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saia-pcd.com
Automation stations – the basics
How does communication influence the application
cycle time?
If the PCD is set as the server (master station), it has lit-
tle or no control over its partner stations. Should these
partner stations send large amounts of data simultane-
ously, the PCD MUST receive them. The receiving/pro-
cessing of these data packages will take priority over
the application cycle time. The cycle time may conse-
quently be increased depending on the workload. The
PCD processing time may be significantly increased
if several partner stations send large volumes of data
simultaneously.
The impact will be minimal if the PCD is set as client
(slave station).
The figures below are based on a PCD3.M5340 with
a program cycle time of 100 ms, excluding additional
communication.
Web server
Displaying a page on a micro browser pan-
el or PC does not have a major impact. Loading a large
file such as a Java applet or an offline trend during the
transfer can increase cycle time by 40…50%. The same
applies when large files are transferred via FTP.
S-Bus or Modbus communication via Ethernet:
Each
partner station running under full load increases the
cycle time by approx. 8%.
Serial S-Bus:
Each slave-type communication at 38.4
kbit/s increases the cycle time by 5% (port #2). In the
case of PCDx.F2xx modules, the increase is approx.
17%.
At 115 Kbits the cycle time is approximately 20%
higher.
Modbus RTU:
A client at 115 kbit/s increases
the cycle time by about 11% (port #2). In the case of
PCDx.F2xx modules, the increase is approx. 45%.
What exactly does MTBF mean?Where can I find the
MTBF values for Saia PCD® controllers?
MTBF stands for Mean Time Between Failures. The time
referred to is the period of operation between two
consecutive failures of a unit (module, device or sys-
tem). The higher the MTBF value, the more “reliable” the
device. On average, a device with a MTBF of 100 hours
will fail more often than a similar device with a MTBF of
1,000 hours. The MTBF value can be calculated in purely
mathematical terms or based on empirical values.
Please bear in mind that the MTBF value of the overall
installation depends on the values of the individual
switch cabinet components.
An overview of the MTBF values of the PCD controllers
is included on our
support site.
The return rate is of greater relevance in practice. We
analyse all the devices that return from the field. The
return rates of the current PCD controllers during the
warranty period (30 months) are as follows:
PCD2.M5xxx:
0.94%
PCD3.M5xxx:
0.99%
PCD3.M3xxx:
1.14%
What part of the memory will be lost if the battery
fails, and how does the PDC react?
In theory, the user memory of the PCD, which contains the
content of the media such as registers, counters, meters,
flags, and the writeable part of the DB and text elements,
will be lost in the event of a failure of the power supply
with a battery that is also weak or defective. We now have
to distinguish between two different types of PCDs.
Controllers equipped with an internal micro SD card store
the user program and associated initial values of the
media in a system partition. Should the user memory be
lost with no backup, the data will be reloaded into the
user memory and the program will be processed again
with the parameters that were defined at the time of the
download in PG5.
Controllers with no internal file system require a backup
containing the user program and associated media. This
backup can be created using PG5 when downloading the
application. As a general rule, there should be a backup
of the PCD of the last download of an application to an
external file system of the PCD to restore the program and
media content in the event of an empty memory.
If a backup of the application of a PCD is available and the
content of the user memory is not feasible, the applica-
tion will be restored from the point at which the backup
was created.
