Re: RS485 is bidirectional does it mean it is fullduplex?

"Steve at fivetrees" <steve@xxxxxxxxxxxxxxxxxxxxx> wrote:
>"Tim Mitchell" <timng@xxxxxxxxxxxxxxxxxxxxx> wrote:
>> RS485 is a 2-wire half duplex system where there is one transmitter and
>> multiple receivers.
>>
>> RS422 is a 4-wire duplex system which goes between two devices, each of
>> which has a transmitter and receiver.
>
>While this is essentially correct, I'd add:
> - Don't forget ground - i.e. 2-wire is actually 3-wire, and 4-wire is
>actually 5-wire.

RS-485 is a differential (balanced) system, and there is no
signal ground connection. The cable used might well include a
frame ground, but that is for noise induction cancellation, not
signal ground.

Hence it actually is a 2-wire or 4-wire link.

Otherwise these are excellent points.

> - RS-422 is also multidrop (10 max drops, IIRC).
> - RS-422 is often implemented using a pair of RS-485 devices these days,
>since the RS-485 spec is superior to the original RS-422 spec. This confuses
>things slightly, but means that RS-485 tends to be used in both 3-wire and
>5-wire configurations.
>
>Finally (pedant mode on), the old RS-422/485 appellation is obsolete:
>strictly speaking it's now EIA-422 and EIA-485. (This may be useful to know
>for Googling purposes.)

Trivia: Even more insignificant (since nobody uses it) the technically correct
appellation apparently is EIA/TIA-485... but I've also seen TIA/EIA-485
and EIA/RS-485 used.

These are pretty good:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/723
http://www.maxim-ic.com/appnotes.cfm/appnote_number/736

--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.

Relevant Pages

  • Re: RS485 is bidirectional does it mean it is fullduplex?
    ... > RS485 is a 2-wire half duplex system where there is one transmitter and ... > multiple receivers. ... > which has a transmitter and receiver. ... since the RS-485 spec is superior to the original RS-422 spec. ...
    (comp.arch.embedded)
  • Re: RS485 is bidirectional does it mean it is fullduplex?
    ... >> RS485 is a 2-wire half duplex system where there is one transmitter and ... >> which has a transmitter and receiver. ... "fail safe" resistors will force the receiver power supply Vcc and Gnd ... The RS-422 impedance levels allows for multiple receivers, ...
    (comp.arch.embedded)