Need Help!- Opto-isolated BOB design

[LZ1TWB]

Hello all,
Since I could not find a sub-forum about BOBs I posted here in general.

I am designing a opto-isolated BOB for my cnc and have some questions about the parallel port connections that I have to make.
I wonder which of the two circuits I have to use to power the optocouplers - the source or the sink one? My parallel port gives 5V at the pins when unloaded but drops to about 3.2V when loaded with about 5mA /I am using UHU servo drive with its optocouplers wired from parallel port pins to ground/.
I suspect I can wire my BOB so the optos are powered from the computer's 5V line and the data pins bring them low. I have read that the sink circuit could give much higher current - in the order of 15-20mA.

The other problem is I have another computer that is generating only 3.3V at its port. Can I wire an opto here to the +5V line or it is not going to be good?
I know some computers produce less voltage at the lpt port pins but wondered why this should be. Or I have to wire it to the 3.3V line then?
The same load which brought the firts lpt port from 5V to 3.2V here made less difference - it dropped from 3.3 to about 3.05V.

I am going to use CNY17-4 devices.

Thanks in advance for any clarification on the subject.

Todor

[epineh]

I always make my BOB switch active low, as you say the parallel port can sink much more than it can source, I am using Elm Chan drives (which the UHU was originally based on) and it works fine.

I haven't tried in the source configuration as it doesn't sound like it would work with opto's unless some buffering is done, just my 2c

Cheers.

Russell.

[irving2008]

You should always use active low on parallel ports with LED returned to +5v rail as this is least susceptible to noise when several lines change at once.

Also, from a logic perspective, active low turns on the LED which then turns on the output transistor in the opto, giving an isolated active low output and preserving the logic polarity, which helps protect against 'stupidity' later (personal experience!)

Finally, active low has a better capacitive current drive and sharper fall times which is better for clock edges, which is why most clock signals act on the negative edge.

In answer to your question re voltages... it depends on the logic chip driving the port. An older TTL device would give 3.3v, a discrete CMOS device off 5v rail would give 4.5v but all in one support chip designs might give only 3.1-3.2v loaded (mosfet off 3.3v rail). Since they are really only intended to drive another TTL/CMOS device anything over 2v or so is a logic 1 and under .8v is a logic 0. ideally therefore you should buffer the signals before using them but the active low approach removes that additional cost.

[LZ1TWB]

So I guess the better way to do this is to put a couple of 74LS244N's in the beginning and drive the optocouplers after that

Todor

[Al_The_Man]

When interfacing to the 'Outside World' I usually use a ULN2803 or for just a couple of outputs, a 2N7000 on each works.
This way you only need the common reference, the final driving supply can be up to 50~60vdc.
My thoughts are, to never use logic voltage out on the machine, I much prefer the industry standard of 24vdc.
Al.

[epineh]

Originally Posted by Al_The_Man My thoughts are, to never use logic voltage out on the machine, I much prefer the industry standard of 24vdc. Al.

I will second that, I setup my machine with 5V limits/homing etc being lazy and got a lot of random trips, changed it all to 24V and the random trips completely dissapeared (except when I crashed it and one time when I leant on the limit switch )

Russell.

[LZ1TWB]

I am going to use 24V for the limit switches as was decided long time ago. I'll also have a proximity switch on each axis that I am going to combine with the index pulse from the encoders to give real accurate homing. It will also be powered off the 24V line.

I am talking about getting +5V out of the computer just for the buffer logic I plan to use. It will not be used anywhere else on the machine, just the BOB.
I've seen many DIY BOB's that use power from the computer through the USB port.

Todor

[Al_The_Man]

If using the PC supply, I generally use a spare 4 pin HD connector etc.
This way you have the common ref as well as the 5v.
Anywhere else, and the power tracks through the MB back to the supply.
Al.

[edo]

Have you considered taking the 5v off of one of your Driver boards?

[Mariss Freimanis]

Most parallel ports use 3.3V logic. You cannot Use 5V for the pull-up voltage; when your parallel port output is at a logic '1' (3.3V), there will be 1.7V across the opto LED and limit resistor (5V - 3.3V = 1.7V). Opto LEDs begin to turn on at 1.3V. This means your LED will never be fully off.

Mariss

[LZ1TWB]

Originally Posted by edo Have you considered taking the 5v off of one of your Driver boards?

I don't want to mess the driver power supply with the computer PS, that why I want an optoisolation. And there will be a ground loop since my driver PS's minus lead is grounded anyway.

The BOB will handle signals from 2 parallel ports because I want more I/O. The second port's data pins being selected for inputs gives me a total of 18 inputs. This is for now, later I consider buying some gamepad and use it in MACH3 through the Keygrabber.

Todor

[epineh]

Originally Posted by Mariss Freimanis Most parallel ports use 3.3V logic. You cannot Use 5V for the pull-up voltage; when your parallel port output is at a logic '1' (3.3V), there will be 1.7V across the opto LED and limit resistor (5V - 3.3V = 1.7V). Opto LEDs begin to turn on at 1.3V. This means your LED will never be fully off. Mariss

But if the P Port is switching to ground wouldn't the pin just float to 5V with the pullup and then once the pin switches go logic low ?

I thought if the port is switching active low then it only sinks current and does not source any voltage at all...

My BOB has three voltage levels, 5V from PC with the P Port switching low, 30V for estop/limit/estop and 5V for logic supply for the opto's to driver cards. It all seems to work fine, not trying to argue, just trying to better understand

Russell.