On Sunday 03 December 2017 19:38:57 Jon Elson wrote: > On 12/03/2017 06:02 PM, Gene Heskett wrote: > > But this teeny little board doesn't want to work unless I > > can figure out where the input bias R's are, and excise > > then. This board has a 120 ohm a/b termination R that I > > can measure from the A terminal to the B terminal. But > > thats one hell of a load on the encoder, which surprises > > me somewhat considering it claims to draw 140 ma. > > Disconnect the encoder from this board, and scope the > > leads from the encoder and I am getting a nice, with > > spikes, nominally ground to 3.75 volts swing on each wire. > > Well, you can do AC termination, just put a 0.1uF cap in > series with the 120 Ohm resistor between the true and > complement output. That should get rid of the spikes > without excessive DC load on the driver. > > Jon > But Jon, I'm not useing it for output. I am using it for a balanced encoder to a cmos input single-ended input on the bob.
Based on wrecking one or more of them, and only having 2 more new ones left, I just bought 5 more of them for $2.29 on fleabay. The very similar board, but with a usb adapter on it, Just Works perfectly if I unplug the flea clips that bridge it to the usb, and take pin 1 of the max485 direct to the bob. Since I only have one of those, but 4 of the one made for ttl i/o on one end, and the usual half duplex rs485 on the other end, I'll see if I can figure out the diff thats making this one so slew rate limited. Its almost as if the bob has a rather large capacitor at its input, but theres none visible when looking over the board. I'll test for that tomorrow too. Maybe this cable if a huge # of pf/foot, its the cable I cut off the encoder because it came with about 7 or 8 feet & I needed less than 4'. My new parts tester ought to be able to look at it as a capacitor? But first, I'll look at it at the a or b wire disconnected from the bob, which should tell me if its 1000pf a foot cable, or a bob that looks like a 5000 pf cap to the wire. I'd put what I thought was a 100 ohm series termination R, between the max485's pin 1 and the cable going toward the bob. That could be marked wrong. a 100k there would certainly explain that, except looking at pin 1 of the max485 shows the fast rise time as about half the total delta. That says the term r is about right, and theres a huge capacitance in the cable or at the bob input. All I have to do is find it. But my legs are done for today, up and down like a jumping jack to the 2nd step on a kitchen ladder. Gets to a diabetics legs because they don't get enough insulin to feed the muscles properly. Don't get old AND diabetic, its not fun. Thanks Jon. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
