Post by thread:[EE] Duplex & How to connect twisted pair wire tog

> Hi all, > > I'm creating an RS485 network consisting of a few devices, used for > temperature sensing and control. I want this to be RS485 because it's > simple and I already have the parts/protocols mostly in place. As of > now, it does not need to be multi master, however, I want to leave the > option open. You also need to specify the maximum speed. > > 1. Should I use a half duplex or full duplex RS485 transceiver? I'm > not really sure how full duplex RS485 would work for multiple nodes (4 > or 5 nodes) and what the advantages would be. Half duplex is two wires + ground and possibly shield. Both transmitter and receiver on all nodes uses the same wires for communication. Only one can talk at a time. Half duplex is the most common for RS485 networks. Full duplex is 4 wires + ground and possibly shield. I have only seen this used in systems where there is a single master and multiple slaves. From the masters point of view it has two dedicated wires for tx and two for rx. This means that the master can receive and transmitt at the same time. The tx on the master is connected to the rx on all slaves and the rx on the master is connected to tx on all slaves. This has the advantage that the master can keep the tx driver enabled all the time and if the master is a PC this can make the software much easier to write since there is no built in mechanism in a PC to handle enabling/disabling of the tx drivers (the software works just like with RS232). The turnaround of the tx lines on the slave nodes still have to be handled though. > > 2. Should I use twisted pair telephone cable or Ethernet cable? Or any > other suggestions? You should use twisted pairs but you also need a third ground wire which connects the ground(plane)s between all your nodes. This can become a problem if the nodes are not floating and/or there is a voltage difference between the grounds on the nodes in the network. A lot of people will tell you that you only need two wires for RS485 and many times this will work. However, a proper RS485 network needs a common ground wire - it is not a current loop. There is more on this in the linked articles below. If there is a problem with different ground levels in your nodes you can use isolated RS485 transcievers. If you can build your nodes with an isolated power supply (mains transformer) or you can supply all nodes from the same power supply and the nodes are not directly connected to other devices you should be fine. If you use a shield, it should be connected at both ends to a solid ground (earth) which is usually the metal frame around a system (chassis). This may or may not be the same as the ground wire (which is connected to the ground plane on the boards). This can also mean that you will get problems with ground currents (ground loops) where the ground (earth) potential is at different levels on different nodes. This can be prevented by DC decoupling the shield with a capacitor (but it is rarely needed if the nodes are within a relatively close area) and it is also usually a lot harder to get a good EMC result with this setup. You can use a shielded or unshielded cable with two twisted pairs where one pair is for communication in the half duplex network and the other pair is for power. > > 3. How should all of the "lines" be connected? How about a "junction > box" which consists of a PCB with a bunch of sockets all connected > together? It's simple and clean enough, but I don't know if the > untwisted traces on the PCB will cause any trouble. You should connect all the nodes in series with short stubs (bus topology) and not as a star network. When I say "in series" I mean on a long bus without stubs or trunks - although the nodes are theoretically connected in parallell. I usually have terminals for an incomming and an outgoing bus line on each node that are actually connected in parallell on the board. If you have lots of nodes the (very low) series resistance introduced in the bus by the terminals could become a problem. Then you could just use one terminal and connect both the incomming and outgoing wires together in that terminal. This is mostly a problem if you supply all boards with power on the bus wires and each node consumes relatively high power. The low series resistance is not a problem for the communication wires. The tx pair should be terminated at the far ends (only one place) with a termination resistor that matches the cable (usually around 120 ohms). If you use half duplex there is only one pair and it should be terminated at both ends. At one place on the bus (usually on the master in a single master system or at one end on the bus) there should also be some weak pullup and down resistors to guarantee that the bus is read as idle when all transmitter drivers are in the off state. I usually have two jumpers on the node boards which are both used together to connect the termination resistor and also the pull up/down resistors. The RS485 transcievers are pretty tolerant for common mode voltages (-7V to +12V) but this is not always enough. I always put some low value resistors and three high powered double (antiparallell) tvs's on each node (two between the communication lines and ground and a third between the lines themselves). For some good, solid information about RS485 networks see the following links: <http://www.national.com/an/AN/AN-1057.pdf> <www.circuitcellar.com/library/ccofeature/perrin0799/c79bppdf.pdf> /Ruben ============================== Ruben Jönsson AB Liros Electronic Box 9124, 200 39 Malmö, Sweden TEL INT +46 40142078 FAX INT +46 40947388 spam_OUTrubenTakeThisOuTpp.sbbs.se ==============================

2010/7/6 Ruben Jönsson <.....rubenKILLspam@spam@pp.sbbs.se>: {Quote hidden}>> Hi all, >> >> I'm creating an RS485 network consisting of a few devices, used for >> temperature sensing and control. I want this to be RS485 because it's >> simple and I already have the parts/protocols mostly in place. As of >> now, it does not need to be multi master, however, I want to leave the >> option open. > > You also need to specify the maximum speed. > >> >> 1. Should I use a half duplex or full duplex RS485 transceiver? I'm >> not really sure how full duplex RS485 would work for multiple nodes (4 >> or 5 nodes) and what the advantages would be. > > Half duplex is two wires + ground and possibly shield. Both transmitter and > receiver on all nodes uses the same wires for communication. Only one can talk > at a time. Half duplex is the most common for RS485 networks. > > Full duplex is 4 wires + ground and possibly shield. I have only seen this used > in systems where there is a single master and multiple slaves. From the masters > point of view it has two dedicated wires for tx and two for rx. This means that > the master can receive and transmitt at the same time. The tx on the master is > connected to the rx on all slaves and the rx on the master is connected to tx > on all slaves. This has the advantage that the master can keep the tx driver > enabled all the time and if the master is a PC this can make the software much > easier to write since there is no built in mechanism in a PC to handle > enabling/disabling of the tx drivers (the software works just like with RS232). > The turnaround of the tx lines on the slave nodes still have to be handled > though. > >> >> 2. Should I use twisted pair telephone cable or Ethernet cable? Or any >> other suggestions? > > You should use twisted pairs but you also need a third ground wire which > connects the ground(plane)s between all your nodes. This can become a problem > if the nodes are not floating and/or there is a voltage difference between the > grounds on the nodes in the network. A lot of people will tell you that you > only need two wires for RS485 and many times this will work. However, a proper > RS485 network needs a common ground wire - it is not a current loop. There is > more on this in the linked articles below. If there is a problem with different > ground levels in your nodes you can use isolated RS485 transcievers. If you can > build your nodes with an isolated power supply (mains transformer) or you can > supply all nodes from the same power supply and the nodes are not directly > connected to other devices you should be fine. > > If you use a shield, it should be connected at both ends to a solid ground > (earth) which is usually the metal frame around a system (chassis). This may or > may not be the same as the ground wire (which is connected to the ground plane > on the boards). This can also mean that you will get problems with ground > currents (ground loops) where the ground (earth) potential is at different > levels on different nodes. This can be prevented by DC decoupling the shield > with a capacitor (but it is rarely needed if the nodes are within a relatively > close area) and it is also usually a lot harder to get a good EMC result with > this setup. > > You can use a shielded or unshielded cable with two twisted pairs where one > pair is for communication in the half duplex network and the other pair is for > power. > >> >> 3. How should all of the "lines" be connected? How about a "junction >> box" which consists of a PCB with a bunch of sockets all connected >> together? It's simple and clean enough, but I don't know if the >> untwisted traces on the PCB will cause any trouble. > > You should connect all the nodes in series with short stubs (bus topology) and > not as a star network. When I say "in series" I mean on a long bus without > stubs or trunks - although the nodes are theoretically connected in parallell. > I usually have terminals for an incomming and an outgoing bus line on each node > that are actually connected in parallell on the board. If you have lots of > nodes the (very low) series resistance introduced in the bus by the terminals > could become a problem. Then you could just use one terminal and connect both > the incomming and outgoing wires together in that terminal. This is mostly a > problem if you supply all boards with power on the bus wires and each node > consumes relatively high power. The low series resistance is not a problem for > the communication wires. > > The tx pair should be terminated at the far ends (only one place) with a > termination resistor that matches the cable (usually around 120 ohms). If you > use half duplex there is only one pair and it should be terminated at both > ends. > > At one place on the bus (usually on the master in a single master system or at > one end on the bus) there should also be some weak pullup and down resistors to > guarantee that the bus is read as idle when all transmitter drivers are in the > off state. I usually have two jumpers on the node boards which are both used > together to connect the termination resistor and also the pull up/down > resistors. > > The RS485 transcievers are pretty tolerant for common mode voltages (-7V to > +12V) but this is not always enough. I always put some low value resistors and > three high powered double (antiparallell) tvs's on each node (two between the > communication lines and ground and a third between the lines themselves). > > For some good, solid information about RS485 networks see the following links: > > <http://www.national.com/an/AN/AN-1057.pdf> > > <http://www.circuitcellar.com/library/ccofeature/perrin0799/c79bppdf.pdf> > > /Ruben Thanks a lot for the detailed reply. I took a look at the links you gave me. The way that the network nodes will be placed will make a star topology the most convenient choice. What's the disadvantage of a star topology? Also, if there are only two devices, and a direct RS485 connection between them (distances anywhere from 1 m to 1000 m) over a twisted pair line, does the line need anything special? That is, does it need to be terminated? If so, how do you terminate a point to point line?

On Wed, Jul 7, 2010 at 11:22 PM, solarwind <x.solarwind.xKILLspamgmail.com> wrote: {Quote hidden}> 2010/7/6 Ruben Jönsson <.....rubenKILLspam.....pp.sbbs.se>: >>> Hi all, >>> >>> I'm creating an RS485 network consisting of a few devices, used for >>> temperature sensing and control. I want this to be RS485 because it's >>> simple and I already have the parts/protocols mostly in place. As of >>> now, it does not need to be multi master, however, I want to leave the >>> option open. >> >> You also need to specify the maximum speed. >> >>> >>> 1. Should I use a half duplex or full duplex RS485 transceiver? I'm >>> not really sure how full duplex RS485 would work for multiple nodes (4 >>> or 5 nodes) and what the advantages would be. >> >> Half duplex is two wires + ground and possibly shield. Both transmitter and >> receiver on all nodes uses the same wires for communication. Only one can talk >> at a time. Half duplex is the most common for RS485 networks. >> >> Full duplex is 4 wires + ground and possibly shield. I have only seen this used >> in systems where there is a single master and multiple slaves. From the masters >> point of view it has two dedicated wires for tx and two for rx. This means that >> the master can receive and transmitt at the same time. The tx on the master is >> connected to the rx on all slaves and the rx on the master is connected to tx >> on all slaves. This has the advantage that the master can keep the tx driver >> enabled all the time and if the master is a PC this can make the software much >> easier to write since there is no built in mechanism in a PC to handle >> enabling/disabling of the tx drivers (the software works just like with RS232). >> The turnaround of the tx lines on the slave nodes still have to be handled >> though. >> >>> >>> 2. Should I use twisted pair telephone cable or Ethernet cable? Or any >>> other suggestions? >> >> You should use twisted pairs but you also need a third ground wire which >> connects the ground(plane)s between all your nodes. This can become a problem >> if the nodes are not floating and/or there is a voltage difference between the >> grounds on the nodes in the network. A lot of people will tell you that you >> only need two wires for RS485 and many times this will work. However, a proper >> RS485 network needs a common ground wire - it is not a current loop. There is >> more on this in the linked articles below. If there is a problem with different >> ground levels in your nodes you can use isolated RS485 transcievers. If you can >> build your nodes with an isolated power supply (mains transformer) or you can >> supply all nodes from the same power supply and the nodes are not directly >> connected to other devices you should be fine. >> >> If you use a shield, it should be connected at both ends to a solid ground >> (earth) which is usually the metal frame around a system (chassis). This may or >> may not be the same as the ground wire (which is connected to the ground plane >> on the boards). This can also mean that you will get problems with ground >> currents (ground loops) where the ground (earth) potential is at different >> levels on different nodes. This can be prevented by DC decoupling the shield >> with a capacitor (but it is rarely needed if the nodes are within a relatively >> close area) and it is also usually a lot harder to get a good EMC result with >> this setup. >> >> You can use a shielded or unshielded cable with two twisted pairs where one >> pair is for communication in the half duplex network and the other pair is for >> power. >> >>> >>> 3. How should all of the "lines" be connected? How about a "junction >>> box" which consists of a PCB with a bunch of sockets all connected >>> together? It's simple and clean enough, but I don't know if the >>> untwisted traces on the PCB will cause any trouble. >> >> You should connect all the nodes in series with short stubs (bus topology) and >> not as a star network. When I say "in series" I mean on a long bus without >> stubs or trunks - although the nodes are theoretically connected in parallell. >> I usually have terminals for an incomming and an outgoing bus line on each node >> that are actually connected in parallell on the board. If you have lots of >> nodes the (very low) series resistance introduced in the bus by the terminals >> could become a problem. Then you could just use one terminal and connect both >> the incomming and outgoing wires together in that terminal. This is mostly a >> problem if you supply all boards with power on the bus wires and each node >> consumes relatively high power. The low series resistance is not a problem for >> the communication wires. >> >> The tx pair should be terminated at the far ends (only one place) with a >> termination resistor that matches the cable (usually around 120 ohms). If you >> use half duplex there is only one pair and it should be terminated at both >> ends. >> >> At one place on the bus (usually on the master in a single master system or at >> one end on the bus) there should also be some weak pullup and down resistors to >> guarantee that the bus is read as idle when all transmitter drivers are in the >> off state. I usually have two jumpers on the node boards which are both used >> together to connect the termination resistor and also the pull up/down >> resistors. >> >> The RS485 transcievers are pretty tolerant for common mode voltages (-7V to >> +12V) but this is not always enough. I always put some low value resistors and >> three high powered double (antiparallell) tvs's on each node (two between the >> communication lines and ground and a third between the lines themselves). >> >> For some good, solid information about RS485 networks see the following links: >> >> <http://www.national.com/an/AN/AN-1057.pdf> >> >> <www.circuitcellar.com/library/ccofeature/perrin0799/c79bppdf.pdf> >> >> /Ruben > > Thanks a lot for the detailed reply. I took a look at the links you gave me. > > The way that the network nodes will be placed will make a star > topology the most convenient choice. What's the disadvantage of a star > topology? > > Also, if there are only two devices, and a direct RS485 connection > between them (distances anywhere from 1 m to 1000 m) over a twisted > pair line, does the line need anything special? That is, does it need > to be terminated? If so, how do you terminate a point to point line? > Also, if I'm going to daisy chain the devices, how do I terminate the line? Another idea is to make a very very simple RS485 hub using (almost nothing but) MAX485 type chips. One can connect the the output of each chip to the input on all the other chips. This way, anything broadcasted by one node will be sent to every other node. However, what are the issues with this. Also, how does one terminate the line in this case?

> > The way that the network nodes will be placed will make a star > topology the most convenient choice. What's the disadvantage of a star > topology? > > Also, if there are only two devices, and a direct RS485 connection > between them (distances anywhere from 1 m to 1000 m) over a twisted > pair line, does the line need anything special? That is, does it need > to be terminated? If so, how do you terminate a point to point line? > A 485 bus needs to be a linear bus with terminations at the ends. You can then drive the bus at any point with a zero ohm driver. The signal goes down the transmission line in both directions. If it were not properly terminated with a termination equal to the characteristic impedance of the line, you'll get reflections which can mess up your data. If you use CAT5 cable, you can make it look like a star but wire it as a bus since the cable has multiple pairs. The signal goes out one pair, hits your equipment, then goes back down the other pair. So, the bus zips back and forth between the center and each transceiver. There's a termination, as discussed above, at each end of the bus. As to the line being "special," it's a transmission line, so it should have a constant characteristic impedance. This is where Ethernet cable is great. Good luck! Harold -- FCC Rules Updated Daily at http://www.hallikainen.com - Advertising opportunities available!

Harold Hallikainen wrote: {Quote hidden}>> The way that the network nodes will be placed will make a star >> topology the most convenient choice. What's the disadvantage of a star >> topology? >> >> Also, if there are only two devices, and a direct RS485 connection >> between them (distances anywhere from 1 m to 1000 m) over a twisted >> pair line, does the line need anything special? That is, does it need >> to be terminated? If so, how do you terminate a point to point line? >> >> > > A 485 bus needs to be a linear bus with terminations at the ends. You can > then drive the bus at any point with a zero ohm driver. The signal goes > down the transmission line in both directions. If it were not properly > terminated with a termination equal to the characteristic impedance of the > line, you'll get reflections which can mess up your data. > > If you use CAT5 cable, you can make it look like a star but wire it as a > bus since the cable has multiple pairs. The signal goes out one pair, hits > your equipment, then goes back down the other pair. So, the bus zips back > and forth between the center and each transceiver. There's a termination, > as discussed above, at each end of the bus. > > As to the line being "special," it's a transmission line, so it should > have a constant characteristic impedance. This is where Ethernet cable is > great. > > Good luck! > > Harold > > > > > In the casino days, I used CAT3 (telco two-pair twisted cable). Only ONE pair was guaranteed to be OK, so I used a system where the Bus master gave the command, then released the bus to hear the reply. It worked pretty well. The reason it was used was that the building was very old and that old cable was all we had to work with. PS, for you who asked, I survived my "starter heart attack" and have recovered much of my strength through an imtense physical therapy program. But I have mostly retired now, and plan to see a few sights out here in the west, like Custer's last stand, Meteor crater, Monument Valley, etc. They are all on my "Bucket List". --Bob A