The mask.sch schematic

The calibration unit motorised mask is required only to move between in-beam and out-of-beam positions. This is achieved physically with a DC motor. The drive electronics is controlled via two TTL voltages from a XYCOM-240 card, one to drive the motor forward and another to drive backward. The 240 card also accepts input from limit switches, located at either end of the full range of travel of the stage.

The software, therefore, need only assert one output line or the other using standard binary output records, and release that line when the corresponding binary input indicates that the mask is in position (either in or out of the optical path).

Records "bo0" and "bo1" are of DTYP "XYCOM-240" and are scan passive. They are used to modify the two required control voltages. Records "bi0" and "bi1", of DTYP "XYCOM-240" and scan i/o intr (process whenever an input changes), are used to sense the limit switch status.

Record "bi2" (DTYP "Soft channel" and scan passive) holds the current requested direction of motion in its VAL field. Engineering screens modify this field (via channel access) to specify "into beam" or "out of beam" directions for the next motion.

The scan passive gensub record, "gsb", services these records. This record runs the function specified in its SNAM field, and is connected to process whenever one of the limit switches changes, the interlock is asserted or the attached CAD record is instructed to START. The code implements a state machine, with the current record state stored in the J input field of the record. State 5, processed whenever the "ilck" binary input record changes, stops the motor regardless of the current state of the mechanism.

A typical operation involving this database would consist of the following:

1. User sets "bi2" VAL field to specify desired direction of motion.
2. User MARKs and STARTs the "cad" record.
3. "gsb" is processed, polls INPA to pick up desired motion direction and asserts VALD or VALE to start the DC motor moving. "gsb" processing ends.
4. "gsb" gets processed again when a limit switch is triggered, thus changing INPB or INPC and triggering the gsb SLNK field. "gsb" releases VALD/VALE to stop the motor, and checks that the mask is in the correct limit.