Hi,
There is no inherent feedback on a step-motor so without some kind of reference switch (or position encoder etc) it's not possible to know where "home" is.

Here's a somewhat expanded version of the code. It's a bit more "modular" than earlier, you can select drive-type and just set the direction and number of steps before calling the Drive subroutine. I've not tested it here (no hardware) so you're on your own there if you decide to use it.

As you might see, I've added a FindHome routined which moves one step at a time until it finds the switch. Just remove it if you're not going to use it.
Code:
'****************************************************************
'*  Name    : StepIndexer.pbp                                   *
'*  Author  : Henrik Olsson                                     *
'*  Notice  : Copyright (c) 2011 Henrik Olsson                  *
'*          : All Rights Reserved                               *
'*  Date    : 2011-09-07                                        *
'*  Version : 1.0                                               *
'*  Notes   : Very simple step motor indexer for unipolar drive.*
'*          :                                                   *
'****************************************************************

Direction   VAR BYTE    ' Forward=1, Back=254
Distance    VAR WORD    ' Number of steps to move
DriveType   VAR BYTE    ' WaveDrive=0, HiTorque=1, HalfStep=2
StepCount   VAR WORD    ' Counts the number of steps moved
Delay       VAR BYTE    ' Delay between steps

HomeSwitch  VAR PORTA.0 ' Reference/home switch connected here

Index       VAR BYTE    ' Index "pointer" used for the lookup tables.
Temp        var BYTE    ' General purpose

FORWARD     CON 1
BACK        CON 254
WaveDrive   CON 0
HiTorque    CON 1
HalfStep    CON 2


TRISB = 0               ' All PortB pins are outputs.
TRISA.0 = 1             ' Reference/home switch connected to RA0
        
pause 1000

CLEAR

DriveType = HiTorque   ' Use the two phase on high torque mode

'---------------------------------------------------------------------------
FindHome:
' Take one step at a time untill the reference switch connected
' to PortA.0 is found, stop there.
  Direction = FORWARD   ' Move forward
  Distance = 1          ' step by step
  Delay = 20            ' at roughly 50Hz
  While HomeSwitch = 0
     GOSUB Drive
  Wend
'---------------------------------------------------------------------------
 
Pause 2500
 
Cycle:
  Direction = FORWARD   ' Go forward...
  Distance = 512        ' for 512 steps...
  Delay = 20            ' at roughly 50Hz...
  Gosub Drive           ' do it

  Pause 2500

  Direction = BACK      ' Go back...
  Distance = 512        ' for 512 steps...
  Delay = 10            ' at roughly 100Hz...
  GOSUB DRIVE           ' do it.

  Pause 2500
Goto Cycle


'---------------------------------------------------------------------------
Drive:
' Move Distance number of steps. Direction gets added to Index each iteration,
' this makes Index count up (when direction=FORWARD (1)) or down
' (when direction=BACK (254)). Then, depending on the desired drive type we
' either mask the lower two or lower three bits in temp and use those as the
' actual pointer into the Lookup-tables for the three different drive sequences
' (Wave drive, Hi-Torque and Half step). The result of the table lookup is put
' on PortB.

  For StepCount = 1 to Distance
     Index = Index + DIRECTION

     Select Case DriveType

       Case WaveDrive
          Temp = Index & 3
          Lookup Temp, [1, 2, 4, 8], PortB
       
       Case HiTorque
          Temp = Index & 3
          Lookup Temp, [3, 6, 12, 9], PortB
       
       Case HalfStep
          Temp = Index & 7
          Lookup Temp, [1, 3, 2, 6, 4, 12, 8, 9], PortB
        
        END SELECT
        
     Pause Delay
  NEXT
Return
'---------------------------------------------------------------------------
/Henrik.