• Opto-Isolation on all input and output pins.
  An opto-isolator (or optical coupler) is a device that uses a short optical transmission path to transfer a signal between elements aof a circuit while keeping them electrically isolated.  Since the signal goes from an electrical signal to an optical signal and then back to an electrical, electrical contact along the path is broken so a power surge does not find a path to your PC parallel port.
  
• Operating frequency.  Up to 1MH, more than enough for controlling a cnc machine from Mach3.

• Safety Charge Pump detection, This board takes advantage of Mach's ability to send a specific frequency through one of the output pins from the parallel port (Pin 17 has been chosen for this board) when the program is in control of the system.  CNC machinery can be very dangerous, you could have the wrisk of the machine doing something different to what you intend the machine to do if the program loses control of you system.
  
• Status LED'S (Light Emitting Diodes)
  Status LED's on all input and outputs so there is no guessing whether an output is being switched.

 Please look at this products data sheet.