On Fri, 15 Apr 2011 14:44:11 -0400, Rich Webb <bbew.ar@mapson.nozirev.ten> wrote:> >Besides, as Grant pointed out, even after you've identified what the >per-spec A and B ends of the cable are, odds are 50-50 that what it is >connecting to got it backwards... ;-)Setting up an RS-485 network is not that hard as long as each device has some kind of RxData LED and at least one device can be configured with "fail safe termination" i.e. pull up/down resistors. Power up a device which is configured for "fail safe termination". Connect the next station to the network. If the RxData LED burns continuously, swap A and B. The RxData LED should be constantly off if no polling is used. Connect the next station to the network and observe the RxData LED and repeat the procedure above. The 2 wire RS-485 system is quite easy to set up, but the 4 wire RS-422 is a bit more problematic, with typically 4 or even 5 badly marked conductors, which wire is A/B/A'/B'/C (signal ground) ?
RS485: two transceivers
Started by ●April 14, 2011
Reply by ●April 15, 20112011-04-15
Reply by ●April 18, 20112011-04-18
Grant Edwards wrote: [...]>> I am not so good in english, so i try to explain: If both, the master >> and the slave have a connection to the GND-level, there may be a >> difference in the voltage level. This will probably cause the problems >> you mentioned. To prevent this, you can have the devices on a floating >> level at least at the rs485 inputs. > >Even if they're floating, you still need a signal ground conneciton. > >I've seen optically isolated RS485 interfaces on industrial control >systems that were floating fail without a signal ground. They would >fail intermittently as the signal ground on one end floated in and out >of the allowed 7V common-mode range for the other end. Maybe there >was something wrong with the isolation, but without a signal ground it >didn't work, and with a signal ground they did.Correct. The (common mode) impedance of a RS485 transceiver is rather high, especially the fractional load types. This high impedance isn't sufficient to keep the isolated circuit in the operating region with lots of (CM) noise. Example: 100pF and 10kOhms yields 160kHz corner frequency...>> In such a case, when there is a level difference, an additional >> gnd-line would cause a current on the ground wire which can also lead >> to problems.Isolation _plus_ a reference conductor is the correct solution in this case. Oliver -- Oliver Betz, Munich despammed.com is broken, use Reply-To:
Reply by ●April 22, 20112011-04-22
>> >> 2) A signal ground is also required. RS485 receivers can typically >> only deal with a common-mode voltage of 7-12V. >> > >If you have signal ground on both sides, you should connect. If not, >don�t care, should work also without GND. > >Best regards > >Stefan DF9BI >Some RS485 transceivers have pullups inside. Alex --------------------------------------- Posted through http://www.EmbeddedRelated.com
Reply by ●April 22, 20112011-04-22
On 2011-04-22, baranov <alexanderbaranov@n_o_s_p_a_m.hotmail.com> wrote:>>> >>> 2) A signal ground is also required. RS485 receivers can typically >>> only deal with a common-mode voltage of 7-12V. >>> >> >>If you have signal ground on both sides, you should connect. If not, >>don't care, should work also without GND.In my experience sometimes it doesn't.> Some RS485 transceivers have pullups inside.I don't see how that obviates the need for a signal ground. -- Grant
Reply by ●April 22, 20112011-04-22
In article <iosam2$cpk$1@reader1.panix.com>, invalid@invalid.invalid says...> On 2011-04-22, baranov <alexanderbaranov@n_o_s_p_a_m.hotmail.com> wrote: > >>> > >>> 2) A signal ground is also required. RS485 receivers can typically > >>> only deal with a common-mode voltage of 7-12V. > >>> > >> > >>If you have signal ground on both sides, you should connect. If not, > >>don't care, should work also without GND. > > In my experience sometimes it doesn't. > > > Some RS485 transceivers have pullups inside. > > I don't see how that obviates the need for a signal ground. >It doesn't. Does anyone that is actually interested in the reliability of their RS- 485 network NOT run a GND?
Reply by ●April 23, 20112011-04-23
On 2011-04-22, WangoTango <Asgard24@mindspring.com> wrote:>> > Some RS485 transceivers have pullups inside. >> >> I don't see how that obviates the need for a signal ground. > > It doesn't. Does anyone that is actually interested in the > reliability of their RS- 485 network NOT run a GND?Yes. It's a not uncommon misconception that "RS485 is two-wire differential" and therefore doesn't need a ground. Just look back in this thread a bit. :)
