====== SERVOLOCK_TTL ======

The ''SERVOLOCK_TTL'' firmware module allows the user to control the stage position bi-directionally using TTL pulses.  When placed in the ''SERVOLOCK_TTL'' state, motors are kept on with servo loop active and the target position is adjusted every time a TTL pulse is received.  The duration of each TTL pulse determines whether a positive or negative adjustment is applied.  The adjustment magnitude is the same as the last relative move and can be changed via serial command even when the ''SERVOLOCK_TTL'' state is active.  Commanded moves and manual (joystick) moves are ignored when the ''SERVOLOCK_TTL'' state is active.

Example applications of the ''SERVOLOCK_TTL'' functionality are:
  * Making slow constant-speed moves by periodically sending TTL pulses and using a small adjustment amount (e.g. a few encoder counts).  It is possible to move slower moves this way than with the normal commanded moves, which by cannot go slower than 1 encoder count every 32 milliseconds.  Similarly, ''SERVOLOCK_TTL'' function can be used to make moves at speeds between the quantized speed values of normal commanded moves.
  * Implementing external feedback control for locking or tracking using a stage (e.g. home-built CRISP-like system).  External equipment determines which direction the ASI stage needs to move and sends an appropriate duration pulse depending on the polarity.  A similar thing can be done using the ''VECTOR'' command, but ''SERVOLOCK_TTL'' does not require any serial commands (sometimes a rate-limiter) and can also make arbitrarily small adjustments.
  * Creating arbitrary continuous movement profiles.  Although the ring buffer can be used, each new position initiates a new discrete move with a ramp up and ramp down which may be unwanted. Furthermore there is no limit on how long the path can be using ''SERVOLOCK_TTL''.

This feature is initially only available on Tiger controllers, but there is nothing preventing the code from being ported to the MS-2000 controller if there is a need.  Because TTL triggering is done on a per-card basis, to have truly independent control of X and Y axes they would be need to be on separate 2-axis cards. ((otherwise either both axes will both react together or only one will react according to the ''RM Y'' setting (but the two axes can have different relative move amounts). ))


===== How to use SERVOLOCK_TTL =====

Engage or disengage the ''SERVOLOCK_TTL'' functionality by using the ''LK'' command without any arguments (if a CRISP-enabled firmware is present, use the ''LK F'' command instead because the ''LK'' without arguments applies to CRISP which takes priority).  

To provide for bi-directional control, short or long TTL pulses can be sent, and the threshold duration is set by the ''RT R'' command (defaults to 0.75 ms; note that there is timing jitter between -0.001 and -0.25 ms so a setting of 0.75 ms will for sure recognize pulses of 0.5 ms as short and 0.75 ms as long but pulses of 0.6 ms may sometimes be recognized a short and sometimes as long).  Internally the implementation as follows: a counter is initialized on rising edge of the TTL pulse, and on a separate interrupt the counter is decremented every 0.25 ms until it reaches 0.  On reaching 0, the state of the TTL IN0 pin is checked again and a decision is taken depending on whether it is high or low.  

During ''SERVOLOCK_TTL'' operation, the TTL IN0 mode (''TTL X'' command) is automatically set to value 11.  Under normal circumstances the TTL IN0 mode will be restored to its prior value the when exiting ''SERVOLOCK_TTL'' operation.  Do not change the TTL IN0 mode while ''SERVOLOCK_TTL'' is engaged.

As a safety provision, the excursion from the initial position (when first locked) is limited.  The maximum distance (in either direction) is set by the ''LR Z''  command and defaults to 1 mm.

Position can be freely queried during ''SERVOLOCK_TTL'' operation.

===== Limitations =====

Experimentally the controller can keep up with TTL pulses at a 1kHz rate.  There is no lower bound on the time between pulses.

It seems in testing that the controller catches every pulse, even when it is responding to serial commands.  If you absolutely require this feature (e.g. creating extended arbitrary move profiles via TTL pulses) then it is worth verifying yourself.


===== Interaction with CRISP =====

The ''SERVOLOCK_TTL'' functionality uses many of the same commands as CRISP and indeed the internal implementation is similar to CRISP except that TTL pulses changes the target stage position instead of the CRISP hardware/firmware.  When the ''SERVOLOCK_TTL'' firmware module is present along with CRISP, CRISP has priority.  However, with CRISP present it is still possible to place the controller in the ''SERVOLOCK_TTL'' state by using the ''LK F'' command, using ''LK F=84'' (ASCII letter T) to turn on ''SERVOLOCK_TTL'' control and ''LK F=90'' (ASCII letter Z) to turn it off when done.


===== Serial Commands =====

Serial commands related to ''SERVOLOCK_TTL'' module:

{{topic>servolock_ttl +commands}}


{{tag>advanced_feature tiger}}