On Sunday 28 March 2010, Steve Robertson wrote: >Hey Guys, > >Been testing my machine and am having trouble with random movement of each >of the axis. I believe the problem is because the input to my homemade >driver boards is high impedance and the STEP and DIRECTION lines are not >terminated. Any voltage spikes or EM interference in the immediate area, >like the laminate trimmer that I'm using as a cutting tool, seems to cause >the problem. > >To supress (hopefully) the problem, I figured I'd just put termination >resistors on the breakout board. So... I'm guessing the value of the >termination resistors would not be critical but, surely, there is a > correct value. What would be the correct value for a resistor between the > STEP / DIRECTION signals and Ground? > Actually, due to the unbalanced amount of pullup vs pulldown available from the average ttl compatible parport, you want the resistors to be from the signal line up to a point that has at least 3 volts solidly available, with the maximum somewhere in the low 4 volt area. Going all the way to the rail can upset some ttl circuitry.
It is much more susceptible to noise pickup when in the logic one condition than it is in the logic zero state. As for the word 'termination', that really is only correctly applied if the terminating resistance equals the surge impedance of the cable, and typical ribbon cable is in the general range of 120 to 124 ohms if every other conductor is grounded, the usual case in most situations that use ribbon cable as interconnection medium. This, unless the drivers are made for that high a service power, is not practical, and no parport drivers that I'm aware of can drive that heavy a load. So rather than 'terminating' the cable to control the signal edge echo's, the best you can do is pull the logic one voltage safely above the guaranteed logic one voltage of 2.4 volts. A 4.7k resistor per line, attached to the cathode terminal of a common silicon power diode whose anode is connected to the 5 volt rail, will give you about 4.35 volts to feed all the resistors, and one such diode can serve all the lines so you only need one of them, should pull you safely above 2.4 volts and give you sufficient noise margin to make for dependable function. Without the added pullup, the logic one voltage will only rarely make it to 3 volts, and with the light pullup, the noise pickup is going to be a problem. The logic zero drive available from most parports is more than enough to pull that 4.7k resistor down to about 50 millivolts, well over a half volt noise margin and do it with 100% dependability. If that isn't sufficient, then grounding and shielding options might have to be explored. >I'll also add some decoupling caps to the Power Supply lines to stop any >potential problems there. Always a good idea, if the supply is analog. Digital supplies do not well absorb the recovery currents a stepper generates, and have been shutdown nuisances in my experiences. Adding capacitance on the outputs of most digital stuff makes for a turn on surge that is seen as an overcurrent, and causes a shutdown also. That doesn't make for good parts. :( Analog Just Works(TM). >TIA, Steve Robertson >steverob [at] ccvn [dot] com > > >-------------------------------------------------------------------------- >---- Download Intel® Parallel Studio Eval >Try the new software tools for yourself. Speed compiling, find bugs >proactively, and fine-tune applications for parallel performance. >See why Intel Parallel Studio got high marks during beta. >http://p.sf.net/sfu/intel-sw-dev >_______________________________________________ >Emc-users mailing list >Emc-users@lists.sourceforge.net >https://lists.sourceforge.net/lists/listinfo/emc-users > -- 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) The trouble with eating Italian food is that five or six days later you're hungry again. -- George Miller ------------------------------------------------------------------------------ Download Intel® Parallel Studio Eval Try the new software tools for yourself. Speed compiling, find bugs proactively, and fine-tune applications for parallel performance. See why Intel Parallel Studio got high marks during beta. http://p.sf.net/sfu/intel-sw-dev _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
