PACSystems™ RX3i and RSTi-EP CPU Reference Manual Section 4
GFK-2222AK October 2019
CPU Operation 99
▪ Infinite execution loop
When selecting a software watchdog value, always set the value higher than the longest
expected sweep time to prevent accidental expiration. For Constant Sweep mode,
allowance for over sweep conditions should be considered when selecting the software
watchdog timer value.
Refer to Section, A-3.6 for EGD Sweep Impact for RX3i CPE302/CPE305/CPE310 and RSTi-EP
CPE100/CPE115 Embedded Ethernet Interface.
The watchdog timer continues during interrupt execution. Queuing of interrupts within a
single sweep may cause watchdog timer expiration.
If the software watchdog timeout value is exceeded, the OK LED blinks, and the CPU goes
to STOP-Halt mode
. Certain functions, however, are still possible. A fault is placed in the
CPU fault table, and outputs go to their default state. The CPU will only communicate
with the programmer; no other communications or operations are possible. To recover,
power must be cycled on the rack or backplane containing the CPU.
To extend the current sweep beyond the software watchdog timer value, the application
program may restart the software watchdog timer using Service Request function #8.
However, the software watchdog timer value may only be changed from the
configuration software.
Note that Service Request Function #8 does not reset the output scan timer implemented
on the Genius Bus Controller.
4.8.3.2 Hardware Watchdog Timer
A backup circuit provides additional protection for the CPU. If this backup circuit
activates, the CPU is immediately Reset. Outputs go to their default states, no
communications of any kind are possible, and the CPU halts. The recovery procedure is
documented below.
There are two basic forms of hardware watchdog:
1) for RX3i CPE302, CPE305, CPE310, CPE330, CPE400, CPL410 and RSTi-EP
CPE100/CPE115, a watchdog reset results in an automatic restart into STOP-Halt
mode;
2) for RX3i CPU310, CPU315, CPU320 and all CPUs, the watchdog reset holds the CPU in
reset until the next power cycle. There is no automatic restart. If a charged battery is
connected, the power cycle will result in a restart into STOP-Halt mode.
For both watchdog reset types, the CPU is power cycled after the energy pack (for the CPE
models), or battery (for the other models listed) has been removed. This procedure gets
the CPU out of STOP-Halt. The backup power source should then be reconnected.
4.8.3.2.1 RX3i CPU Response to a Hardware Watchdog Timeout:
The following responses to a hardware watchdog timeout are common to all RX3i CPU
and CPE models:
▪ While the CPU/CPE is in STOP-Halt mode, you can connect the programmer software
or PACs Analyzer to view the fault tables, including any faults logged before the
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