Delta Tau 5xx-603869-xUxx - Manual (Page 41)

Brick Motion Controller Hardware Reference Manual
Troubleshooting 33
Watchdog Timer
Brick Motion Controller has an on-board watchdog timer. This subsystem provides a fail-safe shutdown
to guard against software and hardware malfunction. To keep it from tripping the hardware circuit for the
watchdog timer requires that two basic conditions be met. First, it must see a DC voltage greater than
approximately 4.75V. If the supply voltage is below this value, the circuit’s relay will trip and the card
will shut down, Brick Motion Controller uses its own DC to DC converter to create 5V and +/-15V from
the user supplied 24VDC. This prevents corruption of registers due to insufficient voltage.
The second necessary condition is that the timer must see a square wave input (provided by the Turbo
PMAC software) of a frequency greater than approximately 25 Hz. In the foreground, the servo-interrupt
routine decrements a counter (as long as the counter is greater than zero), causing the least significant bit
of the timer to toggle. This bit is fed to the timer itself. At the end of each background cycle, the CPU
resets the counter value to a maximum value set by variable I40 (or to 4096 if I40 is set to the default of
0). If the card, for whatever reason, due either to hardware or software problems, cannot set and clear this
bit repeatedly at 25 Hz or greater, the timer will trip and the Turbo PMAC system will shut down.
Actions on Watchdog Timer Trip
When the timer trips due to either under-voltage or under-frequency, the system is latched into a reset
state, with a red LED indicating watchdog failure. The processor stops operating and will not
communicate. All Servo, MACRO, and I/O ICs are forced into their reset states, which force discrete
outputs off, and proportional outputs (DAC, PWM, PFM) to zero-level. In Turbo PMAC2 systems there
is a hard-contact relay with both normally open and normally closed contacts. In a system, these outputs
should be used to drop power to the amplifiers and other key circuitry if the card fails. Once the watchdog
timer has tripped, power to the Turbo PMAC must be cycled off and on, or the INIT/hardware reset line
must be taken low, then high, to restore normal functioning.
Diagnosing Cause of Watchdog Timer Trip
Because the watchdog timer is designed to trip on a variety of hardware and software failures, and the trip
makes it impossible to query the card, it can be difficult to determine the cause of the trip. The following
procedure is recommended to figure out the cause:
1. Reset the Turbo PMAC normally, just power cycle the cycle power. If it does not trip again
immediately, there is an intermittent software or hardware problem. Check for the following:
• Software events that overload the processor at times (e.g. additional servo-interrupt tasks, intensive
lookahead) or possible erroneous instruction (look for firmware or program checksum).
Review the Evaluating the Turbo PMAC’s Computational Load section of the Turbo USERS manual.
• 5V power-supply disturbances
• Loose connections
2. If there is an immediate watchdog timer trip in Step 1, power up with the re-initialization switch
pressed and hold in. If it does not trip now, there is a problem in the servo/phase task loading for the
frequency, or an immediate software problem on the board. Check for the following:
• Phase and servo clock frequencies vs. the number of motors used by Turbo PMAC. These
frequencies may need to be reduced.
• A PLC 0 or PLCC 0 program running immediately on power-up (I5 saved at 1 or 3) and taking too
much time.
• User-written servo or phase program not returning properly.
3. If there is an immediate watchdog timer trip in Step 2, check for hardware issues: