Delta Tau PMAC Mini, Reference Manual

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PMAC Quick Reference Guide
68 Troubleshooting
The hardware circuit for the watchdog timer requires that two basic conditions be met to keep it from
tripping. 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. 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 PMAC
software) of a frequency greater than approximately 25 Hz. If the card, for whatever reason, due either to
hardware or software problems, cannot set and clear this bit repeatedly at this frequency or higher, the
circuit’s relay will trip.
Every RTI, PMAC reads the 12-bit watchdog timer register (Y register $1F) and decrements the value by
8 – this toggles bit 3. If the resulting value is not less than zero, it copies the result into a register that
forces the bit 3 value onto the watchdog timer. Repeated, this process provides a square-wave input to the
watchdog timer.
In the background, PMAC executes one scan through an individual PLC program, then checks to see if
there are any complete commands, responding if there are, then executes the housekeeping functions.
This cycle is repeated endlessly.
Most of the housekeeping functions are safety checks such as following error limits and overtravel limits.
When it is done with these checks, PMAC sets the 12-bit watchdog timer register back to its maximum
value. As long as this occurs regularly at least every 512 RTI cycles, the watchdog timer will not trip.
The purpose of this two-part control of the timer is to make sure all aspects of the PMAC software are
being executed, both in foreground (interrupt-driven) and background. If anything keeps either type of
routine from executing, the watchdog will fail quickly. The only recover for this failure, assuming the 5V
power supply is satisfactory, is to hardware reset PMAC.
Establishing Communications
Either the Executive or Setup program can be used to establish initial communications with the card.
Both programs have menus that tell the PC where to expect to find the PMAC and how to communicate
with it at that location. If told to look for PMAC on the bus, also tell it PMAC’s base address on the bus
(this was set up with jumpers on PMAC). If told to look for PMAC on a COM port, tell it the baud rate
(this was set up with jumpers or switches on the PMAC).
Once the program knows where and how to communicate with PMAC, it will attempt to find PMAC at
that address by sending a query command and waiting for the response. If it gets the expected type of
response, it will report that it has found PMAC and can proceed.
If it does not get the expected type of response after several attempts, it will report that it has not found
PMAC. Check the following:
General
1. Is the green LED (power indicator) on PMAC’s CPU board ON, as it should be? If it is not, find out
why PMAC is not getting a +5V voltage supply.
2. Is the red LED (watchdog timer indicator) on PMAC’s CPU board OFF, as it should be? If it is ON,
make sure PMAC is getting very close to 5V supply – at less than 4.75V, the watchdog timer will
trip, shutting down the card. The voltage can be probed at pins 1 and 3 of the J8 connector (A1 and
A2 on the PMAC VME). If the voltage is satisfactory, inspect PMAC to see that all inter-board
connections and all socketed ICs are well seated. If the card still will not run with the red LED off,
contact the factory.