Re: RS422 interface
- From: rickman <gnuarm@xxxxxxxxx>
- Date: Tue, 8 Jul 2008 21:26:19 -0700 (PDT)
On Jul 8, 4:44 am, Paul Keinanen <keina...@xxxxxx> wrote:
On Mon, 07 Jul 2008 13:53:32 -0500, Grant Edwards <gra...@xxxxxxxx>
wrote:
On 2008-07-07, Paul Keinanen <keina...@xxxxxx> wrote:
On Mon, 07 Jul 2008 12:15:17 -0500, "svyamuna" <svyam...@xxxxxxxxxxx>
wrote:
I am trying to interface two systems through RS422 link. There are only
four lines connected between them (tx+/- & Rx+/-) and no ground.
This is quite OK as long as there is a termination resistor (typically
100..120 ohms) between the Tx+ and Tx- lines and also between the Rx+
and Rx- lines. The ground connection is not needed,
Wrong -- take a look at the receiver's spec for max common-mode
voltage. It's usually only 8-12V. You've got to have a common
ground to make sure you don't exceed the receiver common-mode
voltage spec.
And what would a flimsy signal ground wire help to reduce the common
mode voltage in mains powered systems ?
To understand the situation you have to look how different earthing
systems work e.g.http://en.wikipedia.org/wiki/Earthing_system
In a TN-S system with separate protective earth (PE) and neutral (N)
no current is flowing in normal situations in the PE conductor and
hence all device cases (and the RS-422 transceivers are usually
connected to the case) are at the same potential. In this situation
using a separate signal ground does not make a difference.
For TN-C-S systems, the situation is the same, provided that the two
communicating devices A and B are after the same split point where
the PEN conductor is split into PE and N conductors.
Looking at the TN-C system, in which there is only a single PEN
conductor carrying the neutral current as well as connecting the
equipment case (and usually the RS-422 ground reference), the
situation is more complicated. The current flowing in the PEN
conductor (typically several amps in a non-balanced three phase
system) will cause a voltage drop between mains sockets A and B and
hence also a reference potential difference in the RS-422 transceivers
for equipment A and B, which can be several volts.
If we connect a signal ground (SG) between equipment A and B, this
will create an additional current path parallel to the PEN conductor
and the neutral wire current is shared inversely proportional to the
resistance of the PEN conductor and the SG conductor.
In order to significantly reduce the ground potential difference
between equipments A and B, the SG conductor would have to carry a
significant part of the PEN current (say 1/2) thus the SG conductor
cross section area should be similar to the PEN conductor.
If you make the SG conductor that thick there may flow several amps of
50/60 Hz neutral current and all their harmonics and other noise
currents through the SG wire as well as flow within the equipment
(often through narrow PCB tracks) into the device case, which can have
harmful effects or even burn a narrow PCB track.
In a TN-C system, the separate SG wire does not solve any problems,
but creates some new problems. If the ground potential difference is
larger than the RS-422 transceiver common mode voltage range, the only
alternative is to use a floating (galvanic isolation) system. In a
floating system, the "fail-safe" pull -up/down resistors will pull the
internal signal ground into the middle of the transceiver common mode
voltage range, so there is no need for a SG wire.
as the RS-422 works as a balanced current loop.
Wrong -- it's a differential-mode voltage signal. It's not a
current loop signal.
You can analyze the RS-422 system as a voltage driven system, as a
bipolar current loop or as a transmission line with impedance matching
to the transmission line.
To understand why the SG is not required, it is easiest to look at as
a current loop. The standard requires that the receiver voltage is at
least +/-200 mV and assuming there is a 100 resistor at the receiver,
at least +/- 2 mA will flow in this current loop from Tx+, through the
termination resistor (at the receiver end) and back to Tx-. With a 5 V
transceiver, the maximum current is below +/-50 mA.
Does the unipolar 0/20 mA current loop need a separate signal ground ?
Why would the bipolar RS-422 current loop need a signal ground ?
Paul
I don't want to be argumentative, but you have make a lot of
assumptions in your discussion. For example, you assume that the
chassis of both units "are at the same potential". If that were true,
the OPs circuit should be working. If they are not at the same
potential, then the reason for that needs to be identified. Your
discussion makes assumptions for why that is and makes the assumption
that the chassis and/or the electronics are grounded to the power
ground. Many systems are isolated just for this reason, to prevent
power supply current from changing the ground level of the circuit.
In your discussion of RS-422 as a current loop without a ground, you
don't mention what voltage is present on the conductors relative to
the associated ground, only the current through them. If you ignore
this voltage, you run the risk of blowing out the chips or at least
making them work improperly. A common ground must be established
between an RS-422 driver and receiver. How to best do this depends on
the equipment and the grounding method used. But to say the common
ground conductor is not required is a significant
oversimplification.
Rick
.
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