On Saturday 02 April 2016 07:04:14 Nicklas Karlsson wrote:
> > I had SERIOUS problems with my X200 VFD and RS485 bus- I "mostly"
> > fixed it.
> >
> > Here's the thing- yes, RS485 is differential, but the VFD is
> > probably NOT opto-isolated input. Differential conveys strong noise
> > immunity- but ONLY when both A and B wires' voltages are within the
> > input range of the VFD's bus driver, which might be -0.5v to 7v
> > relative to the ground on the VFD driver chip. Outside that range,
> > it will NOT function.
> >
> > The X200 confuses me greatly. Yes is has an A and B RS485
> > terminals, but no ground on the RJ45 jack. If you don't have a
> > ground to connect to, this problem can easily come up, and DID. I
> > had to take apart the VFD to measure this- like >20v of noise
> > between the PC ground and the VFD driver. That will NOT work, and
> > didn't.
>
> You just hit the major problem with switched power electronics, common
> mode voltages. A common mode choke will increase common mode
> impedance. Increased impedance will not by itself decrease voltage but
> if some current could be conducted away the higher impedance will
> however lower voltage. One problem is even though ground resistance is
> close to zero impedance is not. I remember I have read a value of 50µH
> as maximum power grid inductance but are not totally sure this is
> correct and particular not for all frequencies.
>
> Common mode voltage source is capacitance between switch power
> electronics conductor with a "square" voltage and ground. It is
> probably correct to think about it as a capacitor connected to ground
> which is switched between the two rectified voltage potentials.
>
> There is capacitor between: "square" voltage between inverter
> transistors and cooling fin. In electric motor between phases and
> ground.
>
>
> Different insulation barrier technologies are more or less tolerant to
> common mode voltage. I think capcitive insulation barriers like these
> used in Texas Instruments ISO7421 are tolerant against common mode
> voltage.
Having just trolled thru wikipedia's entries for RS-485, ModBus and
ProfiBus, I have reluctantly come to the conclusion that so much is
unspecified, that you can call most anything RS-485 and get away with
it.
Hell of a way to run a train.
I would start, with everything powered down and unplugged, by checking to
see, with an ohmmeter, if the 120 ohm termination is there, which it
should be ON BOTH ENDS as its a bidirectional protocol in 99% of the use
cases I can think of. This is wire to wire at the terminals on both
devices. Then I would similarly measure from each wire to common looking
for a resistance lower than the 680 shown, could be as low as 390 if the
PSU electrolytic's in both devices aren't displaying any dielectric
absorbtion, where even though discharged, they act somewhat like a low
voltage battery, its quite common. To counter some of that, reverse the
test leads and use the median of the two measurements.
If that termination isn't found, figure out a way to add it. It should
have been a hard and fast part of the specs, but isn't according to what
I read on wikipedia. And neither is a common connection.
That may have been a safety exclusion they didn't state clearly. But some
of the specs show it being implemented at the outputs of a 3.3 volt
capable card. That alone reduces the range of the common mode input
voltage where error free operation can be obtained between two devices
whose only common connection might be the house static ground, here in
the US , the third round pin on the plug. And that can carry lots of HF
noise as its generally a star config, common only at the building
electrical entrance.
Given all that, I do not see how, in a noisy industrial environment, or
even here at the Heskett's home camp, it can be error free unless an
optical translator, bidirectional, is used at BOTH devices terminals.
That, if the devices were carefull might be done on a single fiber by
cross-polarizing the light beams which in typical useage can give 30 db
of isolation between tx and rx. That of course can be degraded by
sharper turns in the fibers path but over the distances used in the shop
environment, could be largely tuned out by allowing one of the devices
to be rotated axially on the fibers axis.
The question then seems to be, is who makes these rs485 to opto fiber (or
even to RJ45 jacks & cat5 or cat6 cable since it doesn't have this
common mode noise problem that I am aware of), bidirectional translators
and at what cost.
When I say "that I am aware of" I have a piece of cat5 running between an
eyehook at the end of my back porch, up across the yard high enough its
overhead, 30 feet or so to an eyehook at the peak of my "shop" building,
from there is smashed by the closing door, hooked to a 4 port hub in
there, and thence to 2, sometimes 3 machines via cat5 from the hub to
them. And its been there since about 2000 when I did the first cnc
install on that toy mill.
Had a power failure last week so the uptime is short. My lathe is on the
other end of that wire:
gene@lathe:~$ uptime
11:04:08 up 3 days, 20:32, 5 users, load average: 0.65, 0.45, 0.58
gene@lathe:~$ sudo ifconfig eth0
[sudo] password for gene:
eth0 Link encap:Ethernet HWaddr 38:60:77:cd:f3:2f
inet addr:192.168.71.5 Bcast:192.168.71.255
Mask:255.255.255.0
inet6 addr: fe80::3a60:77ff:fecd:f32f/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:170309 errors:0 dropped:0 overruns:0 frame:0
TX packets:109287 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:25075162 (23.9 MiB) TX bytes:154003036 (146.8 MiB)
Zero errors. That cable has been abused as it, in 2010, survived a wind
that measured 112 mph and took down every tree in its path except a 50
foot, 25+ yo pin oak in my front yard. The neighbors all teased me that
I needed to build a table for a beer fridge, and a circle of benches
around it because it was the only shade tree left on our street. I hung
it up thinking temporary, intending to bury it when it failed. I have a
hard time believing that its still working, like waiting for the other
shoe to drop, but theres the evidence that it is. Zero errors in nearly
180 megabytes of traffic.
I think that says a heck of a lot for the transceiver chips used to
interface with the RJ45 jack. Or in this case, for the error correction
ability of the linux TCP/IP implementation.
I know I can't change the rest of the world, but RS-485 to me, looks like
an accident that WILL find a place to happen, repeatedly.
I bought a VFD and motor, and time willing, will put it on something like
the Sable for pcb carving. The 1.5 HP is so low torque it would be
worthless on the G0704 even for wood carving. I've looked at the
terminal strip, and the docs, and ANAICT, there is no common point for
its RS485 either. So if and when LCNC controls it, direct speed and dir
will control it. That at least has a common zero reference voltage
connection.
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>
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