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

Paul Keinanen <keinanen@xxxxxx> wrote:
>On Wed, 15 Jun 2005 16:17:56 +0100, "Steve at fivetrees"
><steve@xxxxxxxxxxxxxxxxxxxxx> wrote:
>>"Tim Mitchell" <timng@xxxxxxxxxxxxxxxxxxxxx> wrote in message
>>> 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.
>This is generally true in practice.

It *can't* be true in practice. The presumption is to provide a
signal ground, and it simply doesn't.

Any attempt at providing a signal ground is merely going to connect
frame ground between the two locations, which will very likely cause
more noise induction into the signal pairs than anything else.

>However, if you are using floating devices and the "fail safe"
>termination, the system will work with 2 resp. 4 wires, since the
>"fail safe" resistors will force the receiver power supply Vcc and Gnd
>potential close to the line potential and thus, within the common mode
>range of the receiver (-5..+12 V). Treat the terminated system as a
>bidirectional current loop (through the terminating resistors) and it
>should be easy to analyze how this works.

The circuit impedance is 100 Ohms. It *is* a current loop... :-)

>> - RS-422 is also multidrop (10 max drops, IIRC).
>The RS-422 impedance levels allows for multiple receivers, but I do
>not see how a multidrop bidirectional system could be implemented
>within the RS-422 specification.

What would prevent it? It's a fairly simple 4-wire arrangement
with a single master and 10 slaves plus a single 100 Ohm
termination on each cable.

Floyd L. Davidson <>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx

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