Oce Arizona T220UV User Manual - page 130
Océ | Arizona T220UV Safety Systems
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3010102780-A
means of disabling the control logic and removing power. In addition the carriage
motion is monitored by a two-encoder system, one encoder reads the linear position
of the carriage and the other reads the angular position of the motor shaft. If the
difference between the two-encoder readings becomes significant the DSP will again
execute the shutdown sequence.
4.
Watchdog timer system
The 48-volt and 24-volt power supplies are controlled by the main gate array (as
opposed to the DSP directly) on the DSP board (7500-3007) by means of the
spare_ssr_driver. This driver is required to be switched ON in order to enable the
power supplies. The gate array actually controls the spare_ssr_driver directly, and
the requirements for turning it
on are that a four-bit shift register must have a '1' in the msb, which enables the
output. Every 5-millisecond timer tick shifts all the bits one position to the left and
puts a '0' in the lsb. When the DSP writes to the specified address, the four bit shift
register is automatically filled with ' 1's ' - thus enabling the output for at least 20
milliseconds. If the DSP stops writing to the address in the gate array for any reason
the gate array switches the spare_ssr_driver off after 4 timer ticks (20 milliseconds).
This design provides triple functionality:
•
The DSP can control the power supplies at will (providing means for the
shutdown sequence described above)
•
If the DSP stops running its program, or stops functioning for any reason, the
48 and 24-volt power supplies will automatically get turned off (providing
means for the so-called watchdog timer).
•
The system inherently suppresses power to the motors and motor drivers until
the point in time when the DSP is established in running its control program,
and chooses to enable these power supplies. This prevents a run-away
condition at power-up of the machine, which would otherwise go undetected
by the DSP for the few seconds before it began running its control program
and tracking the encoders in the system.