Thanks Kirk, I did as you suggested, and while I was at it I measured the series current when shorted and found that it was ~2mA. My pulldowns were in fact too high an impedance. I dropped them to ~300 ohms and everything is now working as expected.
N.C. On Feb 05, 2012, at 05:56 PM, Kirk Wallace <kwall...@wallacecompany.com> wrote: > On Sun, 2012-02-05 at 12:41 -0500, N. Christopher Perry wrote: > > I'm bringing up my mini-mill and have encountered a weird problem: > > I'm trying to use pins 10, 11 & 12 on the printer port as limit switch > > inputs for X, Y, Z axes, respectively, but the pins are acting like > > outputs. > > The parallel port 10, 11 and 12 pins should always have high impedance, > so your sensor circuit outputs should act the same way whether they are > connected to these pins or not. If connecting the sensor output wire to > the parallel port pin keeps the signal voltage from toggling, then there > is something wrong with the parallel port. Many motherboard parallel > ports are now running on 3 Volts, I would think the inputs should be 5 > Volt tolerant, but they may be very easy to short out. I avoid using the > motherboard parallel ports. Blowing out a $15 PCI card is much cheaper > than replacing a motherboard, plus these usually run on 5 Volts. > > It should be easy to check these inputs with a wire and a 2.5k Ohm > resistor. Connect the wire to an input pin, then touch the wire to the > PC's ground or a +5 Volt source. You should be able to see the logic > state change with HALmeter, HALscope or the "watch" feature in the "HAL > configuration" window. > > > The limits switches are active high, with ether voltage dividers or > > diodes in line for level correction and short circuit protection. > > When I test the limit switches when not connected to the PC they work > > as expected. > > When connected to the PC a tripped a limit switch might cause the > > voltage to move by 0.5V or so, but the voltage is still held above the > > TTL high threshold. > > > > The X-axis limit switch system consists of two OPB972 optical sensors > > (TTL level output), which have totem-pole output. Both are diode-ORed > > together with a 20K pulldown. > > I would avoid totem-pole outputs. The limit signal should be active low > so that if the sensor, power supply or wire fails, the limit will trip. > Open collector outputs make that easy. One just needs a pull up to the > supply voltage to limit the collector current. A divider would work too, > but I would rather have the full voltage drive an opto-isolator > protecting the parallel port input. > > I avoid optical sensors unless they can be enclosed in a liquid proof > container. > > > > The Y-axis limit switch system consists of two Honeywell 103SR12A-2 > > Hall sensors, which have active source outputs (Open emitter, 12V > > supply, ~12 volt active output, floating otherwise). Both are wired > > together and put through a 5.1K/2.2K resistor divider. > > The 103R's are a nice sensor, but they are expensive. I would go with > the sink version of this sensor, but $.60 can get you a sensor that > works very nearly as well in a hobby environment. > > > > The Z-axis limit switch system consists of two Parker Proprietary Hall > > sensors (TTL level output), which appear to have totem-pole outputs. > > As a precaution, I've diode-ORed them together with a 20K pulldown. > > Another thing, proper soft limits should be setup. With these setup, > there is another layer of safety and they are more convenient because an > axis will come to a controlled stop when it hits a soft limit. One can > just jog away from the limit, whereas hitting a hard limit requires > finding and selecting the limit override, then backing off. Wiring each > limit for each joint to its own input is also more convenient than > or'ing different limits together. PCI parallel ports are cheap and can > provide plenty of extra I/O. > > > -- > Kirk Wallace > http://www.wallacecompany.com/machine_shop/ > http://www.wallacecompany.com/E45/index.html > California, USA > > > ------------------------------------------------------------------------------ > Try before you buy = See our experts in action! > The most comprehensive online learning library for Microsoft developers > is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, > Metro Style Apps, more. Free future releases when you subscribe now! > http://p.sf.net/sfu/learndevnow-dev2 > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ Try before you buy = See our experts in action! The most comprehensive online learning library for Microsoft developers is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, Metro Style Apps, more. Free future releases when you subscribe now! http://p.sf.net/sfu/learndevnow-dev2 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
