THEORY OF OPERATION
4-25
Versapulse Select Service Manual
0621-499-01 01/94
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4.4.14 Safety Processor
The safety processor provides monitoring of system operation independent of the main processor. Refer to 8-
8. Microprocessor U80 is the safety microprocessor. Its software instructions are stored in EPROM's U88 and
U89. U90 and U91 provide 4K of 16 bit nonvolatile memory. Y3 provides an 8 MHZ system clock. U81 detects
bus errors on the address bus, forcing the processor to trap if a bus cycle does not complete. U87 and U86
process interrupt request and acknowledge signals. U73, U75, U83 and U84 provide various address and data
bus signals.
U79 provides 256 bytes of dual channel read/write memory. The main processor and safety processor can
both read or write to this shared memory resource. The memory is essentially a communication link between
the two processors. Both write information in specified areas of this memory which the other can read to
determine status of the other processor.
For example, before firing one of the YAG heads, the main processor writes to the shared memory
indicating which laser head it is about to fire. The safety processor reads this information, and if it is
the same YAG head that the safety processor expects to be fired, it writes that same information back
into the shared memory. The main processor then reads the information written into the shared
memory by the safety processor, checks it against the original information of the head it was
preparing to fire, and, if all checks have agreed, the firing sequence continues. Numerous such cross
checks occur in the software. The two sets of software are written to work together, and to confirm
with each other through this shared memory resource all aspects of safe operation.
Dual UART U85 provides two channels of receive asynchronous communication for the safety processor. The
safety processor uses its RXDA and RXDB receive lines to monitor the asynchronous communication trans-
missions from the touch screen display and remote control. In this manner it can determine operator inputs.
The main processor and safety processor use the shared memory to confirm agreement of all received opera-
tor inputs over these lines. Note that the safety processor dual UART transmit lines are not connected - the
safety processor does not transmit information over the asynchronous bus.
U78 provides the safety processor digital I/O and programmable timer functions.
Its "A" channel lines are used to monitor 7 digital inputs and to provide the /NOFIRE/ output. It
monitors the normally open and normally closed signals from both the footswitch and shutter
(SHTRNO, /SHTRNC/, FTSWNO, /FTSWNC/), the service switch, and the INTEGRATE signal
(generated by the main processor programmable timer).
Whenever the safety processor software detects a potential problem with system operation, it can
assert its digital I/O /NOFIRE/ signal low. This signal will inhibit the laser from firing by:
Forcing the output of U64-3 low. This is the RS ENABLE to the HVPS. A low on this line
disables the HVPS from charging the main charging capacitor.
Forcing the output of U64-6 low. This is the SHUTTER DRIVE signal going to the Shutter
PCB to switch on or off the shutter solenoid. A low turns off the FET switch on the Shutter
PCB, opening the current path for the shutter solenoid. This forces the shutter solenoid into
its de-energized position, blocking the treatment beam path.
Gating off U42. U42 is the multiplexer that connects the /FIREPLS/ signal with one of the
four trigger outputs. /NOFIRE/ low opens all four of the outputs, preventing a trigger from
being generated.
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