[Steve-Tee]

Hi folks,
This is my version of a CNC Breakout box circuit.

Its outputs feature:
Step and Pulse signals for 4 axes,
Coolant control,
Vacuum Clamp control,
Vacuum Cleaner control,
Spindle control.

Its inputs feature:
Home switch detect for X,Y and Z axes,
A 'Spare' input,
Emergency stop and Limits input.

The DC supply for the circuit is 5V @ about 50mA, and this may be obtained from the USB port of the host computer.
Doing this, also ensures that the breakout circuit is referenced to the same signal ground as the host computer is.
Protection for the computer's parallel port is afforded by way of 100 ohm resistors, one for each port line of the computer's DB25 socket.
This is not perfect, but will surely stop the computer from being harmed if there is an 'oops' event when working with the live circuit.
To provide a greater degree of protection, one could raise the values of the resistors to 470 ohms.
This should still work, but needs to be tested on your system before it can be trusted to reliably report system status.
High intensity Amber Leds are attached to all of the buffered lines. This permits the operator to quickly check that the parallel port is working correctly. This comes in handy when trouble shooting an aberrant system.
In my build, all of the 100 ohm resistors that connect to the parallel port are on their own circuit board, and are enclosed in a little box that also houses the DB25 connector.
Leading from this box, is a ribbon cable that connects the free ends of those resistors to the rest of the circuit, housed in a separate box.
This arrangement helps to ensure that the 'oops' effect does not get past those 100 ohm safety resistors.
The 6 chips that make up this circuit are all the same. These are hex inverting schmitt triggered buffers.
By using these schmitt triggers, any noise in the digital input/output signals is cleaned up before it gets an opportunity to interfere with the operation of the computer interface.
As these chips are in sockets, replacement is a simple matter if someone lets the 'magic smoke' out of them.
I am sure there are better designs out there, but this one offers simplicity and a good, but not perfect, degree of protection for the host computer.

As at 22/10/07, this circuit has been tested, and is working very nicely



Here's hoping this will be helpful.

Best wishes,
Steve-tee.