TRACE Focus Mechanism Documentation

One step takes 192 ms.
Motor is powered for 64 ms.
Focus Motion: 0.00035 inches (9 microns) per step. 
Total range: About +/- .032 inches (+/- 95 steps)
Motor: Inland Motor P/N RBE-00502-D08, same as MDI Alignment Motor.

FOCUS ENCODER
The focus encoder has 2 channels (FOCEDGE & FOCCTR). FOCEDGE detects when
the mechanism is at either end of its range. FOCCTR tells which half of
the range the mechanism is in. Additionally, the FOCCTR signal can be read
by the A-D board, and since it does not change abruptly from 0 to 1 it
can be used to determine the focus position if the motor is within a few
steps of the center position:
             _______                             _________
   FOCEDGE          \                           /
                     \_________________________/
                                   _______________________  
   FOCCTR                         /
             ____________________/
             focus motion   dec (-) <----->  inc (+)

FOCEDGE & FOCCTR are used by the focus electronics to keep from running
the focus motor off the edge of the screw. If a command is sent that is
not allowed, no power will be applied to the motor.

   FOCEDGE  |  FOCCTR  |  Allowed Operation
   ---------+----------+-------------------
       1    |    1     |  DEC
       1    |    0     |  INC
       0    |    X     |  INC or DEC

FOCUS STATUS WORD   
S0 thru S4 indicate which state the focus electronics state machine is
in.  Normally (if the electronics is waiting for a command) S will be
between 1 and 6. If S is 1 and an INC command is sent, S will be 2
after the INC command is done.

              Description 
   ------------------------
   IN7        Always 0
   IN6        FOCCTR
   IN5        FOCEDGE
   IN4        S0
   IN3        S1
   IN2        S2
   IN1        S3
   IN0        S4

COMMANDS

   Function  | F2 | F1 | F0             Card Select #6
   ----------+----+----+---- 
   DEC       |  0 |  1 |  X 
   INC       |  1 |  0 |  X  (secondary further from the primary)
   RESET     |  1 |  1 |  X
   NOP       |  0 |  0 |  X

A focus motion can be divided into three 64 ms steps:
    1) No motion 
    2) Motor step 
    3) No motion

Bits 5 & 6 of the focus status word (FOCCTR & FOCEDGE) are only
meaningful during steps 2 or 3, so he focus status word should be read
then. The lower 5 bits of the status word may be read at any time.

A focus reset energizes the focus encoder for 64 ms. A reset does not
move the focus motor. A reset synchronizes the focus state machine with
the rotational position of the motor. FOCCTR & FOCEDGE may be read
during the 64 ms reset.