I am trying to interface two systems through RS422 link. There are only
four lines connected between them (tx+/- & Rx+/-) and no ground.
Independently both systems work fine but when interconnected and i try to
communicate with each other there are some problems.  initially master
units Tx lines have been set to  a known state (3.6v app). if i transmit 4
bytes from master unit only 3 bytes are received at the slave unit, though
when captured the waveform in CRO shows all the 4 bytes.iam glowing
separate LED's for each byte received.the 4th byte LED doesnt glow at all
but when the master is powered off, then the fourth byte LED glows. what
could be the problem.

On 2008-07-07, svyamuna <s...@yahoo.co.in> wrote:

> I am trying to interface two systems through RS422 link. There
> are only four lines connected between them (tx+/- & Rx+/-) and
> no ground.

That's probably what's wrong: you need to connect the two
system's grounds together.

> Independently both systems work fine but when interconnected
> and i try to communicate with each other there are some
> problems.  initially master units Tx lines have been set to a
> known state (3.6v app). if i transmit 4 bytes from master unit
> only 3 bytes are received at the slave unit, though when
> captured the waveform in CRO shows all the 4 bytes.iam glowing
> separate LED's for each byte received.the 4th byte LED doesnt
> glow at all but when the master is powered off, then the
> fourth byte LED glows. what could be the problem.

-- 
Grant Edwards                   grante             Yow! ... bleakness
                                  at               ... desolation ... plastic
                               visi.com            forks ...

On Mon, 07 Jul 2008 12:15:17 -0500, "svyamuna" <s...@yahoo.co.in>
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, as the RS-422
works as a balanced current loop.
 
>Independently both systems work fine but when interconnected and i try to
>communicate with each other there are some problems.  initially master
>units Tx lines have been set to  a known state (3.6v app). if i transmit 4
>bytes from master unit only 3 bytes are received at the slave unit, 

Sounds like an Rx ISR problem.

Paul

On 2008-07-07, Paul Keinanen <k...@sci.fi> wrote:
> On Mon, 07 Jul 2008 12:15:17 -0500, "svyamuna" <s...@yahoo.co.in>
> 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.

> 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.

-- 
Grant Edwards                   grante             Yow! I want EARS!  I want
                                  at               two ROUND BLACK EARS
                               visi.com            to make me feel warm
                                                   'n secure!!

Paul Keinanen 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, 

Ground connection is required unless the transceivers are floating.

> as the RS-422
> works as a balanced current loop.

???
RS-422 has voltage output.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

On 2008-07-07, Vladimir Vassilevsky <a...@hotmail.com> wrote:
>
>
> Paul Keinanen 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, 
>
> Ground connection is required unless the transceivers are
> floating.

Even when they are, I've had problems unless there's a 5th
wire.  In theory they should float together and work fine, but
in my experience you can't count on it.

-- 
Grant Edwards                   grante             Yow! I just had a NOSE
                                  at               JOB!!
                               visi.com            

In article <QoWdnZl_Ce8o9-_V...@posted.visi>, 
g...@visi.com says...
> On 2008-07-07, Vladimir Vassilevsky <a...@hotmail.com> wrote:
> >
> >
> > Paul Keinanen 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, 
> >
> > Ground connection is required unless the transceivers are
> > floating.
> 
> Even when they are, I've had problems unless there's a 5th
> wire.  In theory they should float together and work fine, but
> in my experience you can't count on it.

Yep, gotta' have a reference for the signals, even if they are 
"differential", at least in this case.
You hit it on the head when you made the comparison to a current loop.
I'll bet you dimes to doughnuts that once a good common is run between 
the units all the problems go away.  At the very least that is one more 
thing to mark off the checklist.

                            Jim

>I forgot to mention one thing, one unit is deriving power from the other
unit i.e. the slave unit is powered up from the master. in such a case is
it required to connect a gnd line between them? But i have even tried this
out by connecting one more gnd line but that doesnt solve my problem. 
yamuna
In article <QoWdnZl_Ce8o9-_V...@posted.visi>, 
>g...@visi.com says...
>> On 2008-07-07, Vladimir Vassilevsky <a...@hotmail.com>
wrote:
>> >
>> >
>> > Paul Keinanen 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, 
>> >
>> > Ground connection is required unless the transceivers are
>> > floating.
>> 
>> Even when they are, I've had problems unless there's a 5th
>> wire.  In theory they should float together and work fine, but
>> in my experience you can't count on it.
>
>Yep, gotta' have a reference for the signals, even if they are 
>"differential", at least in this case.
>You hit it on the head when you made the comparison to a current loop.
>I'll bet you dimes to doughnuts that once a good common is run between 
>the units all the problems go away.  At the very least that is one more 
>thing to mark off the checklist.
>
>                            Jim
>

On Mon, 07 Jul 2008 13:53:32 -0500, Grant Edwards <g...@visi.com>
wrote:

>On 2008-07-07, Paul Keinanen <k...@sci.fi> wrote:
>> On Mon, 07 Jul 2008 12:15:17 -0500, "svyamuna" <s...@yahoo.co.in>
>> 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
 

On Jul 7, 12:33 pm, James Beck <j...@reallykillersystems.com> wrote:
> In article <QoWdnZl_Ce8o9-_VnZ2dnUVZWhedn...@posted.visi>,
> gra...@visi.com says...
>
>
>
> > On 2008-07-07, Vladimir Vassilevsky <antispam_bo...@hotmail.com> wrote:
>
> > > Paul Keinanen 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,
>
> > > Ground connection is required unless the transceivers are
> > > floating.
>
> > Even when they are, I've had problems unless there's a 5th
> > wire.  In theory they should float together and work fine, but
> > in my experience you can't count on it.
>
> Yep, gotta' have a reference for the signals, even if they are
> "differential", at least in this case.
> You hit it on the head when you made the comparison to a current loop.
> I'll bet you dimes to doughnuts that once a good common is run between
> the units all the problems go away.  At the very least that is one more
> thing to mark off the checklist.
>
>                             Jim

I've found the same thing. Ya gotta have a common ground for reliable
operation. It has always seemed to me that it should work with twisted
pair shielded cable grounded at one end - but it doesn't in practice :
(

Bob