SBC PCD1 Series - 3.19 Monitoring the CPU (watchdog); 3.19.1 PCD1 hardware watchdog

Saia-Burgess Controls AG
Manual Manual PCD 1 / PCD 2 Series │ Document 26 / 737 EN22 │ 2013-11-26
CPUs and expansion housings
3-56
Monitoring the CPU (watchdog)
3
3.19 Monitoring the CPU (watchdog)
The watchdog monitoring unit can be used to monitor the correct processing of the
user program with a high level of reliability; in the event of errors, effective safety
measures can be triggered, e.g. to switch off parts of the installation.
3.19.1 PCD1 hardware watchdog
On the PCD1, this can be achieved with an external time relay (SBC KOP128j) with
a timing range of 1 second. The time relay has a retriggerable drop-out delay; the
B1 input to the time relay is connected to an output from the PCD1 (e.g. to an output
from a PCD2.A400 card) .
t
U
S
Output
LED
A1/A2
B1
15
18
16
18
16
15
S
A1 B1
A2
L/+
N/-
InSaiaPCD
®
userprograms,theoutputissettoashing.
Example:
COB 0 ;or1 … 15
0
STL WD_Flag ;Inverthelpag
OUT WD_Flag
OUT O 255 ;Setoutput255toashing
: :
: :
ECOB
Withthecodeshownintheexample,thewatchdogdropsoutinthecaseofloops
causedbytheprogrammer.Withregardtothecycletimeoftheuserprogram,please
note:
● Withveryshortcycletimes,thetimerelaymaynotbeabletodetectthepulse
reliably
● Withverylongcycletimes,eitherthecodesequencemustberepeatedseveral
times in the user program, or the drop-out time for the time relay must be set to a
longer interval, to prevent drop-out of the watchdog in normal operation.