11
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®
5
Switch cabinet
components
4
Consumer
data acquisition
3
Dedicated
room controllers
2
Operation
and monitoring
1
Automation
stations
Automation stations – the basics
The user program consists of one or more organisation blocks that are executed by the PCD Interpreter. Each user program has at least
one cyclical organisation block, COB, the 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 special role here and is processed as a core task. In addition to the user program, any communication
tasks and server functions (web, FTP) are processed. The CPU capacity is allocated accordingly. The cycle time for the user program not
only depends on the length of the program itself, but also on the simultaneous additional load.
Execution of the user program
Cycle time with no additional communication
Cycle time with BACnet® communication and interrupt (XOB)
Cycle time with BACnet® communication
Examples:
The COSinus operating system ensures that all tasks are processed. An intelligent load balance must be maintained
between the user program and communication. This actually occurs in planning practice. It is only problematic
if the contractor uses a lower performing Saia PCD® CPU than planned to save money or is “saving” on CPUs by
concentrating tasks on one CPU.
Cold start
Application
Application
Application
Application
Application
Application
Application
Application
Application
Application
Cold start
Cold start
The more communication takes place, the longer the cycle time (t
x
), which may result in variations in the cycle time. If this variation in
the cycle time is not required, for example because regulation must take place in a fixed time period and ideally without jitters, make
sure 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 and the execution of the
BACnet® task.
