From: Gene H. <ghe...@wd...> - 2013-09-28 15:31:08
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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 -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Diplomacy is to do and say, the nastiest thing in the nicest way. -- Balfour A pen in the hand of this president is far more dangerous than 200 million guns in the hands of law-abiding citizens. |