Hardware Manual PCD 1 / PCD 2 Series│Document 26 / 737│Edition EN 15│2008-07-22
Saia-Burgess Controls AG
Monitoring the CPU (watchdog)CPUs and expansion housings3
3.17 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.3.17.1 PCD1 hardware watchdog
On the PCD1, this can be achieved with an external time relay (Saia 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
(^151816)
18 16
15
S
A1 B1
A2
L/+
N/-
In PCD user programs, the output is set to flashing.
Example:
COB 0 ; or 1 ... 15
0
ACC H ;Invert output 64
COM O 64
: :
: :
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.