hi all system: 9600bps rs485 half-duplex with single master and multiple slaves; both master and slave connector have 3 pins (A,B,RTN) cable: about 500mt shielded cable with 2 twisted pairs inside, one pair used for A/B signals other pair used for RTN the system works well questions: 1. the shield of the cable is connected to earth ground (yellow-green wire) at master side only: is this right? 2. the function of RTN wire is keeping the master and slaves own logic- ground at the same potential and avoid dangerous voltage floating? thanks
rs485 and RTN
Started by ●February 19, 2012
Reply by ●February 19, 20122012-02-19
On Sun, 19 Feb 2012 02:39:53 -0800 (PST), ice <iceeyes82@gmail.com> wrote:>hi all > >system: 9600bps rs485 half-duplex with single master and multiple >slaves; both master and slave connector have 3 pins (A,B,RTN) >cable: about 500mt shielded cable with 2 twisted pairs inside, one >pair used for A/B signals other pair used for RTN > >the system works well > >questions: >1. the shield of the cable is connected to earth ground (yellow-green >wire) at master side only: is this right?There is no universal answer. Connecting the shield to frame ground at the source is, however, a common practice and may have been used in this specific instance.>2. the function of RTN wire is keeping the master and slaves own logic- >ground at the same potential and avoid dangerous voltage floating?Probably. Not so much from "danger" issues (one hopes) but to help to keep the A/B lines within the your installation's specified common-mode voltage range. B&B Electronics has some very readable appnotes and guides for understanding and setting up 422 and 485 systems. Take a look at <http://www.bb-elec.com/technical_library.asp> -- Rich Webb Norfolk, VA
Reply by ●February 19, 20122012-02-19
ice wrote:> hi all > > system: 9600bps rs485 half-duplex with single master and multiple > slaves; both master and slave connector have 3 pins (A,B,RTN) > cable: about 500mt shielded cable with 2 twisted pairs inside, one > pair used for A/B signals other pair used for RTN > > the system works well > > questions: > 1. the shield of the cable is connected to earth ground (yellow-green > wire) at master side only: is this right? > 2. the function of RTN wire is keeping the master and slaves own logic- > ground at the same potential and avoid dangerous voltage floating? > > thanks1. Correct. This prevents current loops which other wise can cause damage and magnetic coupling of other signals. 2. I shield is a shield is a shield, as it is stated. Just keeps the inside and out side noise apart and not to converge. Jamie
Reply by ●February 19, 20122012-02-19
On Sun, 19 Feb 2012 02:39:53 -0800 (PST), ice <iceeyes82@gmail.com> wrote:>hi all > >system: 9600bps rs485 half-duplex with single master and multiple >slaves; both master and slave connector have 3 pins (A,B,RTN) >cable: about 500mt shielded cable with 2 twisted pairs inside, one >pair used for A/B signals other pair used for RTN > >the system works well > >questions: >1. the shield of the cable is connected to earth ground (yellow-green >wire) at master side only: is this right?It's better to chassis-ground the shield on both ends. That prevents EMI and ESD and crud from being coupled into the signal conductors on the end where the shield is floating.>2. the function of RTN wire is keeping the master and slaves own logic- >ground at the same potential and avoid dangerous voltage floating?Yes, but if both ends have local earth grounds, the resistance of the RTN pair will get overwhelmed by the much lower ground impedance. But it never hurts. RS485 is fairly forgiving of common-mode potentials.
Reply by ●February 19, 20122012-02-19
John Larkin wrote:> On Sun, 19 Feb 2012 02:39:53 -0800 (PST), ice <iceeyes82@gmail.com> > wrote: > > >>hi all >> >>system: 9600bps rs485 half-duplex with single master and multiple >>slaves; both master and slave connector have 3 pins (A,B,RTN) >>cable: about 500mt shielded cable with 2 twisted pairs inside, one >>pair used for A/B signals other pair used for RTN >> >>the system works well >> >>questions: >>1. the shield of the cable is connected to earth ground (yellow-green >>wire) at master side only: is this right? > > > It's better to chassis-ground the shield on both ends. That prevents > EMI and ESD and crud from being coupled into the signal conductors on > the end where the shield is floating.John, this is where I must disagree with you. The above assertion is incorrect! You DC connect only one end, the other, you can use a small signal cap if it makes you feel better, but never tied two different devices together via their shields on a ground path. Argue all you want, I've been there many times over the years, it's basic 101 practice. If you look in a lot of equipment, the grounding of such transmission lines are cap coupled to the chassis, not direct and for good reason.> >>2. the function of RTN wire is keeping the master and slaves own logic- >>ground at the same potential and avoid dangerous voltage floating? > > > Yes, but if both ends have local earth grounds, the resistance of the > RTN pair will get overwhelmed by the much lower ground impedance. But > it never hurts. > > RS485 is fairly forgiving of common-mode potentials. > >Jamie
Reply by ●February 19, 20122012-02-19
On Sun, 19 Feb 2012 11:19:17 -0500, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:>John Larkin wrote: > >> On Sun, 19 Feb 2012 02:39:53 -0800 (PST), ice <iceeyes82@gmail.com> >> wrote: >> >> >>>hi all >>> >>>system: 9600bps rs485 half-duplex with single master and multiple >>>slaves; both master and slave connector have 3 pins (A,B,RTN) >>>cable: about 500mt shielded cable with 2 twisted pairs inside, one >>>pair used for A/B signals other pair used for RTN >>> >>>the system works well >>> >>>questions: >>>1. the shield of the cable is connected to earth ground (yellow-green >>>wire) at master side only: is this right? >> >> >> It's better to chassis-ground the shield on both ends. That prevents >> EMI and ESD and crud from being coupled into the signal conductors on >> the end where the shield is floating. > > John, this is where I must disagree with you. The above assertion is >incorrect!It's perfectly correct. Imagine a long shielded cable with the shield open on one end. Induce a nasty current spike, EMI or ESD, into the shield at the open end. That will make a big voltage spike in the shield... it's far away, many wavelengths and many ohms and many microhenries, from earth ground. That nasty voltage spike is directly coupled into the signal conductors. And driving the signal lines will induce voltages into the ungrounded shield, radiating EMI. Bad news all around.> > You DC connect only one end, the other, you can use a small signal cap >if it makes you feel better, but never tied two different devices >together via their shields on a ground path.I almost always do. And my stuff works.> > Argue all you want, I've been there many times over the years, it's >basic 101 practice.In other words, hearsay and superstition. If you look in a lot of equipment, the grounding of>such transmission lines are cap coupled to the chassis, not direct and >for good reason.Disagree, sorry. Shielded Ethernet is grounded on both ends. So are standard shielded RS232 and VGA cables and RCA audio/video, and lots of other stuff. You *want* ground loop current in the shield, because it induces compensating common-mode voltages into the differential signal conductors. The cable acts like a balun at higher frequencies Why leave one end of the shield open, then add caps to local ground? It's better to just ground it. I'm having this argument with a major customer now. I'm selling him lots of boxes that interconnect over shielded cables. I'm earth grounding all the PC board ground planes with lots of spacers to the chassis, and also grounding the cable shields at both ends. They want me to float the boards, add an R||C to ground at every board, and half-ground the shields. I'm not gonna do it.
Reply by ●February 19, 20122012-02-19
On Sun, 19 Feb 2012 09:36:09 -0800, John Larkin wrote:> On Sun, 19 Feb 2012 11:19:17 -0500, Jamie > <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: > >>John Larkin wrote: >> >>> On Sun, 19 Feb 2012 02:39:53 -0800 (PST), ice <iceeyes82@gmail.com> >>> wrote: >>> >>> >>>>hi all >>>> >>>>system: 9600bps rs485 half-duplex with single master and multiple >>>>slaves; both master and slave connector have 3 pins (A,B,RTN) cable: >>>>about 500mt shielded cable with 2 twisted pairs inside, one pair used >>>>for A/B signals other pair used for RTN >>>> >>>>the system works well >>>> >>>>questions: >>>>1. the shield of the cable is connected to earth ground (yellow-green >>>>wire) at master side only: is this right? >>> >>> >>> It's better to chassis-ground the shield on both ends. That prevents >>> EMI and ESD and crud from being coupled into the signal conductors on >>> the end where the shield is floating. >> >> John, this is where I must disagree with you. The above assertion is >>incorrect! > > It's perfectly correct. Imagine a long shielded cable with the shield > open on one end. Induce a nasty current spike, EMI or ESD, into the > shield at the open end. That will make a big voltage spike in the > shield... it's far away, many wavelengths and many ohms and many > microhenries, from earth ground. That nasty voltage spike is directly > coupled into the signal conductors. > > And driving the signal lines will induce voltages into the ungrounded > shield, radiating EMI. Bad news all around. > > > >> You DC connect only one end, the other, you can use a small signal cap >>if it makes you feel better, but never tied two different devices >>together via their shields on a ground path. > > I almost always do. And my stuff works. > > > >> Argue all you want, I've been there many times over the years, it's >>basic 101 practice. > > In other words, hearsay and superstition. > > If you look in a lot of equipment, the grounding of >>such transmission lines are cap coupled to the chassis, not direct and >>for good reason. > > Disagree, sorry. Shielded Ethernet is grounded on both ends. So are > standard shielded RS232 and VGA cables and RCA audio/video, and lots of > other stuff. You *want* ground loop current in the shield, because it > induces compensating common-mode voltages into the differential signal > conductors. The cable acts like a balun at higher frequencies > > Why leave one end of the shield open, then add caps to local ground? > It's better to just ground it. > > I'm having this argument with a major customer now. I'm selling him lots > of boxes that interconnect over shielded cables. I'm earth grounding all > the PC board ground planes with lots of spacers to the chassis, and also > grounding the cable shields at both ends. They want me to float the > boards, add an R||C to ground at every board, and half-ground the > shields. I'm not gonna do it.Well, IMHO, you're both wrong. Partially. And you're both right, partially. If you have a setup where the various boxes are fairly close together, the grounding is thought out well enough to allow for ground-loops in the shielding, and the EMI environment isn't so severe that you'll melt the shielding, then having the shielding stay continuous from end to end is a good thing. Presumably this is the world that John is used to. If you have a setup where the cable runs are long and ground loops are uncontrolled, then connecting the ground of one machine to that of another two hundred feet away may not just be a bad idea from a signal perspective, it may start a fire. Presumably this is the world that Jamie is used to. I've seen different grounding schemes used to varying degrees of success -- each one has its place, many of them can work well, and few of them work well when you start mixing and matching. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
Reply by ●February 19, 20122012-02-19
On Sun, 19 Feb 2012 12:15:37 -0600, Tim Wescott <tim@seemywebsite.please> wrote:>On Sun, 19 Feb 2012 09:36:09 -0800, John Larkin wrote: > >> On Sun, 19 Feb 2012 11:19:17 -0500, Jamie >> <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: >> >>>John Larkin wrote: >>> >>>> On Sun, 19 Feb 2012 02:39:53 -0800 (PST), ice <iceeyes82@gmail.com> >>>> wrote: >>>> >>>> >>>>>hi all >>>>> >>>>>system: 9600bps rs485 half-duplex with single master and multiple >>>>>slaves; both master and slave connector have 3 pins (A,B,RTN) cable: >>>>>about 500mt shielded cable with 2 twisted pairs inside, one pair used >>>>>for A/B signals other pair used for RTN >>>>> >>>>>the system works well >>>>> >>>>>questions: >>>>>1. the shield of the cable is connected to earth ground (yellow-green >>>>>wire) at master side only: is this right? >>>> >>>> >>>> It's better to chassis-ground the shield on both ends. That prevents >>>> EMI and ESD and crud from being coupled into the signal conductors on >>>> the end where the shield is floating. >>> >>> John, this is where I must disagree with you. The above assertion is >>>incorrect! >> >> It's perfectly correct. Imagine a long shielded cable with the shield >> open on one end. Induce a nasty current spike, EMI or ESD, into the >> shield at the open end. That will make a big voltage spike in the >> shield... it's far away, many wavelengths and many ohms and many >> microhenries, from earth ground. That nasty voltage spike is directly >> coupled into the signal conductors. >> >> And driving the signal lines will induce voltages into the ungrounded >> shield, radiating EMI. Bad news all around. >> >> >> >>> You DC connect only one end, the other, you can use a small signal cap >>>if it makes you feel better, but never tied two different devices >>>together via their shields on a ground path. >> >> I almost always do. And my stuff works. >> >> >> >>> Argue all you want, I've been there many times over the years, it's >>>basic 101 practice. >> >> In other words, hearsay and superstition. >> >> If you look in a lot of equipment, the grounding of >>>such transmission lines are cap coupled to the chassis, not direct and >>>for good reason. >> >> Disagree, sorry. Shielded Ethernet is grounded on both ends. So are >> standard shielded RS232 and VGA cables and RCA audio/video, and lots of >> other stuff. You *want* ground loop current in the shield, because it >> induces compensating common-mode voltages into the differential signal >> conductors. The cable acts like a balun at higher frequencies >> >> Why leave one end of the shield open, then add caps to local ground? >> It's better to just ground it. >> >> I'm having this argument with a major customer now. I'm selling him lots >> of boxes that interconnect over shielded cables. I'm earth grounding all >> the PC board ground planes with lots of spacers to the chassis, and also >> grounding the cable shields at both ends. They want me to float the >> boards, add an R||C to ground at every board, and half-ground the >> shields. I'm not gonna do it. > >Well, IMHO, you're both wrong. > >Partially. > >And you're both right, partially. > >If you have a setup where the various boxes are fairly close together, >the grounding is thought out well enough to allow for ground-loops in the >shielding, and the EMI environment isn't so severe that you'll melt the >shielding, then having the shielding stay continuous from end to end is a >good thing. > >Presumably this is the world that John is used to. > >If you have a setup where the cable runs are long and ground loops are >uncontrolled, then connecting the ground of one machine to that of >another two hundred feet away may not just be a bad idea from a signal >perspective, it may start a fire. > >Presumably this is the world that Jamie is used to. > >I've seen different grounding schemes used to varying degrees of success >-- each one has its place, many of them can work well, and few of them >work well when you start mixing and matching.I've had to contend with 60VAC gnd-to-gnd high-rise building-to-building. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●February 19, 20122012-02-19
Tim Wescott wrote:> On Sun, 19 Feb 2012 09:36:09 -0800, John Larkin wrote: > > >>On Sun, 19 Feb 2012 11:19:17 -0500, Jamie >><jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: >> >> >>>John Larkin wrote: >>> >>> >>>>On Sun, 19 Feb 2012 02:39:53 -0800 (PST), ice <iceeyes82@gmail.com> >>>>wrote: >>>> >>>> >>>> >>>>>hi all >>>>> >>>>>system: 9600bps rs485 half-duplex with single master and multiple >>>>>slaves; both master and slave connector have 3 pins (A,B,RTN) cable: >>>>>about 500mt shielded cable with 2 twisted pairs inside, one pair used >>>>>for A/B signals other pair used for RTN >>>>> >>>>>the system works well >>>>> >>>>>questions: >>>>>1. the shield of the cable is connected to earth ground (yellow-green >>>>>wire) at master side only: is this right? >>>> >>>> >>>>It's better to chassis-ground the shield on both ends. That prevents >>>>EMI and ESD and crud from being coupled into the signal conductors on >>>>the end where the shield is floating. >>> >>> John, this is where I must disagree with you. The above assertion is >>>incorrect! >> >>It's perfectly correct. Imagine a long shielded cable with the shield >>open on one end. Induce a nasty current spike, EMI or ESD, into the >>shield at the open end. That will make a big voltage spike in the >>shield... it's far away, many wavelengths and many ohms and many >>microhenries, from earth ground. That nasty voltage spike is directly >>coupled into the signal conductors. >> >>And driving the signal lines will induce voltages into the ungrounded >>shield, radiating EMI. Bad news all around. >> >> >> >> >>> You DC connect only one end, the other, you can use a small signal cap >>>if it makes you feel better, but never tied two different devices >>>together via their shields on a ground path. >> >>I almost always do. And my stuff works. >> >> >> >> >>> Argue all you want, I've been there many times over the years, it's >>>basic 101 practice. >> >>In other words, hearsay and superstition. >> >> If you look in a lot of equipment, the grounding of >> >>>such transmission lines are cap coupled to the chassis, not direct and >>>for good reason. >> >>Disagree, sorry. Shielded Ethernet is grounded on both ends. So are >>standard shielded RS232 and VGA cables and RCA audio/video, and lots of >>other stuff. You *want* ground loop current in the shield, because it >>induces compensating common-mode voltages into the differential signal >>conductors. The cable acts like a balun at higher frequencies >> >>Why leave one end of the shield open, then add caps to local ground? >>It's better to just ground it. >> >>I'm having this argument with a major customer now. I'm selling him lots >>of boxes that interconnect over shielded cables. I'm earth grounding all >>the PC board ground planes with lots of spacers to the chassis, and also >>grounding the cable shields at both ends. They want me to float the >>boards, add an R||C to ground at every board, and half-ground the >>shields. I'm not gonna do it. > > > Well, IMHO, you're both wrong. > > Partially. > > And you're both right, partially. > > If you have a setup where the various boxes are fairly close together, > the grounding is thought out well enough to allow for ground-loops in the > shielding, and the EMI environment isn't so severe that you'll melt the > shielding, then having the shielding stay continuous from end to end is a > good thing. > > Presumably this is the world that John is used to. > > If you have a setup where the cable runs are long and ground loops are > uncontrolled, then connecting the ground of one machine to that of > another two hundred feet away may not just be a bad idea from a signal > perspective, it may start a fire. > > Presumably this is the world that Jamie is used to. > > I've seen different grounding schemes used to varying degrees of success > -- each one has its place, many of them can work well, and few of them > work well when you start mixing and matching. >We've never had a fire or melt down but what happens is magnetic coupling occurs in the components under the shield and takes out the inputs/outputs of the devices. Our lab actually test for these conditions when doing product developing. They apply various levels of current through shields for destructive testing and you'd me amazed what the simplest methods of protection can do for you. You would also be amazed of what kind of crap also makes it through what you think should be a good shield. Most of the devices I've had open on the bench for view actually support a signal ground, which does not directly connect to the chassis. Those eng must've done their home work because we normally don't have any issues with that aspect of the equipment. This whole thread reminds of how we had to remove a system ground from a AB power supply due to getting damaged components when ever mother nature decided to pass by us. The next revision of that supply from them had a designed change, I wonder why? Must of been the cost they were incurring on warranty cost. Jamie
Reply by ●February 19, 20122012-02-19
On Sun, 19 Feb 2012 12:15:37 -0600, Tim Wescott <tim@seemywebsite.please> wrote:>On Sun, 19 Feb 2012 09:36:09 -0800, John Larkin wrote: > >> On Sun, 19 Feb 2012 11:19:17 -0500, Jamie >> <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: >> >>>John Larkin wrote: >>> >>>> On Sun, 19 Feb 2012 02:39:53 -0800 (PST), ice <iceeyes82@gmail.com> >>>> wrote: >>>> >>>> >>>>>hi all >>>>> >>>>>system: 9600bps rs485 half-duplex with single master and multiple >>>>>slaves; both master and slave connector have 3 pins (A,B,RTN) cable: >>>>>about 500mt shielded cable with 2 twisted pairs inside, one pair used >>>>>for A/B signals other pair used for RTN >>>>> >>>>>the system works well >>>>> >>>>>questions: >>>>>1. the shield of the cable is connected to earth ground (yellow-green >>>>>wire) at master side only: is this right? >>>> >>>> >>>> It's better to chassis-ground the shield on both ends. That prevents >>>> EMI and ESD and crud from being coupled into the signal conductors on >>>> the end where the shield is floating. >>> >>> John, this is where I must disagree with you. The above assertion is >>>incorrect! >> >> It's perfectly correct. Imagine a long shielded cable with the shield >> open on one end. Induce a nasty current spike, EMI or ESD, into the >> shield at the open end. That will make a big voltage spike in the >> shield... it's far away, many wavelengths and many ohms and many >> microhenries, from earth ground. That nasty voltage spike is directly >> coupled into the signal conductors. >> >> And driving the signal lines will induce voltages into the ungrounded >> shield, radiating EMI. Bad news all around. >> >> >> >>> You DC connect only one end, the other, you can use a small signal cap >>>if it makes you feel better, but never tied two different devices >>>together via their shields on a ground path. >> >> I almost always do. And my stuff works. >> >> >> >>> Argue all you want, I've been there many times over the years, it's >>>basic 101 practice. >> >> In other words, hearsay and superstition. >> >> If you look in a lot of equipment, the grounding of >>>such transmission lines are cap coupled to the chassis, not direct and >>>for good reason. >> >> Disagree, sorry. Shielded Ethernet is grounded on both ends. So are >> standard shielded RS232 and VGA cables and RCA audio/video, and lots of >> other stuff. You *want* ground loop current in the shield, because it >> induces compensating common-mode voltages into the differential signal >> conductors. The cable acts like a balun at higher frequencies >> >> Why leave one end of the shield open, then add caps to local ground? >> It's better to just ground it. >> >> I'm having this argument with a major customer now. I'm selling him lots >> of boxes that interconnect over shielded cables. I'm earth grounding all >> the PC board ground planes with lots of spacers to the chassis, and also >> grounding the cable shields at both ends. They want me to float the >> boards, add an R||C to ground at every board, and half-ground the >> shields. I'm not gonna do it. > >Well, IMHO, you're both wrong. > >Partially. > >And you're both right, partially. > >If you have a setup where the various boxes are fairly close together, >the grounding is thought out well enough to allow for ground-loops in the >shielding, and the EMI environment isn't so severe that you'll melt the >shielding, then having the shielding stay continuous from end to end is a >good thing. > >Presumably this is the world that John is used to.60 to 80 meters, three floors of a building, in the current case. One end is my controller, the other end is a megawatt of pulsed CO2 laser.> >If you have a setup where the cable runs are long and ground loops are >uncontrolled, then connecting the ground of one machine to that of >another two hundred feet away may not just be a bad idea from a signal >perspective, it may start a fire.If the grounds are that many volts far apart, go fiber; the signals won't work anyhow. Anybody heard of shielded CAT6 catching fire?
