On Monday, May 28, 2012 03:10:38 AM Jon Elson did opine: > gene heskett wrote: > > Jon, while the encoder I built for the lathe spindle was pretty crude, > > one of the things I did in my research was to make it an given that > > the opto device chosen had an active, high speed CMOS rail to rail > > output. While the qradrature may not have been accurate enough to be > > usable as a velocity detector, the output of all 3 channels is > > absolutely noise free. I may yet carve me another wheel, from even > > thinner material which would allow me to use a pcb sized drill for > > the mill to cut it with. > > > > I think, when any of us is shopping for an encoder, any encoder with a > > TTL level only, or open collector type output should be removed from > > our purchase consideration. Even differential outputs, which need > > twice the wires, should not be immune to this purchasing rule. > > > > They are simply too susceptible to stray noise pickup to even be > > considered when they are in a high impedance state 50% of the time. > > I certainly never advocated buying an open-collector encoder. > I have used a few, and they work, but I feel a lot more comfortable > with "totem-pole" drivers that are always driving the signal > at a low impedance. I don't think there is anything wrong with > "TTL" drivers as opposed to CMOS. Well, anything made after > 2000 is goign to be CMOS for sure. > > Jon
But there are still plenty of the older TTL stuffs gathering dust on the shelves at Newark et all. The advantage of CMOS is to me, the much larger noise immunity. Running on 5 volt rails, noise has to be nearly half that 5 volts in order to even get into the active regions of the logic. TTL only needs about .6 volts of noise to get into the active area if the total swing, which running on 5 volts is from .1 volts for a zero, and about 3.1 volts for a logic 1 when loaded to its rated fanout. Typical guaranteed logic zero might extend as high as .6 volts, and logic 1 is guaranteed only for inputs above 2.4 volts. CMOS outputs are relatively low impedance drive, swinging to within 1-10 millivolts of the supply rails. Draw enough current from a CMOS gate to pull it 100 milivolts away from the rail and you are likely exceeding the gates ability to deliver more current and it will begin to resistively heat, but it can gobble up an amazing amount of noise doing that. But in the case where more drive is needed, paralleling the gates can achieve quite respectable output currents, I have driven relay coils with the heavier of the two cmos 4000 family hex gates, using one gate to get the inversion I needed, driving the other 5 to drive the coil, with a protective diode of course. The chip did not get more than 5F warmer than ambient, and could even still be working 35 years later for all I know. Maybe, the transmitter that needed that particular bit of control was such a piece of crap that it cannot possibly be still running. The company that made it certainly has expired as a google search discloses. Usually rated for 15 volts, I've found in one case where a 4028, a 3 to 8 decoder, ran a bit warm, but ran otherwise happily on 28 volts. That 28 volts gave me the switching speed I needed to add colored borders to the characters of an old 3M character generator, extending its useful life several years at KRCR-TV in Redding CA in the late 70's. Whether the slight heat was from the voltage excess, or from the high speed switching it was doing was never determined, but it Just Worked(TM). 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) My web page: <http://coyoteden.dyndns-free.com:85/gene> One nuclear bomb can ruin your whole day. ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
