Re: [CAD_CAM_EDM_DRO] Re: The mechanical part of my router is done!

On Fri, 18 Jul 2003 16:15:33 -0000, you wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, Charles Knight <charles@i...>
>wrote:
>> OK, guys...I need a little bit of help. Who's good with
>electronics?
>>
>>
>
>Chuck: Harvey saved me a lot of typing. Make the mods he lists and
>you will have an opto isolated driver card. Make sure the logic
>outputs on the parallel port pins are active low.

Yep, that's the way it works best.

You do want the signals to be active low.

>Parallel ports
>sink current to ground better than they source current. Using the
>resistors shown (R1;R2) the port will have to sink almost 20ma per
>pin.

I'm not sure about that. There are 2 leds in series here, the one in
the isolator, and the "warm fuzzy" one on the board. That gives us a
3.2 volt drop right there, and with a 5 volt supply, I think we get
around 7.5 ma. That's within spec.

Assuming a transfer ratio of 30%, we have a 10 K to 5 volts driving at
worst an LS TTL input, so that's about 660 ua or so, no problem here.

> That may be asking a lot from a motherboard P-Pot. You want
>things to happen on the low going pulse from the computer. Set your
>software to output active low signals.

Correct. on the pulse.

>
>One of the things that has me scratching my head about the circuit is
>that pin 18 on the 297 does not show to have a pull-up to the on-
>board logic supply. I would think that this pin needs a 10K just
>like 17 unless it internally has a pull-up. It needs to swing
>between the on-board VSS +5 and the circuit ground presented when OCA
>is turned on after your mods.

I think that I left that out in the description, but hopefully "just
like OCRB would handle it. You're right, it does need a 10K pullup.

>
>Let us know how it works out.

Ditto.

Thanks for the corrections.

Harvey

>
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