On 2011-04-22, WangoTango <Asgard24@mindspring.com> wrote:
>> > Some RS485 transceivers have pullups inside.
>>
>> I don't see how that obviates the need for a signal ground.
>
> It doesn't. Does anyone that is actually interested in the
> reliability of their RS- 485 network NOT run a GND?
Yes. It's a not uncommon misconception that "RS485 is two-wire
differential" and therefore doesn't need a ground. Just look back in
this thread a bit. :)
Reply by WangoTango●April 22, 20112011-04-22
In article <iosam2$cpk$1@reader1.panix.com>, invalid@invalid.invalid
says...
> On 2011-04-22, baranov <alexanderbaranov@n_o_s_p_a_m.hotmail.com> wrote:
> >>>
> >>> 2) A signal ground is also required. RS485 receivers can typically
> >>> only deal with a common-mode voltage of 7-12V.
> >>>
> >>
> >>If you have signal ground on both sides, you should connect. If not,
> >>don't care, should work also without GND.
>
> In my experience sometimes it doesn't.
>
> > Some RS485 transceivers have pullups inside.
>
> I don't see how that obviates the need for a signal ground.
>
It doesn't.
Does anyone that is actually interested in the reliability of their RS-
485 network NOT run a GND?
Reply by Grant Edwards●April 22, 20112011-04-22
On 2011-04-22, baranov <alexanderbaranov@n_o_s_p_a_m.hotmail.com> wrote:
>>>
>>> 2) A signal ground is also required. RS485 receivers can typically
>>> only deal with a common-mode voltage of 7-12V.
>>>
>>
>>If you have signal ground on both sides, you should connect. If not,
>>don't care, should work also without GND.
In my experience sometimes it doesn't.
> Some RS485 transceivers have pullups inside.
I don't see how that obviates the need for a signal ground.
--
Grant
Reply by baranov●April 22, 20112011-04-22
>>
>> 2) A signal ground is also required. RS485 receivers can typically
>> only deal with a common-mode voltage of 7-12V.
>>
>
>If you have signal ground on both sides, you should connect. If not,
>don�t care, should work also without GND.
>
>Best regards
>
>Stefan DF9BI
>
Some RS485 transceivers have pullups inside.
Alex
---------------------------------------
Posted through http://www.EmbeddedRelated.com
Reply by Oliver Betz●April 18, 20112011-04-18
Grant Edwards wrote:
[...]
>> I am not so good in english, so i try to explain: If both, the master
>> and the slave have a connection to the GND-level, there may be a
>> difference in the voltage level. This will probably cause the problems
>> you mentioned. To prevent this, you can have the devices on a floating
>> level at least at the rs485 inputs.
>
>Even if they're floating, you still need a signal ground conneciton.
>
>I've seen optically isolated RS485 interfaces on industrial control
>systems that were floating fail without a signal ground. They would
>fail intermittently as the signal ground on one end floated in and out
>of the allowed 7V common-mode range for the other end. Maybe there
>was something wrong with the isolation, but without a signal ground it
>didn't work, and with a signal ground they did.
Correct.
The (common mode) impedance of a RS485 transceiver is rather high,
especially the fractional load types. This high impedance isn't
sufficient to keep the isolated circuit in the operating region with
lots of (CM) noise.
Example: 100pF and 10kOhms yields 160kHz corner frequency...
>> In such a case, when there is a level difference, an additional
>> gnd-line would cause a current on the ground wire which can also lead
>> to problems.
Isolation _plus_ a reference conductor is the correct solution in this
case.
Oliver
--
Oliver Betz, Munich
despammed.com is broken, use Reply-To:
>
>Besides, as Grant pointed out, even after you've identified what the
>per-spec A and B ends of the cable are, odds are 50-50 that what it is
>connecting to got it backwards... ;-)
Setting up an RS-485 network is not that hard as long as each device
has some kind of RxData LED and at least one device can be configured
with "fail safe termination" i.e. pull up/down resistors.
Power up a device which is configured for "fail safe termination".
Connect the next station to the network. If the RxData LED burns
continuously, swap A and B. The RxData LED should be constantly off if
no polling is used.
Connect the next station to the network and observe the RxData LED and
repeat the procedure above.
The 2 wire RS-485 system is quite easy to set up, but the 4 wire
RS-422 is a bit more problematic, with typically 4 or even 5 badly
marked conductors, which wire is A/B/A'/B'/C (signal ground) ?
Reply by Mark Borgerson●April 15, 20112011-04-15
In article <pc1hq61hu701i0b1uj08uof8cgu4ku299i@4ax.com>,
bbew.ar@mapson.nozirev.ten says...
>
> On Fri, 15 Apr 2011 10:15:51 -0700, Mark Borgerson
> <mborgerson@comcast.net> wrote:
>
>
> >Hey, I can see some potential sales for a small (DVM-size) box to
> >which you connect the two wires. A pushbutton would cycle through
> >the possibilities of A/B and baud rate. An LCD would show decoded
> >text. when you recogize the data, the box would tell you which is
> >A and which is B and the baud rate. For known data formats (
> >GPS, etc.) the box could auto-search. Shouldn't take anything
> >more than a few receiver chips, a small MPU and a display.
> >If you put a USB interface on it, you could program it to
> >recognize custom protocols. I'd pay $25 for one of those.
>
> Actually, I had one of those designed and ready to go, with autobaud,
> auto-polarity, and 232/422 recognition as well. But we're tuned for low
> volume and high cost, so the Powers That Be pulled the plug.
>
> Besides, as Grant pointed out, even after you've identified what the
> per-spec A and B ends of the cable are, odds are 50-50 that what it is
> connecting to got it backwards... ;-)
Ah well. It would seem a logical product for B&B, and I'm sure their
engineers could design it. Perhaps the problem you noted would make
it less useful if you couldn't force both ends of the circuit to
speak on command.
Mark Borgerson
Reply by Rich Webb●April 15, 20112011-04-15
On Fri, 15 Apr 2011 10:15:51 -0700, Mark Borgerson
<mborgerson@comcast.net> wrote:
>Hey, I can see some potential sales for a small (DVM-size) box to
>which you connect the two wires. A pushbutton would cycle through
>the possibilities of A/B and baud rate. An LCD would show decoded
>text. when you recogize the data, the box would tell you which is
>A and which is B and the baud rate. For known data formats (
>GPS, etc.) the box could auto-search. Shouldn't take anything
>more than a few receiver chips, a small MPU and a display.
>If you put a USB interface on it, you could program it to
>recognize custom protocols. I'd pay $25 for one of those.
Actually, I had one of those designed and ready to go, with autobaud,
auto-polarity, and 232/422 recognition as well. But we're tuned for low
volume and high cost, so the Powers That Be pulled the plug.
Besides, as Grant pointed out, even after you've identified what the
per-spec A and B ends of the cable are, odds are 50-50 that what it is
connecting to got it backwards... ;-)
--
Rich Webb Norfolk, VA
Reply by Mark Borgerson●April 15, 20112011-04-15
In article <2mogq6tbhh79n7c99kgdj9m15q7j7va425@4ax.com>,
bbew.ar@mapson.nozirev.ten says...
>
> On Fri, 15 Apr 2011 08:13:09 -0700, Mark Borgerson
> <mborgerson@comcast.net> wrote:
>
> >In article <jhieq6paekk7ioevigmgam02lh3igpf6vu@4ax.com>,
> >bbew.ar@mapson.nozirev.ten says...
> >>
> >> On Thu, 14 Apr 2011 12:15:01 -0700, Mark Borgerson
> >> <mborgerson@comcast.net> wrote:
> >>
> >> >In article <io6kto$6p8$1@dont-email.me>, aleksazr@gmail.com says...
> >> >>
> >> >> "aleksa" <aleksazr@gmail.com> wrote in message news:io6k61$upc$1@dont-email.me...
> >> >> >I have two CPUs, master and slave.
> >> >> > They are (will be, actually) connected with one SN75176B IC
> >> >> > on each side, with two wires connecting them.
> >> >> >
> >> >> > After reset, the master will be in transmit mode, slave in recv mode.
> >> >> >
> >> >> > When the master sends a command to slave, the master will
> >> >> > switch into recv mode and slave into transmit mode.
> >> >> >
> >> >> > Now, what happens while they are switching? At one moment,
> >> >> > both sides may be in input mode, and the wires will float,
> >> >> > maybe generating a start impulse.
> >> >> >
> >> >> > How is this situation usually handled?
> >> >> >
> >> >> > I could probably solve this with software, by waiting
> >> >> > some time, then discarding the byte received, if any.
> >> >> > The slave would have to wait even longer and then start sending.
> >> >> >
> >> >>
> >> >> Do I just place a pull-up one one line, and a pull-down on the other?
> >> >
> >> >That's the most common solution. I suggest that you read through the
> >> >B&B RS-485 application note:
> >> >
> >> >http://www.bb-elec.com/tech_articles/rs422_485
> >> >_app_note/table_of_contents.asp
> >>
> >> Should be on the required reading list.
> >>
> >> >(The link may need to be reassembled).
> >>
> >> For many (most?) modern newsreaders, putting the link in angle brackets
> >> <http://www.bb-elec.com/tech_articles/rs422_485_app_note/table_of_contents.asp>
> >> prevents the line break.
> >>
> >Hmm. When I replied to your message, the link remained intact. However,
> >if I copy and paste the link, I get:
> >
> ><http://www.bb-elec.com/tech_articles/rs422_485
> >_app_note/table_of_contents.asp>
> >
> >(Broken into two lines on my screen).
>
> Here, too.
>
> >I'm using MicroPlanet Gravity 3.0. I'll check to see if there is a
> >setting that can be changed.
> >
> >Perhaps the link will be restored to one line when the message is sent.
>
> Nope. But, as you say, there may be a configuration setting that will
> retain long links. Or, it's a great excuse to diddle with the source...
>
> >> Now if I could just understand why UARTs idle on mark 'high' and both
> >> 232 and the 422/485 'A' line idle on mark 'low' but the standard
> >> symbology for 422/485 drivers has the inversion circle on the 'B' line.
> >
> >The saving graces of both RS-232 and RS-485 are that getting the lines
> >mixed up doesn't let the smoke out of the usual driver chips an you
> >have a 50% probability of getting it right the first time! ;-)
>
> So true. It's a PITA for the field service crew, though. If you aren't
> seeing the expected $GPGLL is it because A/B are swapped? Or the sender
> is at 38400? You forgot to enable that port altogether? Or you just
> grabbed the wrong pair? Life is too short! ;-)
Hey, I can see some potential sales for a small (DVM-size) box to
which you connect the two wires. A pushbutton would cycle through
the possibilities of A/B and baud rate. An LCD would show decoded
text. when you recogize the data, the box would tell you which is
A and which is B and the baud rate. For known data formats (
GPS, etc.) the box could auto-search. Shouldn't take anything
more than a few receiver chips, a small MPU and a display.
If you put a USB interface on it, you could program it to
recognize custom protocols. I'd pay $25 for one of those.
Mark Borgerson
Reply by Rich Webb●April 15, 20112011-04-15
On Fri, 15 Apr 2011 08:13:09 -0700, Mark Borgerson
<mborgerson@comcast.net> wrote:
>In article <jhieq6paekk7ioevigmgam02lh3igpf6vu@4ax.com>,
>bbew.ar@mapson.nozirev.ten says...
>>
>> On Thu, 14 Apr 2011 12:15:01 -0700, Mark Borgerson
>> <mborgerson@comcast.net> wrote:
>>
>> >In article <io6kto$6p8$1@dont-email.me>, aleksazr@gmail.com says...
>> >>
>> >> "aleksa" <aleksazr@gmail.com> wrote in message news:io6k61$upc$1@dont-email.me...
>> >> >I have two CPUs, master and slave.
>> >> > They are (will be, actually) connected with one SN75176B IC
>> >> > on each side, with two wires connecting them.
>> >> >
>> >> > After reset, the master will be in transmit mode, slave in recv mode.
>> >> >
>> >> > When the master sends a command to slave, the master will
>> >> > switch into recv mode and slave into transmit mode.
>> >> >
>> >> > Now, what happens while they are switching? At one moment,
>> >> > both sides may be in input mode, and the wires will float,
>> >> > maybe generating a start impulse.
>> >> >
>> >> > How is this situation usually handled?
>> >> >
>> >> > I could probably solve this with software, by waiting
>> >> > some time, then discarding the byte received, if any.
>> >> > The slave would have to wait even longer and then start sending.
>> >> >
>> >>
>> >> Do I just place a pull-up one one line, and a pull-down on the other?
>> >
>> >That's the most common solution. I suggest that you read through the
>> >B&B RS-485 application note:
>> >
>> >http://www.bb-elec.com/tech_articles/rs422_485
>> >_app_note/table_of_contents.asp
>>
>> Should be on the required reading list.
>>
>> >(The link may need to be reassembled).
>>
>> For many (most?) modern newsreaders, putting the link in angle brackets
>> <http://www.bb-elec.com/tech_articles/rs422_485_app_note/table_of_contents.asp>
>> prevents the line break.
>>
>Hmm. When I replied to your message, the link remained intact. However,
>if I copy and paste the link, I get:
>
><http://www.bb-elec.com/tech_articles/rs422_485
>_app_note/table_of_contents.asp>
>
>(Broken into two lines on my screen).
Here, too.
>I'm using MicroPlanet Gravity 3.0. I'll check to see if there is a
>setting that can be changed.
>
>Perhaps the link will be restored to one line when the message is sent.
Nope. But, as you say, there may be a configuration setting that will
retain long links. Or, it's a great excuse to diddle with the source...
>> Now if I could just understand why UARTs idle on mark 'high' and both
>> 232 and the 422/485 'A' line idle on mark 'low' but the standard
>> symbology for 422/485 drivers has the inversion circle on the 'B' line.
>
>The saving graces of both RS-232 and RS-485 are that getting the lines
>mixed up doesn't let the smoke out of the usual driver chips an you
>have a 50% probability of getting it right the first time! ;-)
So true. It's a PITA for the field service crew, though. If you aren't
seeing the expected $GPGLL is it because A/B are swapped? Or the sender
is at 38400? You forgot to enable that port altogether? Or you just
grabbed the wrong pair? Life is too short! ;-)
--
Rich Webb Norfolk, VA