Parallel port Isolation

[NC Cams]

I learned my lesson about optoisolation on my Fanuc's.

The lesson: Use OPTOISOLATION.

Can be a PITA to do as you have to be careful NOT to connnect grounds 'tween the PC and the device being connected to. It may also be difficult to get 5vdc "out" to power the opto on either side.

You can use diode array to pull power from the signals on LPT side and dump to a capacitor - plenty enough power to run the opto.

With a bit of creativity, you can also get power on "outside" via wall transformer to power the isolation circuit. Worst case, a 9V battery will last years.

I learned subsequently that it costs roughly $60 to have 3 a opto isolated boards etched and drilled from ExpressPCB. The design and layout software is FREE. Add another $5 for parts - figure $20 worst case.

In comparison, my Fanucs' have several THOUSAND in repairs due to NOT using proper isolation of LPT port and the Fanuc M/B's.

Don't even TRY to persuade me that you can't afford to do it right.

Having done it myself and learned in the process, if you fry a computer, the LPT port or an expensive CNC controller, you can find SYMPATHY in the dictionary somewhere between CRAP and VENERIAL DISEASE......

[UKRobotics]

Is it possible to purchase a parrallel port opto-isolator circuit with a db25 connector on each end, a power supply and all the relevant opto electronic gubbins in the middle to make it all work in a simple plug and play turnkey solution ?

[DanC]

I just read through this entire post with a little fear and some hope. I am slowly putting together the hardware for my first CNC machine and have been trying to absorb and learn as much as I can. I think I will pick up the book that NC Cams recommended.

Right now all I have is a computer with controller software. I decided to buy the parallel interface board from CNC4PC. http://www.cnc4pc.com/Parallel_Port_Interface_Card.htm I just ordered it a few days ago, so I have not recieved it yet. I haven't decided what servo drives to use, that's why I've been reading in the Rutex forum. I am going to use MCG ID33003 brush motors.

Anyway, there has been alot of food for thought here.

[kreutz]

Some of the stepper and servo controllers come with input signal opto-isolation (e.g. Gecko's). In that case you don't need to isolate again those pins on the port. Make sure you isolate all the inputs (limit and home switches) as well as the rest of the outputs from the parallel port (spindle on-off, cooling control, etc.)

For those of you who can manufacture your own printed circuit boards there are schematics on the Internet. Just make sure you keep separate PC port ground and pins from CNC equipment ground. Also you can provide +5volt from the USB port to supply the PC part of the interface, keep it separated physically on the board from +5 and/or +12 volt necessary to supply the CNC part of the interface.

Use high speed opto-isolators for the Step and Dir signals. Spindle on-off, coolant control, etc can use low speed opto-isolators , solid state or electro-mechanical relays.

Hugo.

[pminmo]

Originally Posted by kreutz Make sure you isolate all the inputs (limit and home switches) Hugo.

If the inputs are just mechanical switches it is not necessary to add the extra silicon, you just use the PC side 5V and the pc side ground, and don't mix the switch input wiring grounds with the machine side.

However, if it is an expensive machine and the input switch wiring is exposed where it could be damaged, then yes the input optos would make sense. But you shouldn't have exposed wiring anyway.

[kreutz]

Phil, you are right.

Due to the nature of CNC work the environment is not ideal for electrical wiring integrity, eventually wires will get wet, damaged, broken or micro switches contacts will short against the machine body. The safest and, in the long time, less expensive way is using isolation.

There are alternatives to opto-isolators they are called "i-couplers" from Analog Devices. I guess opto-isolators are easily available to more people around the world.

Hugo.

[pminmo]

[QUOTE=kreutz;199307]

Due to the nature of CNC work the environment is not ideal for electrical wiring integrity, eventually wires will get wet, damaged, broken or micro switches contacts will short against the machine body. QUOTE]

The machine body better be safety grounded. And in that case, unless there is some AC ground fault on the circuit the PC is plugged into, it will make no difference.

I'm not discounting isolation, but bottom line, it takes a fault or miswire situation for isolation to be of benefit. That situation should be detected and fixed before anything is hooked up! I'd much rather have a circuit fail than an injured (or worse) operator.

(p.s. I've seen the I-couplers, but my fear is they are single source)

[BCwanderer]

http://www.cel.com/prod/prod_optoapp.pdf
Here is an interesting article that shows how to speed up a slow optocoupler with a resistor to the base pin of the opto-transistor.

I tried it with a SFH601 and a Siemens IL5 optocoupler and it works.
The unit was choking on the turnoff time.
When the 200,000 ohm resistor was added between the base and ground, the speed increased about 5 times.
I tried a lower value resistor and the output voltage dropped to half the previous value.
I was able to get a good signal up to 12 msec wide. At 8 msec It was getting pretty sloppy but it made it to the top and down again to the bottom.
This would be fast enough for most of the medium speed applications.
The screamers with 1/16 step and high speed would still have to upgrade to high speed optos.
The best signal was with an emitter follower load of 1100 ohms.
Input resistor was best at 270 ohms for the SFH601 and 320 ohms for the IL5
The sweet spot for the SFH601 base resistor with emitter follower was 320k.
The sweet spot for the SFH601 base resistor with the load on the collecter was 65K
The IL5 took much less resistor.
This was with a signal of about 3 volts output swing with about 10 ma input across the LED.
That would be good enough for a A3986.

More testing to come.

Cheers

[xtal]

Heres an untested parallel port isolator,,,I'm planning to use with a
HobbyCncPro board ....

note the added attempt for adding some filtering to the input lines
can also mount 2 7805 regs if wanted ...

DIY cost would not be to great...

I plan to use PS2501 [4 pin isolator from Jameco]
Many of the resistors are vertically mounted with a common voltage bus
connecting resistors then connect to pcb
The zip file is the expresspcb layout

Again this is untested,,but I see no reason why It wouldn't work...