On Saturday 02 December 2017 10:54:58 Jon Elson wrote: > On 12/02/2017 05:12 AM, Frederic RIBLE wrote: > > On 2017-12-02 11:24, Gene Heskett wrote: > >> I'd have really serious doubts anybody ever considered > >> the transmission > >> line termination characteristics needed to preserve a > >> perfect square > >> wave anyway. And if I asked that of the guy at Lowes > >> spooling off 100 > >> feet of that cable for me, he'd think I was suddenly > >> talking in swahili. > > > > Many MODBUS/RS485 papers are talking about 120 ohms bus > > termination and signal reflection effects. > > http://www.ti.com/lit/an/slla272c/slla272c.pdf > > Most twisted pair cables, over a wide range of wire sizes, > come out to somewhere around 110 Ohm differential > impedance. So, anything between 100 and 120 Ohms works out > pretty well for termination. > > Jon > I just got it put together, more or less as a breadboard.
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. With 12 volts on the motor so the spindle is doing about 1rps. BUT, the a/b inputs are biased so heavily that it never hits the swing threshold to change the output state. A is sitting at about 3.5 volts, and B is sitting at 1.02 volts. I can probably find and remove those R's, but it will be easier to see if the LC Electrinics web site has some docs. They were nice enough to silk screen the site on the bottom of the pcb. <http://www.lctech-inc.com>. Nope, they show the module, but no downloads about it are available. These things might work on a real rs485 circuit, but sure aren't going to work for this without some changes. Grrrrr. I hate trying to unsolder a grain of sand. My hands aren't as steady as they once were. I did get rid of the 120 ohm R, and tied the /RE and OE lines to ground. But this made the output look pretty much like the input, with a huge slew rate limit. Works ok at 13 volts to the motor=297 rpm at the spindle, but I've serious doubts it will make it to 1000 rpms, let alone 3000. This is u-max485 whereas the max485 on the other much larger convertor is the next size bigger, but still a surface mount, package. There are 2 chip caps on the board, but they are across the rails for bypassing. The bias unbalance went away when I enabled the receiver section by grounding the /RE terminal, and killed its output with the same ground on the OE terminal. I'll mess around tomorrow & see if I can find the src of the slew rate limit. These things are supposed to be schmidt triggers I thought. Thanks all. 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
