Manual

3-28 Detailed Descriptions of ACL Commands
FH [<only if needed> [, <z first> ]] Find Home Switches
If no parameters are specified the machine finds the Y Home switch, then the X Home
switch. The XY motors are left at full power afterwards. (See the PM command.) The
point where both switches close becomes the new (0,0) microstep coordinate, or Home
position. The No Reference status bit is cleared. (See the OS command.) If either switch is
not found within 32767 microsteps then that motor stops, the appropriate status bit is set, an
error is logged, and that motor's present position is taken to be zero microsteps for that axis.
Before either the X or Y motor travels toward its Home switch the first time, they both move
250 microsteps in the opposite direction. This is in case the carriage is already beyond the
switch closure point.
The entire XY motion sequence is actually performed twice -- the first time at high speed,
the second at a lower speed for better accuracy. During the second sequence the motors
back away only 100 steps before seeking the switches.
The <only if needed> parameter is optional. If it is specified and is nonzero then the
System checks the No Reference status bit before performing any motion. (See the OS
command.) The XY switches are only sought if they have not already been found (i.e., if the
No Reference status bit is set).
For example, if you send "FH 1;" at the beginning of your move sequence then the System
finds home the first time the sequence is executed but not subsequent times. This prevents
unnecessary motion and delay.
If the <only if needed> parameter is omitted it defaults to zero; i.e., the Find Home action
is performed regardless of the No Reference status bit.
<z first> is also optional but may only be specified if <only if needed> is also specified. If
<z first> is present and is nonzero then the Z axis seeks its home switch before X and Y.
For a description of this motion see the FZ command. If you have a three-axis system you
may wish to use <z first> to prevent collisions with tooling fixtures, etc., during the XY
motion.
If <z first> is omitted it defaults to zero, meaning that only the X and Y switches are found.
You can change this default via the PE command, for example, to prevent collisions during
the manual re-reference operation. See below.
If both <only if needed> and <z first> are nonzero then the System checks both the No
Reference and No Z Reference status bits. If both XY Home and Z Home have already been
found then no motion occurs. Otherwise, if either Home has not been found, the Z Home is
found.
All of the distances and speeds associated with the FH motion can be changed via the PE
command. The speed and acceleration of the back-off vectors can be changed via the SR
and AC commands. If FH causes the motors to move away from the switches instead of
towards them you can swap the motor direction via the PE command.
If the STOP button is pressed (or an external Emergency Stop switch closes) while either
motor is moving, the motors stop immediately and the present position becomes the Home
position. If the machine is already in the Emergency Stopped state when the FH command
is received the command is ignored.