication and interrupt (XOB)
S
Cycle time with BACnet® communication
Examples
The COSinus operating system ensures that all tasks are processed. A sensible load balance must be maintained
between the user program and communication. This actually always comes up in planning practice. It is only prob-
lematic if the contractor uses a weaker Saia PCD® CPU than planned in order to save money or completely “saves” on
CPUs by concentrating tasks on one CPU.
The more communication takes place, the longer the cycle time (t
x
) and this may result in variation in the cycle time.
If this variation in the cycle time is not desired, for example because regulation must take place in a fixed time period and
ideally without any jitter, then you should ensure that this part of the program is executed in an XOB. The priority of
the XOBs is higher than that of the COBs and higher than many other operating system tasks. The above example shows
that a periodic XOB interrupts the cyclic program as well as the execution of the BACnet® task.
Cold start
Application
Application
Application
Application
Application
Application
Application
Application
Application
Application
Cold start
Cold start
