On Saturday 02 December 2017 03:47:14 Frederic RIBLE wrote: > On 2017-12-01 22:44, Gene Heskett wrote: > > So 2 of those solves my diff to single ended signal problems with > > bandwidth to throw away. Almost dc to light. And the duty cycle is > > 50%, plus or minus 2% worst case due to the motors wibblies at > > nearly 8500 rpms. > > Great! > Additionally, some of those RS-422 transceivers have integrated ESD > protection for long wires. > You may consider adding a termination resistor to have the > transmission line working in good conditions. > I think you can check that with a scope on the transmission line: you > should see a good square shape, with not overshoot. > Frederic > http://cnc.f1oat.org > There is overshoot in both directions, that to me translates to group delay errors, mainly due to the scopes ground lead being clipped to the main box. Probably better if I was grounded to the cable shield. We'll see what it looks like once assembled.
Don't forget that it will be clipped and buffered in the bob also, which has 10ns rise and fall times. And the ribbon cable carrying that signal is about 42" long getting to the 5i25. As a transmission line, I've no clue what the characteristic impedance of 3 feet of the shielded cable I am using is. The ribbon cable is fairly well known at about 120 ohms if every other wire is ground, but I'd be amazed to actually find it properly terminated, and its for sure not equiped with inter-digitated grounds. There is a chip r between pin 1 of the max485 and the output pin but it measures at around an ohm. Another 100 ohms there might be a good idea, absorbing some of the echo from the bob's high impedance input. As for looking at it with a good TDR, I expect it would look pretty bad for that last foot or so, from the disconnect plug on the end of the main box on in to the bob itself, probably another 18" of cable there. 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. TDR's can be handier than bottled beer when you've a fried teflon disk someplace in the middle of a 2000' run of 6.125" line running up the tv tower. You look at it with with the TDR, and tell the tower crew which joint of the line to open up, and usually be within 10 feet of the fire. The fire of course crawls toward the src on the energy, so there will be smoke above it to swab out, and possibly another 200 feet below to dis and replace singed teflon. That sure beats hell out of tearing it down from the ground up when all you know is the VSWR, which needs to be well below 1.04 to 1.00, is up to 1.09/1 and the transmitter is protecting itself by an automatic shutdown. And all because the Rosemount ultrasonic ice detector has failed and you can see 20 tons of rime ice hanging off the lee side of the antenna after a Nebraska blizzard because the deicers didn't work when I turned them on manually. Off the air for about 3 weeks that time because a tower crew, if dumb enough to climb with that much ice on things, would have had to chisel it off to get to the burned out joint in the deicer cabling inside the antenna itself. So you wait till the field glasses tell you the ice has ablated away and a crew can actually get to the problem. Just another of my war stories from 40 years in broadcast engineering. :) 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
