Hi I have a requirement to communicate between a slave device and a master where the distance between them is approximately 3km. I don't need high speed comms, 10-20kbaud is sufficient. Unfortunately I don't have the luxury of being able to use repeaters, fibre optics or wireless. If RS422 was spec'd beyond 1.2km then that would be ideal. It seems that CAN will work up to 6km at bit rates of 10kbaud but I'd rather not have the complexity required to implement CAN especially as I only need point-to-point comms. I was thinking of using CAN transceivers & connecting the Rx/Tx pins to UARTs at either end. Does anyone have any experience of such an application or have any suggestions ? Thanks Dave
Long distance comms
Started by ●April 8, 2006
Reply by ●April 8, 20062006-04-08
Some of the metering busses are designed for this kind of rate/distance. One that comes to mind has Master -> slave comms voltage modulated (12-24V) and slave to master comms current modulated . Each slave needs to have a constant current drain for this to work. -Andrew M "Daveb" <dave.bryan@gmail.com> wrote in message news:1144493923.566431.168140@u72g2000cwu.googlegroups.com...> Hi > > I have a requirement to communicate between a slave device and a master > where the distance between them is approximately 3km. I don't need high > speed comms, 10-20kbaud is sufficient. Unfortunately I don't have the > luxury of being able to use repeaters, fibre optics or wireless. If > RS422 was spec'd beyond 1.2km then that would be ideal. > > It seems that CAN will work up to 6km at bit rates of 10kbaud but I'd > rather not have the complexity required to implement CAN especially as > I only need point-to-point comms. I was thinking of using CAN > transceivers & connecting the Rx/Tx pins to UARTs at either end. > > Does anyone have any experience of such an application or have any > suggestions ? > > Thanks > Dave >
Reply by ●April 8, 20062006-04-08
Daveb scrobe on the papyrus:> Hi > > I have a requirement to communicate between a slave device and a > master where the distance between them is approximately 3km. I don't > need high speed comms, 10-20kbaud is sufficient. Unfortunately I > don't have the luxury of being able to use repeaters, fibre optics or > wireless. If RS422 was spec'd beyond 1.2km then that would be ideal. > > It seems that CAN will work up to 6km at bit rates of 10kbaud but I'd > rather not have the complexity required to implement CAN especially as > I only need point-to-point comms. I was thinking of using CAN > transceivers & connecting the Rx/Tx pins to UARTs at either end. > > Does anyone have any experience of such an application or have any > suggestions ? > > Thanks > DaveA current loop with opto isolators should do the trick. You might be limited to a lower baud rate. -- John B
Reply by ●April 8, 20062006-04-08
Andrew Thanks for the reply. I searched for metering busses & the only one I can find is M-Bus which is restricted to 350m without repeaters. Do you know of any which go up to 3km ? Thanks Dave
Reply by ●April 8, 20062006-04-08
On 8 Apr 2006 11:57:56 GMT, "John B" <spamj_baraclough@blockerzetnet.co.uk> wrote:>Daveb scrobe on the papyrus: > >> Hi >> >> I have a requirement to communicate between a slave device and a >> master where the distance between them is approximately 3km. I don't >> need high speed comms, 10-20kbaud is sufficient. Unfortunately I >> don't have the luxury of being able to use repeaters, fibre optics or >> wireless. If RS422 was spec'd beyond 1.2km then that would be ideal. >> >> It seems that CAN will work up to 6km at bit rates of 10kbaud but I'd >> rather not have the complexity required to implement CAN especially as >> I only need point-to-point comms. I was thinking of using CAN >> transceivers & connecting the Rx/Tx pins to UARTs at either end. >> >> Does anyone have any experience of such an application or have any >> suggestions ? >> >> Thanks >> Dave > >A current loop with opto isolators should do the trick. You might be >limited to a lower baud rate.Using current loop would also be my first suggestion with a sufficient voltage headroom (24-60 V) in the constant current generator. However, the RS-422/485 is specified for 1200 m at 100 kbit/s for a specific wire gauge with the terminating resistors present. If you can use a thicker wire and possibly get away with the terminating resistor due to the low data rate, the link might be doable with standard RS-422/485 hardware. Due to ground potential differences (especially during thunderstorms), it is essential that the systems have a galvanic isolation regardless which technology is used. Paul
Reply by ●April 8, 20062006-04-08
Daveb wrote:> Hi > > I have a requirement to communicate between a slave device and a master > where the distance between them is approximately 3km. I don't need high > speed comms, 10-20kbaud is sufficient. Unfortunately I don't have the > luxury of being able to use repeaters, fibre optics or wireless. If > RS422 was spec'd beyond 1.2km then that would be ideal. > > It seems that CAN will work up to 6km at bit rates of 10kbaud but I'd > rather not have the complexity required to implement CAN especially as > I only need point-to-point comms. I was thinking of using CAN > transceivers & connecting the Rx/Tx pins to UARTs at either end. > > Does anyone have any experience of such an application or have any > suggestions ? > > Thanks > Dave >Look up DSL line drivers. donald
Reply by ●April 8, 20062006-04-08
Thanks for all the suggestions. I realise my requirement isn't a common one but I'm hoping to find a comms standard I can use that will save time in designing/testing the physical implementation from scratch. I've seen mentioned in several overviews of CAN that it will work up to 6km @ 10kbps but can't find any specs that confirm this. Could anyone point me in the direction of a spec that details this ? Thanks all Dave
Reply by ●April 8, 20062006-04-08
Daveb <dave.bryan@gmail.com> wrote:> I realise my requirement isn't a common one but I'm hoping to find a > comms standard I can use that will save time in designing/testing the > physical implementation from scratch. I've seen mentioned in several > overviews of CAN that it will work up to 6km @ 10kbps but can't find > any specs that confirm this. Could anyone point me in the direction of > a spec that details this ?There may not be one. CAN is primarily a *protocol*, with more than one physical line specification choice to run it over. What limits the length of a CAN bus depends on the baudrate *and* the physical medium choosen. The 6km @ 10kbps is probably a protocol limit --- basically the speed-of-light barrier applied to bit timing requirements of CAN. That doesn't automatically guarantee that a plain vanilla two-wire CAN bus transmitter will actually be able to drive a line of that length. IOW, you may be chasing a red herring. -- Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de) Even if all the snow were burnt, ashes would remain.
Reply by ●April 8, 20062006-04-08
Daveb wrote:> > Thanks for all the suggestions. > > I realise my requirement isn't a common one but I'm hoping to find > a comms standard I can use that will save time in designing/testing > the physical implementation from scratch. I've seen mentioned in > several overviews of CAN that it will work up to 6km @ 10kbps but > can't find any specs that confirm this. Could anyone point me in > the direction of a spec that details this ?The opto-isolated current loop is by far the simplest, and has been working over far greater distances for nearly two centuries. I have used it myself in noisy environments (hospitals, with elevators and other machinery, over leased pairs, for several miles). No common mode worries, just control the single point ground and have adequate opto-isolators. -- "If you want to post a followup via groups.google.com, don't use the broken "Reply" link at the bottom of the article. Click on "show options" at the top of the article, then click on the "Reply" at the bottom of the article headers." - Keith Thompson More details at: <http://cfaj.freeshell.org/google/> Also see <http://www.safalra.com/special/googlegroupsreply/>
Reply by ●April 8, 20062006-04-08
Hans-Bernhard Broeker wrote:> Daveb <dave.bryan@gmail.com> wrote: > > >>I realise my requirement isn't a common one but I'm hoping to find a >>comms standard I can use that will save time in designing/testing the >>physical implementation from scratch. I've seen mentioned in several >>overviews of CAN that it will work up to 6km @ 10kbps but can't find >>any specs that confirm this. Could anyone point me in the direction of >>a spec that details this ? > > > There may not be one. CAN is primarily a *protocol*, with more than > one physical line specification choice to run it over. What limits > the length of a CAN bus depends on the baudrate *and* the physical > medium choosen. The 6km @ 10kbps is probably a protocol limit --- > basically the speed-of-light barrier applied to bit timing > requirements of CAN. That doesn't automatically guarantee that a > plain vanilla two-wire CAN bus transmitter will actually be able to > drive a line of that length. > > IOW, you may be chasing a red herring. >AFAIK that is the case -- when you read the CAN document itself it's pretty adamant about _not_ being a spec for the physical layer. There _is_ an SAE spec for a physical layer which uses 1/2 VCC on both wires for 1 and pulls the wires to 0V and VCC for 0 -- but I would expect that it wouldn't be any better than RS-485 for really long distances. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/