[yassser]

Hello ,

I'm a computer engineering student , I'm trying to make a small hobbyist cnc machine using mach3 , the manual says that I have to put an isolated breakout board between the parallel port of the computer and the motor drivers to save the computer motherboard from damage if something went wrong in the high power drivers , can any one confirm its construction , and what is the difference between using buffers and optical couplers .

thanks in advance.

[bonmotwang]

Normally the motor driver inputs are opto isolated. Please double check the spec.
So for the break out board, just get a <$20 one, none isolated.


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[yassser]

thanks for reply , but I made the motor drivers , I didn't buy them , and they are not isolated .

[bonmotwang]

Ok.
Buffer here is only for voltage level conversion and low impedance output, or higher current driving ability. In your case, it is not isolated.
Opto isolation is what you will need.

But I am surprised that you can make your own stepper driver, but not the break out board :-D

But anyway search on eBay to get a break out board says isolated and confirm with the seller before you pay.




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[rgammage]

Hello

The 'normal' 3 axis machine uses 6 signals, Step and Direction each for XY&Z.

Using low current Opto Isolators < 2mA on each of these lines, and the reverse for any limit switches etc.

I attach a simple sketch I did some while ago and did not build ..... it will give you an idea. The biggest issue is to ensure that the optos do not invert signals or if they do, you invert them again before use. Check and modify first please.

I recommend powering the PC end from a USB connector (+5v @ 100mA) and find another for the other end ... maybe its USB (?) what ever they have to be isolated so the large voltages and currents cannot get back and blow up your mother board!

I hope this helps ...

Richard

[brownfox]

the manual says that I have to put an isolated breakout board between the parallel port of the computer and the motor drivers to save the computer motherboard from damage

The manual says the truth. However, with some precautions, it may not be absolutely necessary - though very disirable.
It greatly depends on what is the power supply for your stepper motors. If it's not very powerful, I would connect it directly.
What is absolutely necessary:
- provide a reliable common ground between PC and stepper drivers, maybe with a wire to the computer case. Be sure to separate signal (logic) ground and high power ground in the driver board, then connect it in a single point.
- ensure no direct electrical connection between any circuit and the machine frame. The frame must be grounded with a separate wire (this is a personal safety requirement). Think of it again when working with touch probe.
- always connect or disconnect the LPT connector when both the computer and driver are completely off. I repeat: _completely off_. That means not just the power switch is off, but the wall socket disconnected (remember the filtering capacitors in parallel with AC input). Fix the connectors with screws.
- always switch on the PC first, then the driver; switch off in reverse.
Now we have a medium-size machine (workspace 500x800 mm) connected as described here, and it works just well.
Anyway, you can add the breakout board later.

[Al_The_Man]

I recommend powering the PC end from a USB connector (+5v @ 100mA) and find another for the other end ... maybe its USB (?) what ever they have to be isolated so the large voltages and currents cannot get back and blow up your mother board!

Richard

Notice on the print the P.P. commons are taken to Earth Ground, they are also taken to earth ground inside the PC via the PC P.S. via M.B. ground connections to case.
This external ground reference shown should be taken to the central earth ground point.
Al.