Omi Input; Patient Positioners; Limit Switches; X-Ray Source Control / Mechanical Interlocks - GE Medical Systems PRODIGY Service Manual

2.5.20 OMI Input

2.5.21 Patient Positioners

2.5.22 Limit Switches

2.5.23 X-ray Source Control / Mechanical Interlocks

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PRODIGY Service Manual (Rev C - 2000)
The optical motion interrupt (OMI) sub-system connects to the transverse and
longitudinal OMI CCA's. The OMI CCA's are located on the far end (the gear
end farthest from the motor) of each drive axis. When the axis is in motion a
small toothed wheel spins through the OMI opto's beam and pulses are
sensed back on the cSBC. In this manner the system can sense a drive
circuit, motor, or belt failure which might otherwise result is a concentrated
exposure point during a patient scan. The OMI inputs are similar to those
used on the DPX-IQ. 74LS14's are added for hysteresis which provides a
clean direct interface to the PLD. As on Prodigy, logic in the FLEX PLD is
used to qualify the CH A and CH B inputs into a single 'valid motion' output.
Based on the phases of the square wave inputs on CH's A&B, the FLEX is
able to sense a change in direction. The circuit provides hysteresis to reject
false motion inputs resulting from scanner vibration when a wheel edge stops
in the center of the opto beam at the end of a move.
Four optically isolated inputs are provided for patient positioning. These are
used by the firmware to implement a joystick mode which is used in
conjunction with the laser to position the X-ray beam as desired over the
patient immediately prior to a scan. 74LS14 inverters are used to provide
hysteresis and isolate the FLEX PLD inputs from the slow rise times of the
opto output signals.
Four optically isolated inputs are provided for limit switches. These are used
by the firmware to define the transverse and longitudinal table limits. 74LS14
inverters are used to provide hysteresis and isolate the FLEX PLD inputs from
the slow rise times of the opto output signals.
The cSBC is designed such that a high on the scanner reset net disables all
scanner functionality and assures a know, fail safe, state. All scanner control
outputs are driven by the FLEX PLD. The FLEX outputs drive the low side of a
PS2501 opto coupler emitter. The far side of all these opto circuits are
configured such that the opto has to be energized for the scanner function to
be active. As such the FLEX has to sink current to energize the opto and
activate the desired scanner function. Internally the FLEX code defines these
outputs as tri-state buffers, each of which has their enable line tied to the
scanner reset net. As such a scanner reset will force all these FLEX scanner
outputs to a high impedance state, de-energizing the opto's and disabling the
scanner.
The FLEX device is SRAM based and hence must be reprogrammed by the
CPU at power up. When the device is not programmed all I/O pins default to
the high impedance state. As such the scanner will also be in a fail safe state
when the FLEX is not programmed. The CPU port 1, pin 2 also runs directly to
the FLEX's ENABLE pin.
System Overview 2-59