>>> >"Correct termination" is a can of worms. Almost all current RS485
>>> >devices have the A/B circuits reversed re the spec. Also, simply
>>> >terminating across the A/B lines will result in an indeterminate state.
>> That's why I consider EIA-485 broken by design.
>
>Nothing wrong with the spec. Plenty wrong with the devices which followed.
I disagree. Requesting a DC bias was the wrong way IMNSHO. The better
design choice is to make 0V a defined state, as CAN does.
Then I can use passive terminations at the ends of the bus and I don't
need to select a bus node to provide DC bias.
[...]
>>> >You need some bleed current in there to ensure a MARK state when idle.
>>> >Hint: AC-coupling the termination often helps with the bleed current.
>> Do you have a working real world example?
>>
>> I can't imagine that AC termination works if the bit rate is not way
>> beyond the propagation time. Either it's time constant is too short
>> for long cables, or it's too long for very biasing patterns.
>
>I fear you misunderstand me. Given the lowish value of the terminator,
>the bleed to/from rails results in fairly high DC current. A
>transmission line is only an issue with AC ;). So, AC-coupling the
>terminator resolves the termination issue, while resolving the DC
>current issue.
That's what I understood. I think it doesn't work under the conditions
I pointed out.
Oliver
--
Oliver Betz, Muenchen (oliverbetz.de)
Reply by Steve at fivetrees●February 18, 20112011-02-18
On 18/02/2011 08:24, Oliver Betz wrote:
> Steve at fivetrees wrote:
>
> [...]
>
>> >"Correct termination" is a can of worms. Almost all current RS485
>> >devices have the A/B circuits reversed re the spec. Also, simply
>> >terminating across the A/B lines will result in an indeterminate state.
> That's why I consider EIA-485 broken by design.
Nothing wrong with the spec. Plenty wrong with the devices which followed.
Although - I presume your point is that, with termination and no bleeds,
there's that indeterminate state. Yes, the specs could have been clearer
about this. But it's not *that* hard. I've been using RS485 without
problems (apart from those danged chips) for nearly 30 years now...
(Eeeeep.)
>
>> >You need some bleed current in there to ensure a MARK state when idle.
>> >Hint: AC-coupling the termination often helps with the bleed current.
> Do you have a working real world example?
>
> I can't imagine that AC termination works if the bit rate is not way
> beyond the propagation time. Either it's time constant is too short
> for long cables, or it's too long for very biasing patterns.
>
I fear you misunderstand me. Given the lowish value of the terminator,
the bleed to/from rails results in fairly high DC current. A
transmission line is only an issue with AC ;). So, AC-coupling the
terminator resolves the termination issue, while resolving the DC
current issue.
There was an app note from my old company detailing this online, but
they're rebuilding their site, and it's now "under construction"... gah...
Steve
--
http://www.fivetrees.com
Reply by WangoTango●February 18, 20112011-02-18
In article <ijl52h$25e$1@news.eternal-september.org>,
tauno.voipio@notused.fi.invalid says...
>
> > Thanks. The tests I did were with termination - a 150 ohm resistor
> > across the A B terminals, plus a 47K resistor from A to VCC (I think
> > this was for biasing, whatever that is).
>
> A single 47k is not enough to guarantee a mark signal
> on an idle line. You need neough bleed current from
> both lines, one to voltage supply and another to ground,
> to create a guaranteed differential signal across the
> termination.
True.
A 680 to Vcc on 'A' a 680 to Vss on 'B' and a 150 terminating is a lot
better.
Reply by WangoTango●February 18, 20112011-02-18
In article <4d5e0a6d$0$10361$607ed4bc@cv.net>,
NeilKurzm@worldnet.att.net says...
> On 2/17/2011 5:46 AM, Shane williams wrote:
> > Hi
> >
> > I'm a software engineer wondering about some hardware. We have a
> > circuit board with RS485 circuitry and Motorola 68302 processor using
> > an LTC491 RS485 chip and MOC5007 optocoupler for electrical
> > isolation. The RS485 circuit is arranged to run as either 4 wire
> > multidrop (with contention detection circuitry) or as two by 2 channel
> > half duplex point to point, with each channel 2 wires.
> >
> > The contention circuitry seems to cause some complications, so
> > assuming that it's not there, what is the fastest speed I could expect
> > to run 2 wire point to point RS485 with these chips over 1000 metres
> > using reasonable cable?
> >
> > On two boards, I've been able to do 250 Kbps with no problems and 200
> > Kbps over 1400 metres. A fellow hardware engineer tells me that the
> > RS485 chip and optocoupler may vary significantly from chip to chip
> > and also over time/ temperature so that predicting a reliable baud
> > rate is difficult. Currently we use 57600 bps. I would like to go at
> > twice that.
> >
> > Does the fact that 250 Kbps worked indicate that 115200 bps should be
> > reliable?
> >
> > TIA
>
> You Know they Make RS-485 Transceivers with the Isolation built-in.
>
Reply by Shane williams●February 18, 20112011-02-18
On Feb 18, 7:54=A0pm, Tauno Voipio <tauno.voi...@notused.fi.invalid>
wrote:
> On 18.2.11 7:57 , Shane williams wrote:
>
>
>
> > Thanks. =A0The tests I did were with termination - a 150 ohm resistor
> > across the A B terminals, plus a 47K resistor from A to VCC (I think
> > this was for biasing, whatever that is).
>
> A single 47k is not enough to guarantee a mark signal
> on an idle line. You need neough bleed current from
> both lines, one to voltage supply and another to ground,
> to create a guaranteed differential signal across the
> termination.
>
> > The optocoupler datasheet says "high data rate 1MHz typical NRZ".
> >http://pdf1.alldatasheet.com/datasheet-pdf/view/5043/MOTOROLA/MOC5007...
> > We use the 68302 DDCMP mode. =A0The error rate at 250 Kbps over 1000 M
> > non twisted cable was very small / negligible.
>
> For the speeds you're aiming at, the wavelength is below the
> 1000 m cable length, and you have to have a decent transmission
> line. Using a non-twisted pair and/or unknown insulation is
> inviting trouble.
>
The non twisted cable was just what I was testing with because we have
1000 M of it here. We can specify whatever kind of cable we like for
use in a real system.
Reply by Oliver Betz●February 18, 20112011-02-18
Steve at fivetrees wrote:
[...]
>"Correct termination" is a can of worms. Almost all current RS485
>devices have the A/B circuits reversed re the spec. Also, simply
>terminating across the A/B lines will result in an indeterminate state.
That's why I consider EIA-485 broken by design.
>You need some bleed current in there to ensure a MARK state when idle.
>Hint: AC-coupling the termination often helps with the bleed current.
Do you have a working real world example?
I can't imagine that AC termination works if the bit rate is not way
beyond the propagation time. Either it's time constant is too short
for long cables, or it's too long for very biasing patterns.
Oliver
--
Oliver Betz, Munich
despammed.com is broken, use Reply-To:
Reply by Tauno Voipio●February 18, 20112011-02-18
On 18.2.11 7:57 , Shane williams wrote:
> On Feb 18, 12:53 pm, Steve at fivetrees<st...@NOSPAMTAfivetrees.com>
> wrote:
>> On 17/02/2011 10:46, Shane williams wrote:
>>
>>> The contention circuitry seems to cause some complications, so
>>> assuming that it's not there, what is the fastest speed I could expect
>>> to run 2 wire point to point RS485 with these chips over 1000 metres
>>> using reasonable cable?
>>
>> Assuming correct termination, and decent common-mode (presumably taken
>> care of by your [no doubt high-speed] optos), it should be as Wikipedia
>> says:
>>
>> "It offers high data transmission speeds (35 Mbit/s up to 10 m and 100
>> kbit/s at 1200 m)."
>>
>> <http://en.wikipedia.org/wiki/EIA-485>
>>
>> "Correct termination" is a can of worms. Almost all current RS485
>> devices have the A/B circuits reversed re the spec. Also, simply
>> terminating across the A/B lines will result in an indeterminate state.
>> You need some bleed current in there to ensure a MARK state when idle.
>> Hint: AC-coupling the termination often helps with the bleed current.
>>
>> Without termination, you're at the mercy of cable capacitance and
>> transmission-line reflections. From experience, you can probably get
>> away with 4800baud over that distance, if you're lucky.
>>
>> >> A fellow hardware engineer tells me that the RS485 chip and
>> optocoupler may vary significantly from chip to chip and also over time/
>> temperature so that predicting a reliable baud rate is difficult.<<
>>
>> True for optos, not true for RS485. The "SOA" (distance vs speed) is
>> part of the RS485 spec. If it doesn't meet it, it's non-compliant.
>>
>> Do you have the EIA-485 spec? If not, you should.
>>
>> Steve
>>
>> --http://www.fivetrees.com
>
> Thanks. The tests I did were with termination - a 150 ohm resistor
> across the A B terminals, plus a 47K resistor from A to VCC (I think
> this was for biasing, whatever that is).
A single 47k is not enough to guarantee a mark signal
on an idle line. You need neough bleed current from
both lines, one to voltage supply and another to ground,
to create a guaranteed differential signal across the
termination.
For the speeds you're aiming at, the wavelength is below the
1000 m cable length, and you have to have a decent transmission
line. Using a non-twisted pair and/or unknown insulation is
inviting trouble.
--
Tauno Voipio
Reply by Shane williams●February 18, 20112011-02-18
On Feb 18, 3:57=A0am, "RCIngham" <robert.ingham@n_o_s_p_a_m.gmail.com>
wrote:
> >Hi
>
> >I'm a software engineer wondering about some hardware. =A0We have a
> >circuit board with RS485 circuitry and Motorola 68302 processor using
> >an LTC491 RS485 chip and MOC5007 optocoupler for electrical
> >isolation. =A0The RS485 circuit is arranged to run as either 4 wire
> >multidrop (with contention detection circuitry) or as two by 2 channel
> >half duplex point to point, with each channel 2 wires.
>
> >The contention circuitry seems to cause some complications, so
> >assuming that it's not there, what is the fastest speed I could expect
> >to run 2 wire point to point RS485 with these chips over 1000 metres
> >using reasonable cable?
>
> >On two boards, I've been able to do 250 Kbps with no problems and 200
> >Kbps over 1400 metres. =A0A fellow hardware engineer tells me that the
> >RS485 chip and optocoupler may vary significantly from chip to chip
> >and also over time/ temperature so that predicting a reliable baud
> >rate is difficult. =A0Currently we use 57600 bps. =A0I would like to go =
at
> >twice that.
>
> >Does the fact that 250 Kbps worked indicate that 115200 bps should be
> >reliable?
>
> >TIA
>
> If you read the fine datasheet athttp://cds.linear.com/docs/Datasheet/491=
fa.pdf, Figure 12 suggests that -
> connected directly to the wires - you should be able to get a bit over 10=
0k
> bps at a bit over 300 ft (i.e. 1000 m).
>
> The effect of the optocouplers are an extra confounding factor. Can you
> define a nearly-worst-case combination of conditions to do a long
> bit-error-rate test?
>
> --------------------------------------- =A0 =A0 =A0 =A0
> Posted throughhttp://www.EmbeddedRelated.com
Thanks. I guess you meant 3000 feet as that's what Figure 12 shows.
I ran a test at 250 Kbps for several hours over 1000M with very few
errors but I don't have a worst case optocoupler or a worst case RS485
chip.
I thought that speed / distance rating thing was for when there are
multiple devices on the wire. We have only two devices on the wire.
Reply by Neil●February 18, 20112011-02-18
On 2/17/2011 5:46 AM, Shane williams wrote:
> Hi
>
> I'm a software engineer wondering about some hardware. We have a
> circuit board with RS485 circuitry and Motorola 68302 processor using
> an LTC491 RS485 chip and MOC5007 optocoupler for electrical
> isolation. The RS485 circuit is arranged to run as either 4 wire
> multidrop (with contention detection circuitry) or as two by 2 channel
> half duplex point to point, with each channel 2 wires.
>
> The contention circuitry seems to cause some complications, so
> assuming that it's not there, what is the fastest speed I could expect
> to run 2 wire point to point RS485 with these chips over 1000 metres
> using reasonable cable?
>
> On two boards, I've been able to do 250 Kbps with no problems and 200
> Kbps over 1400 metres. A fellow hardware engineer tells me that the
> RS485 chip and optocoupler may vary significantly from chip to chip
> and also over time/ temperature so that predicting a reliable baud
> rate is difficult. Currently we use 57600 bps. I would like to go at
> twice that.
>
> Does the fact that 250 Kbps worked indicate that 115200 bps should be
> reliable?
>
> TIA
You Know they Make RS-485 Transceivers with the Isolation built-in.
Reply by Shane williams●February 18, 20112011-02-18
On Feb 18, 12:53=A0pm, Steve at fivetrees <st...@NOSPAMTAfivetrees.com>
wrote:
> On 17/02/2011 10:46, Shane williams wrote:
>
> > The contention circuitry seems to cause some complications, so
> > assuming that it's not there, what is the fastest speed I could expect
> > to run 2 wire point to point RS485 with these chips over 1000 metres
> > using reasonable cable?
>
> Assuming correct termination, and decent common-mode (presumably taken
> care of by your [no doubt high-speed] optos), it should be as Wikipedia
> says:
>
> "It offers high data transmission speeds (35 Mbit/s up to 10 m and 100
> kbit/s at 1200 m)."
>
> <http://en.wikipedia.org/wiki/EIA-485>
>
> "Correct termination" is a can of worms. Almost all current RS485
> devices have the A/B circuits reversed re the spec. Also, simply
> terminating across the A/B lines will result in an indeterminate state.
> You need some bleed current in there to ensure a MARK state when idle.
> Hint: AC-coupling the termination often helps with the bleed current.
>
> Without termination, you're at the mercy of cable capacitance and
> transmission-line reflections. From experience, you can probably get
> away with 4800baud over that distance, if you're lucky.
>
> =A0>> A fellow hardware engineer tells me that the RS485 chip and
> optocoupler may vary significantly from chip to chip and also over time/
> temperature so that predicting a reliable baud rate is difficult. <<
>
> True for optos, not true for RS485. The "SOA" (distance vs speed) is
> part of the RS485 spec. If it doesn't meet it, it's non-compliant.
>
> Do you have the EIA-485 spec? If not, you should.
>
> Steve
>
> --http://www.fivetrees.com
Thanks. The tests I did were with termination - a 150 ohm resistor
across the A B terminals, plus a 47K resistor from A to VCC (I think
this was for biasing, whatever that is).
The optocoupler datasheet says "high data rate 1MHz typical NRZ".
http://pdf1.alldatasheet.com/datasheet-pdf/view/5043/MOTOROLA/MOC5007.html
We use the 68302 DDCMP mode. The error rate at 250 Kbps over 1000 M
non twisted cable was very small / negligible.