Re: [Emc-users] Modbus wiring

[Danny Miller]

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 might think "there's that 'L' terminal in front for 'Logic Ground', 
I'll tie the RS485 ground together there".  But I measured, it's not 
tied to the RS485 driver's ground- which is confusing and weird but 
that's what it is.  Clearly Hitachi did NOT intend for it to be ground, 
since it's not on the RJ45 jack. It'd be awkward to have a removable 
RJ45 and a nonpluggable ground.

I ended up using an ADAM isolated RS232-RS485 bridge (needed to get 
RS232 to RS485 anyhow).  This is still questionable because I STILL only 
floated the RS485 ground- but that's better than having it referenced to 
something guaranteed to be incompatible with the X200's ground.  
Snooping the bus driver, the Modbus does get a bad transaction like 10% 
of the time but that's good enough.

I honestly don't know how Hitachi intended for that X200 to work. In the 
WJ200 manual, they show "L" IS supposed to be the ground for RS485 comm, 
though.  I went through the manual and can't find the pinout for the 
RJ45 jack, I don't know if they included a ground or not.  I guess 
that's what they intended for the X200 too, to use the "L" as gnd?

But anyhow-

The X200 and WJ200 at least have optional internal termination 
resistors.  But you're only supposed to build a modbus in a line and 
only the end device can have a termination resistor.

"Quality" of wire will NOT matter much especially in short lengths like 
<3m or whatever.

Danny

On 3/30/2016 11:13 AM, andy pugh wrote:
> On 9 February 2016 at 11:23, andy pugh  wrote:
>> This is where things get iffy. Imagine that I have the 2-terminal
>> dongle plugged into a laptop. What do I use for a gnd? Stuff a wire
>> down the side of the USB connector? trap the wire in the hinge? :-)
> In the end, I fitted a different VFD which has a pre-written HAL driver.
> It also only has two connections for RS485, so is less confusing.
> I found a way to connect the dongle to the motherboard header:
> https://forum.linuxcnc.org/forum/18-computer/30675-on-motherboard-modbus-rs485-connection
> They are so cheap that I was prepared to go through a couple
> experimenting, but it works fine.
>


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Re: [Emc-users] Wheezy for Old PCs?

[Ricardo Moscoloni]

one thing to note (as already i witness it), is that wheezy as light is on
memory resources (100mb or so for running), something screw latency in old
pc's when ubuntu 10.04lts is not.

old pcs are:
p4 on an asrock 775i65G mb
amd athlon64 not remember mb

regards
rck


2016-03-31 15:34 GMT-03:00 Gene Heskett :

> On Thursday 31 March 2016 11:45:01 John Thornton wrote:
>
> > Why not just use Ubuntu 10.04?
> >
> > JT
>
> The newer install iso is basically a debian wheezy-7.8, with full access
> to the debian repos to add almost anything you might need when its not
> carving parts.  Lots better than the *buntu's IMO.
>
> > On 3/31/2016 8:58 AM, Todd Zuercher wrote:
> > > I was rummaging around with synaptic, and noticed that the Linuxcnc
> > > Wheezy Repository has a linux-image-3.4-9-rtai-486 kernel and
> > > headers. I tried to install it to see how/if it might work. It
> > > booted ok but when I tried to run Linuxcnc it said that I must use
> > > the 686-pae kernal. The whole point of my experiment was to try not
> > > to use the 686-pae for some old uni-processor machines. What else am
> > > I missing? Is this even possible to make work? If the non pae kernel
> > > is useless why is it even in the repo?
> >
> > --
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>
> Cheers, Gene Heskett
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>
>
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Re: [Emc-users] Modbus wiring

[Gene Heskett]

On Friday 01 April 2016 17:17:16 John Thornton wrote:

> You're going to turn copper to steel?
>
Puzzled me too John, until I remembered the date.  Today I don't believe 
a thing I read, and only about 5% of what I see.

> On 4/1/2016 3:57 PM, Nicklas Karlsson wrote:
> >> On 1 April 2016 at 20:07, Dave Cole  wrote:
> >>> Profibus is basically an RS485 network as well.I have no idea
> >>> why they used a purple jacket color.
> >>
> >> There is an awful lot of purple wiring on the London Underground.
> >> Perhaps that is data cable.
> >
> > What do you plan to do with it? Steel it?
> >
> > 
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Re: [Emc-users] Nets and parameters in *.hal files

[andy pugh]

On 1 April 2016 at 23:51, Jeff Epler  wrote:

> This is not really feasible in the general case anyway.  What pins does
> "hm2_eth.so" create?  This depends on the connected hardware, and even
> the initial prefix of the pins/parameters is not known until the
> attached hardware is probed.

Ditto anything with a smart-serial device attached. The pins and
params depend entirely on the the firmware on the board.


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Re: [Emc-users] Nets and parameters in *.hal files

[Jeff Epler]

No, there is no way to take a component and find out the pins
and parameters it will create, except for loading it and looking at the
lists halcmd can print.

This is not really feasible in the general case anyway.  What pins does
"hm2_eth.so" create?  This depends on the connected hardware, and even
the initial prefix of the pins/parameters is not known until the
attached hardware is probed.

Jeff

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Re: [Emc-users] Modbus wiring

[Nicklas Karlsson]

I can turn gold to lead, do you want see. It should have been that kind of 
steal.


> You're going to turn copper to steel?
> 
> On 4/1/2016 3:57 PM, Nicklas Karlsson wrote:
> >> On 1 April 2016 at 20:07, Dave Cole  wrote:
> >>> Profibus is basically an RS485 network as well.I have no idea why
> >>> they used a purple jacket color.
> >> There is an awful lot of purple wiring on the London Underground.
> >> Perhaps that is data cable.
> > What do you plan to do with it? Steel it?
> >
> > --
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> 
> 
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Re: [Emc-users] Modbus wiring

[Dave Cole]

On 4/1/2016 1:48 PM, andy pugh wrote:
> On 1 April 2016 at 20:07, Dave Cole  wrote:
>> Profibus is basically an RS485 network as well.I have no idea why
>> they used a purple jacket color.
> There is an awful lot of purple wiring on the London Underground.
> Perhaps that is data cable.

Siemens makes subway cars and rail control systems.If they did the 
control system, then it is likely Profibus cable.  Its frequently used 
for I/O systems.

Dave

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Re: [Emc-users] Modbus wiring

[John Thornton]

You're going to turn copper to steel?

On 4/1/2016 3:57 PM, Nicklas Karlsson wrote:
>> On 1 April 2016 at 20:07, Dave Cole  wrote:
>>> Profibus is basically an RS485 network as well.I have no idea why
>>> they used a purple jacket color.
>> There is an awful lot of purple wiring on the London Underground.
>> Perhaps that is data cable.
> What do you plan to do with it? Steel it?
>
> --
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Re: [Emc-users] Modbus wiring

[Nicklas Karlsson]

> On 1 April 2016 at 20:07, Dave Cole  wrote:
> > Profibus is basically an RS485 network as well.I have no idea why
> > they used a purple jacket color.
> 
> There is an awful lot of purple wiring on the London Underground.
> Perhaps that is data cable.

What do you plan to do with it? Steel it?

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Re: [Emc-users] Modbus wiring

[Gene Heskett]

On Friday 01 April 2016 13:58:55 Stephen Dubovsky wrote:

> You can not include the termination or pull up/down resistors on a
> RS422/485 network in the drivers (unless they can by disabled.)  You
> don't know where (or how many) are located on the bus.  They are
> multi-drop networks after all.  Only terminate the two far ends.
>
Thank you Stephen, I hadn't remembered that as I only had one instance of 
it, point to point at the tv station, and it Just Worked(TM).

Can I blame it on aged wet ram?  You are of course correct.
>
> On Fri, Apr 1, 2016 at 1:37 PM, Nicklas Karlsson <
>
> nicklas.karlsso...@gmail.com> wrote:
> > > > > Twisted pair is a transmission line and subject to all the
> > > > > vagaries of load matching and such.
> > > > >
> > > > > That cables impedance is not listed there,
> > > >
> > > > Yes it is. 120 Ohms.
> > >
> > > Resistance is NOT impedance, Andy.  And I first read that as 1200
> > > because my screen is high res and the ohm sign looks like another
> > > 0 at first glance, I mentaly threw that whole line out as being
> > > technically impractical, written by someone who obviously did not
> > > understand anything about transmission line theory.  My bad for
> > > the miss read of 120 ohms as 1200 ohms, but still the use of the
> > > word resistance renders it null and void in my technical mind.  If
> > > they meant impedance, they should have written impedance. If the
> > > loads J dactor is 0, then they would directly correspond, but that
> > > J=0 generally would be quite rare in the real world.
> > >
> > > However, my comments about the load termination, in this case with
> > > a 120 ohm resistor from wire to wire at the load end are still
> > > valid.
> >
> > https://www.expertdaq.com/static/img/info/RS485-termination-120-ohm.
> >gif
> >
> > I think the extra bias network to power rails is necessary to get
> > correct level in between data is sent.
> >
> > > Properly done, thats taken care of in the rs485 interface in the
> > > device, but you never know when some bean counter between the
> > > engineer and the production line will delete that seemingly
> > > useless part as a cost saver.
> >
> > They save beans in the short but have to spend more of them later.
> > By investing the beans in rather good fast growing soil they figured
> > out they could return more of them later but still have some left
> > for them self.
> >
> >
> > 
> >-- Transform Data into Opportunity.
> > Accelerate data analysis in your applications with
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Re: [Emc-users] Modbus wiring

[Dave Cole]

Well that is true.   It does tend to stand out from the generic gray 
control cabling.

Dave

On 4/1/2016 1:26 PM, Peter Blodow wrote:
> ... because this will be different from all other kinds of cables.
> Imagine large facilities, dozens of buildings, with thousands of devices
> being controlled by a central control room, wired by random cable
> colours. These are being mixed with current supply, LAN, telephone and
> all other cables together on a cable way... I was one of these customers
> who urged the companies and the DIN authorities to use easily
> identifiable cables for building control.
>
> Peter Blodow
>
>
>
> Am 01.04.2016 21:07, schrieb Dave Cole:
>> Profibus is basically an RS485 network as well. I have no idea why
>> they used a purple jacket color. Dave
> ---
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Re: [Emc-users] Modbus wiring

[andy pugh]

On 1 April 2016 at 20:07, Dave Cole  wrote:
> Profibus is basically an RS485 network as well.I have no idea why
> they used a purple jacket color.

There is an awful lot of purple wiring on the London Underground.
Perhaps that is data cable.

-- 
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designed for the especial use of mechanical geniuses, daredevils and
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Re: [Emc-users] Modbus wiring

[Peter Blodow]

... because this will be different from all other kinds of cables. 
Imagine large facilities, dozens of buildings, with thousands of devices 
being controlled by a central control room, wired by random cable 
colours. These are being mixed with current supply, LAN, telephone and 
all other cables together on a cable way... I was one of these customers 
who urged the companies and the DIN authorities to use easily 
identifiable cables for building control.

Peter Blodow



Am 01.04.2016 21:07, schrieb Dave Cole:
> Profibus is basically an RS485 network as well. I have no idea why 
> they used a purple jacket color. Dave 

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Re: [Emc-users] Modbus wiring

[Dave Cole]

On 4/1/2016 11:55 AM, andy pugh wrote:
> On 1 April 2016 at 18:33, Dave Cole  wrote:
>> Unless you are running at 60+ Kbaud and up and long distances you likely
>> won't need anything but twisted pair cabling.
> It was mainly the purple thing.
Ha ha...

Well,  I must like purple also.I have the remains of a 1000 ft spool 
of some purple Siemens Profibus cable that looks almost identical to 
that cable.
Profibus is basically an RS485 network as well.I have no idea why 
they used a purple jacket color.

Dave
>

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Re: [Emc-users] Modbus wiring

[Nicklas Karlsson]

Yes termination with pull up/down in both ends.


On Fri, 1 Apr 2016 13:58:55 -0400
Stephen Dubovsky  wrote:

> You can not include the termination or pull up/down resistors on a
> RS422/485 network in the drivers (unless they can by disabled.)  You don't
> know where (or how many) are located on the bus.  They are multi-drop
> networks after all.  Only terminate the two far ends.
> 
> 
> On Fri, Apr 1, 2016 at 1:37 PM, Nicklas Karlsson <
> nicklas.karlsso...@gmail.com> wrote:
> 
> > > > > Twisted pair is a transmission line and subject to all the vagaries
> > > > > of load matching and such.
> > > > >
> > > > > That cables impedance is not listed there,
> > > >
> > > > Yes it is. 120 Ohms.
> > >
> > > Resistance is NOT impedance, Andy.  And I first read that as 1200 because
> > > my screen is high res and the ohm sign looks like another 0 at first
> > > glance, I mentaly threw that whole line out as being technically
> > > impractical, written by someone who obviously did not understand
> > > anything about transmission line theory.  My bad for the miss read of
> > > 120 ohms as 1200 ohms, but still the use of the word resistance renders
> > > it null and void in my technical mind.  If they meant impedance, they
> > > should have written impedance. If the loads J dactor is 0, then they
> > > would directly correspond, but that J=0 generally would be quite rare in
> > > the real world.
> > >
> > > However, my comments about the load termination, in this case with a 120
> > > ohm resistor from wire to wire at the load end are still valid.
> >
> > https://www.expertdaq.com/static/img/info/RS485-termination-120-ohm.gif
> >
> > I think the extra bias network to power rails is necessary to get correct
> > level in between data is sent.
> >
> > > Properly done, thats taken care of in the rs485 interface in the device,
> > > but you never know when some bean counter between the engineer and the
> > > production line will delete that seemingly useless part as a cost saver.
> >
> > They save beans in the short but have to spend more of them later. By
> > investing the beans in rather good fast growing soil they figured out they
> > could return more of them later but still have some left for them self.
> >
> >
> > --
> > Transform Data into Opportunity.
> > Accelerate data analysis in your applications with
> > Intel Data Analytics Acceleration Library.
> > Click to learn more.
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> >
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Re: [Emc-users] Modbus wiring

[Stephen Dubovsky]

You can not include the termination or pull up/down resistors on a
RS422/485 network in the drivers (unless they can by disabled.)  You don't
know where (or how many) are located on the bus.  They are multi-drop
networks after all.  Only terminate the two far ends.


On Fri, Apr 1, 2016 at 1:37 PM, Nicklas Karlsson <
nicklas.karlsso...@gmail.com> wrote:

> > > > Twisted pair is a transmission line and subject to all the vagaries
> > > > of load matching and such.
> > > >
> > > > That cables impedance is not listed there,
> > >
> > > Yes it is. 120 Ohms.
> >
> > Resistance is NOT impedance, Andy.  And I first read that as 1200 because
> > my screen is high res and the ohm sign looks like another 0 at first
> > glance, I mentaly threw that whole line out as being technically
> > impractical, written by someone who obviously did not understand
> > anything about transmission line theory.  My bad for the miss read of
> > 120 ohms as 1200 ohms, but still the use of the word resistance renders
> > it null and void in my technical mind.  If they meant impedance, they
> > should have written impedance. If the loads J dactor is 0, then they
> > would directly correspond, but that J=0 generally would be quite rare in
> > the real world.
> >
> > However, my comments about the load termination, in this case with a 120
> > ohm resistor from wire to wire at the load end are still valid.
>
> https://www.expertdaq.com/static/img/info/RS485-termination-120-ohm.gif
>
> I think the extra bias network to power rails is necessary to get correct
> level in between data is sent.
>
> > Properly done, thats taken care of in the rs485 interface in the device,
> > but you never know when some bean counter between the engineer and the
> > production line will delete that seemingly useless part as a cost saver.
>
> They save beans in the short but have to spend more of them later. By
> investing the beans in rather good fast growing soil they figured out they
> could return more of them later but still have some left for them self.
>
>
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Re: [Emc-users] Modbus wiring

[Nicklas Karlsson]

> > > Twisted pair is a transmission line and subject to all the vagaries
> > > of load matching and such.
> > >
> > > That cables impedance is not listed there,
> >
> > Yes it is. 120 Ohms.
> 
> Resistance is NOT impedance, Andy.  And I first read that as 1200 because 
> my screen is high res and the ohm sign looks like another 0 at first 
> glance, I mentaly threw that whole line out as being technically 
> impractical, written by someone who obviously did not understand 
> anything about transmission line theory.  My bad for the miss read of 
> 120 ohms as 1200 ohms, but still the use of the word resistance renders 
> it null and void in my technical mind.  If they meant impedance, they 
> should have written impedance. If the loads J dactor is 0, then they 
> would directly correspond, but that J=0 generally would be quite rare in 
> the real world.
> 
> However, my comments about the load termination, in this case with a 120 
> ohm resistor from wire to wire at the load end are still valid.

https://www.expertdaq.com/static/img/info/RS485-termination-120-ohm.gif

I think the extra bias network to power rails is necessary to get correct level 
in between data is sent.

> Properly done, thats taken care of in the rs485 interface in the device, 
> but you never know when some bean counter between the engineer and the 
> production line will delete that seemingly useless part as a cost saver.

They save beans in the short but have to spend more of them later. By investing 
the beans in rather good fast growing soil they figured out they could return 
more of them later but still have some left for them self.

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[Emc-users] Nets and parameters in *.hal files

[Nicklas Karlsson]

I have read thru the source. Are there any method to find out from source name 
of block connection points used to set parameters with "setp" and connect 
signals with "net" in *.hal files?

I have seen they are documented in manual but machine readable from source 
would be good.

Regards Nicklas Karlsson

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Re: [Emc-users] Modbus wiring

[andy pugh]

On 1 April 2016 at 16:58, John Dammeyer  wrote:
> Which cable is the Van Damme?
> Can you post an ebay # or link?

http://www.ebay.co.uk/itm/141838984494

-- 
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Re: [Emc-users] Modbus wiring

[Gene Heskett]

On Friday 01 April 2016 11:33:18 andy pugh wrote:

> On 1 April 2016 at 16:22, Gene Heskett  wrote:
> >> http://www.rapidonline.com/cables-connectors/belden-9841-data-cable
> >>-for-rs-485-51-6887
> >
> > Twisted pair is a transmission line and subject to all the vagaries
> > of load matching and such.
> >
> > That cables impedance is not listed there,
>
> Yes it is. 120 Ohms.

Resistance is NOT impedance, Andy.  And I first read that as 1200 because 
my screen is high res and the ohm sign looks like another 0 at first 
glance, I mentaly threw that whole line out as being technically 
impractical, written by someone who obviously did not understand 
anything about transmission line theory.  My bad for the miss read of 
120 ohms as 1200 ohms, but still the use of the word resistance renders 
it null and void in my technical mind.  If they meant impedance, they 
should have written impedance. If the loads J dactor is 0, then they 
would directly correspond, but that J=0 generally would be quite rare in 
the real world.

However, my comments about the load termination, in this case with a 120 
ohm resistor from wire to wire at the load end are still valid.  
Properly done, thats taken care of in the rs485 interface in the device, 
but you never know when some bean counter between the engineer and the 
production line will delete that seemingly useless part as a cost saver.

I learned 25 years ago to never, ever, discount that possibility when 
something wasn't working right.  The places where a well designed 
product was made into a wholesale failure make up quite a few of my "war 
stories".

> It is also specifically described as RS-485 cable.
>
> But I also noticed that Van Damme list their DMX cable as suitable for
> RS-485 and that that is available from eBay at fairly low cost, so
> ordered some in on a "just in case" basis.
> It is also a pretty purple colour.

Chuckle.  The "oh look, a pony" thing. ;)  The electrons are color blind.
Which is a good thing IMO, since if you see that color 5 years later, you 
should recall that the cable is an RS485 circuit, and thats a Good 
Thing(TM).

Cheers, Gene Heskett
-- 
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 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page 

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Re: [Emc-users] Modbus wiring

[andy pugh]

On 1 April 2016 at 18:33, Dave Cole  wrote:
> Unless you are running at 60+ Kbaud and up and long distances you likely
> won't need anything but twisted pair cabling.

It was mainly the purple thing.

-- 
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designed for the especial use of mechanical geniuses, daredevils and
lunatics."
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Re: [Emc-users] Modbus wiring

[Dave Cole]

On 4/1/2016 10:33 AM, andy pugh wrote:
> On 1 April 2016 at 16:22, Gene Heskett  wrote:
>>> http://www.rapidonline.com/cables-connectors/belden-9841-data-cable-for-rs-485-51-6887
>> Twisted pair is a transmission line and subject to all the vagaries of
>> load matching and such.
>>
>> That cables impedance is not listed there,
> Yes it is. 120 Ohms.
> It is also specifically described as RS-485 cable.
>
> But I also noticed that Van Damme list their DMX cable as suitable for
> RS-485 and that that is available from eBay at fairly low cost, so
> ordered some in on a "just in case" basis.
> It is also a pretty purple colour.

Unless you are running at 60+ Kbaud and up and long distances you likely 
won't need anything but twisted pair cabling.

Ethernet cable will work fine.

Dave



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Re: [Emc-users] Modbus wiring

[John Dammeyer]

Which cable is the Van Damme?  
Can you post an ebay # or link?
Thanks
John

> -Original Message-
> From: andy pugh [mailto:bodge...@gmail.com]
> Sent: April-01-16 8:33 AM
> To: Enhanced Machine Controller (EMC)
> Subject: Re: [Emc-users] Modbus wiring
> 
> On 1 April 2016 at 16:22, Gene Heskett  wrote:
> >> http://www.rapidonline.com/cables-connectors/belden-9841-data-
> cable-for-rs-485-51-6887
> >
> > Twisted pair is a transmission line and subject to all the vagaries of
> > load matching and such.
> >
> > That cables impedance is not listed there,
> 
> Yes it is. 120 Ohms.
> It is also specifically described as RS-485 cable.
> 
> But I also noticed that Van Damme list their DMX cable as suitable for
> RS-485 and that that is available from eBay at fairly low cost, so
> ordered some in on a "just in case" basis.
> It is also a pretty purple colour.
> 
> --
> atp
> "A motorcycle is a bicycle with a pandemonium attachment and is
> designed for the especial use of mechanical geniuses, daredevils and
> lunatics."
> � George Fitch, Atlanta Constitution Newspaper, 1916
> 
> --
> Transform Data into Opportunity.
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Re: [Emc-users] Modbus wiring

[andy pugh]

On 1 April 2016 at 16:22, Gene Heskett  wrote:
>> http://www.rapidonline.com/cables-connectors/belden-9841-data-cable-for-rs-485-51-6887
>
> Twisted pair is a transmission line and subject to all the vagaries of
> load matching and such.
>
> That cables impedance is not listed there,

Yes it is. 120 Ohms.
It is also specifically described as RS-485 cable.

But I also noticed that Van Damme list their DMX cable as suitable for
RS-485 and that that is available from eBay at fairly low cost, so
ordered some in on a "just in case" basis.
It is also a pretty purple colour.

-- 
atp
"A motorcycle is a bicycle with a pandemonium attachment and is
designed for the especial use of mechanical geniuses, daredevils and
lunatics."
— George Fitch, Atlanta Constitution Newspaper, 1916

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Re: [Emc-users] Modbus wiring

[Gene Heskett]

On Friday 01 April 2016 07:02:14 andy pugh wrote:

> On 1 April 2016 at 11:46, Erik Christiansen  
wrote:
> > Incidentally, is that cable in the last picture twisted pair?
> > Shielded or no, is its characteristic impedance controlled?
>
> It is just what I cold find in Maplin that evening.
> I will see how well it works, and if there is a problem I can buy some
> of this:
> http://www.rapidonline.com/cables-connectors/belden-9841-data-cable-fo
>r-rs-485-51-6887

Twisted pair is a transmission line and subject to all the vagaries of 
load matching and such.

That cables impedance is not listed there, but should be on beldens own 
site. It looks quite similar to a black jacketed version used for 
balanced audio in a broadcast facility, with total usage often measured 
in miles.  And while its great cable for short runs, its characteristic 
impedance is only 60 ohms, which when driven by the usual amplifier 
expecting to find a "ma bell" style 600 ohm load, the cables capacitance 
can and will cause an high frequency rolloff.

So when I needed to re-design that amplifier rack because the existing 
shop made amps were power hungry and had a high failure rate, I turned 
them into op-amps which at unity or thereabouts gain, have an output 
impedance of almost zero ohms, then built it out to nominally 60 ohms 
with a 27 ohm resistor in series with each side of a driven line.

The difference in audio quality on the air could very easily be heard. 
The HF losses at the far end of a 180 foot run to an editing booth in 
the news room was only a small, almost unmeasureable loss at 15 
kilohertz. Perhaps .1 db. The old design had some .25% distortion, the 
new design was very close to .05%.  And I suspect that was at least half 
accounted for in the ohmage losses of the cable conductors.

With the amplifiers I could buy for broadcasting use at the time, that HF 
loss was almost 20 db in that same 180 feet of cable!

An rs485 circuit should be treated similarly, particularly at the load 
end if the run is long enough that you can see the echo step coming back 
when you're looking at the source end of the circuit.  With modern 
higher speed logic, that might be as little as 5 feet!  If a 62 ohm 
resistor across the two wires at the load end reduces the errors, that 
would be somewhat crude way to check if a scope isn't handy.

Cheers, Gene Heskett
-- 
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Genes Web page 

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Re: [Emc-users] Modbus wiring

[Gene Heskett]

On Friday 01 April 2016 06:46:02 Erik Christiansen wrote:

> On 30.03.16 17:13, andy pugh wrote:
> > In the end, I fitted a different VFD which has a pre-written HAL
> > driver. It also only has two connections for RS485, so is less
> > confusing. I found a way to connect the dongle to the motherboard
> > header:
> > https://forum.linuxcnc.org/forum/18-computer/30675-on-motherboard-mo
> >dbus-rs485-connection They are so cheap that I was prepared to go
> > through a couple experimenting, but it works fine.
>
> Andy,
>
> That is an inspired solution to the ungrounded dangling-dongle problem
> - well worth emulating.
>
> Incidentally, is that cable in the last picture twisted pair? Shielded
> or no, is its characteristic impedance controlled?
>
> I wonder if the dongles have a 110 or 120 ohm termination, or you need
> to add that yourself?
>
> It's usual to put a 100 ohm resistor in a ground wire run between
> ends, to take up a difference due to leakage, but not cause big ground
> loops. That's in data networks - I can't say for sure it's least
> imperfect in a machine situation.
>
> Erik

I've not had to resort to that but its a good idea. Use a 1/8 watt 100 
ohm resistor, and if the "ground loop" is strong enough to burn that 
out, then its safe to say that there are far more serious grounding 
problems in your present wiring config that need to be addressed as they 
are a shock hazard at best, and possibly lethal.

In my latest build, I have a bare twisted grounding wire connecting all 
the machine frames together. But I maintain also a "star" ground config 
in that all shielded cables have the shield connected to one point in 
the "electronics" box, and trimmed back, no connection, at the far end 
of the cable. That connection in the electronics box is also its case 
ground and the machine ground is there also.

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Re: [Emc-users] Modbus wiring

[andy pugh]

On 1 April 2016 at 11:46, Erik Christiansen  wrote:
>
> Incidentally, is that cable in the last picture twisted pair? Shielded
> or no, is its characteristic impedance controlled?

It is just what I cold find in Maplin that evening.
I will see how well it works, and if there is a problem I can buy some of this:
http://www.rapidonline.com/cables-connectors/belden-9841-data-cable-for-rs-485-51-6887

-- 
atp
"A motorcycle is a bicycle with a pandemonium attachment and is
designed for the especial use of mechanical geniuses, daredevils and
lunatics."
— George Fitch, Atlanta Constitution Newspaper, 1916

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Re: [Emc-users] Modbus wiring

[Erik Christiansen]

On 30.03.16 17:13, andy pugh wrote:
> In the end, I fitted a different VFD which has a pre-written HAL driver.
> It also only has two connections for RS485, so is less confusing.
> I found a way to connect the dongle to the motherboard header:
> https://forum.linuxcnc.org/forum/18-computer/30675-on-motherboard-modbus-rs485-connection
> They are so cheap that I was prepared to go through a couple
> experimenting, but it works fine.

Andy,

That is an inspired solution to the ungrounded dangling-dongle problem -
well worth emulating.

Incidentally, is that cable in the last picture twisted pair? Shielded
or no, is its characteristic impedance controlled?

I wonder if the dongles have a 110 or 120 ohm termination, or you need
to add that yourself?

It's usual to put a 100 ohm resistor in a ground wire run between ends,
to take up a difference due to leakage, but not cause big ground loops.
That's in data networks - I can't say for sure it's least imperfect in a
machine situation.

Erik

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