RS485 chip LTC491; optocoupler MOC5007

Started by Shane williams February 17, 2011
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
>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 >
If you read the fine datasheet at http://cds.linear.com/docs/Datasheet/491fa.pdf, Figure 12 suggests that - connected directly to the wires - you should be able to get a bit over 100k 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? --------------------------------------- Posted through http://www.EmbeddedRelated.com
"RCIngham" <robert.ingham@n_o_s_p_a_m.gmail.com> wrote in message 
news:0pudnZLk0dLpqsDQnZ2dnUVZ_qqdnZ2d@giganews.com...
> >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 >> > > If you read the fine datasheet at > http://cds.linear.com/docs/Datasheet/491fa.pdf, Figure 12 suggests that - > connected directly to the wires - you should be able to get a bit over > 100k > bps at a bit over 300 ft (i.e. 1000 m).
Eh? 300 m is approx 1000 ft. It looks like your 10x too short.
> 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? >
Peter
Em 17/2/2011 12:57, RCIngham escreveu:
[snipped]

> > If you read the fine datasheet at > http://cds.linear.com/docs/Datasheet/491fa.pdf, Figure 12 suggests that - > connected directly to the wires - you should be able to get a bit over 100k > bps at a bit over 300 ft (i.e. 1000 m).
Not really, in fact ten percent of that distance in meters: 91,44 meters, for a 12 inches foot more exactly. -- Cesar Rabak GNU/Linux User 52247. Get counted: http://counter.li.org/
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
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.
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.
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.
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.
> 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.
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
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: