On Saturday 28 September 2013 11:24:35 Kirk Wallace did opine:
> On 09/28/2013 05:38 AM, Charles Steinkuehler wrote:
> > On 9/28/2013 4:45 AM, Richard Thornton wrote:
> >> Hi,
> >>
> >> I have seen a bunch of info on using BBB with capes and a lot of 3D
> >> printing specific stuff like Replicape and BeBoPr++.
> >>
> >> My thought was can the BBB replace the PC in LinuxCNC and drive the
> >> G540 directly?
> >
> > The gecko will happily work with 3.3V signals (as will just about
> > anything designed to connect to a standard PC parallel port...they
> > have all had 3.3V signaling for ages). So if you want, you can just
> > hook up some wires from the BeagleBone connectors to a DB25, maybe
> > using a prototype "cape" if you want to be fancy:
> >
> > http://www.adafruit.com/products/572
>
> The bottom line is, that for some G540 inputs one only needs to turn on
> and off the LED in a LTV846 opto-coupler.
>
> http://www.wallacecompany.com/machine_shop/G540/G540_upper_bottom-2.jpg
> http://media.digikey.com/pdf/Data%20Sheets/Lite-On%20PDFs/LTV-8x6.pdf
>
> The forward voltage for the LTV is typically 1.2 V. there is a 200 Ohm
> resistor in series with the input which will also need to be considered.
> At 3.3 V, V=IR, I=V/R, I=(3.3 - 1.2)/200=0.0105 A= 10.5mA. It looks
> like 5mA should be enough to drive the LED, so if the BBB output can
> drive 10mA and survive, a direct connection should work.
>
> The charge pump signal is different.
> http://www.wallacecompany.com/machine_shop/G540/G540_upper_bottom_z-1b.p
> ng
>
> The input charges a capacitor and diode bridge which needs a lot more
> current and is more sensitive to voltage, frequency and duty. This shows
> a parallel port signal in 5 V SPP (low drive) mode, which doesn't work:
> http://www.wallacecompany.com/machine_shop/G540/g540_mb_spp.jpg
>
> Here it is in 5 V EPP (high drive) mode, which does work:
> http://www.wallacecompany.com/machine_shop/G540/g540_mb_spp.jpg
Those 2 look identical because they are Kirk. :)
> This signal peaks at just over 3 Volts and takes its time getting there.
> The SPP signal peaks at 2 Volts.
>
> The situation gets more difficult when one considers the length and type
> of cable used. Noise can put significant + and - voltages on the signal,
> so reverse voltage protection, filtering, conditioning and termination
> will need to be considered.
>
> So direct connecting a BBB and a G540? ... It depends.
It should work well IF the + side of the optos are at Vcc, while the - side
is connected to the BBB's output signal. That assumes the BBB has a TTL-
like output stage, which I do not know for a fact.
Cheers, Gene
--
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-Ed Howdershelt (Author)
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