RS-485 Troubleshooting

Hi,
I am using Rabbit development board which has separate ports for RS232
and Rs485. Right now i am using a single port both as RS232 as well as
RS485.
The port works fine for RS232 signals but for RS485 signals, the
differential output is not proper.I think there is some problem with
the differential voltages.
Please help.

"RahulS" <rahuls@kpitcummins.com> schreef in bericht 
news:1138186965.580233.127170@o13g2000cwo.googlegroups.com...
> Hi,
> I am using Rabbit development board which has separate ports for RS232
> and Rs485. Right now i am using a single port both as RS232 as well as
> RS485.
> The port works fine for RS232 signals but for RS485 signals, the
> differential output is not proper.I think there is some problem with
> the differential voltages.

Well if you suspect the differential voltages,
have you taken a look at hem?
An oscilloscope would be the most appropriate tool.
But what makes you think it is the voltage levels?
If you are using a proper driver chip
with proper supply voltages,
and only correct load on the RS-485 bus,
why then should the levels go wrong? 

I am using SP483EN chip for RS485 signals. I used oscilloscope to test
the differential signals.
The signals were not differential in nature. When i short the CRO
ground and RS485+, it works.
When i monitor the waveform between RS485+ and GND, the waveform is
correct.
When i monitor the waveform between RS485- and GND, the waveform is
correct.But when  i monitor the waveform between RS485+ and RS485-, the
waveform is incorrect.
Could this be the case that the RS485 driver IC is not functioning
properly
Regards

"RahulS" <rahuls@kpitcummins.com> wrote in message
news:1138188073.191302.290970@z14g2000cwz.googlegroups.com...
> I am using SP483EN chip for RS485 signals. I used oscilloscope to test
> the differential signals.
> The signals were not differential in nature. When i short the CRO
> ground and RS485+, it works.
> When i monitor the waveform between RS485+ and GND, the waveform is
> correct.
> When i monitor the waveform between RS485- and GND, the waveform is
> correct.But when  i monitor the waveform between RS485+ and RS485-, the
> waveform is incorrect.

How do you measure that? Not by clipping the probe on RS485+ and the ground
clip on RS485- I hope?

> Could this be the case that the RS485 driver IC is not functioning

The only proper way of measuring this is by using a dual channel CRO, one
channel on each RS485 pin and checking if the waveforms are eachothers'
opposite.

Meindert

I have already done that, after which i was able to conclude that
differential signals are not correct

"RahulS" <rahuls@kpitcummins.com> schreef in bericht 
news:1138189568.656826.165680@g47g2000cwa.googlegroups.com...
>I have already done that, after which i was able to conclude that
> differential signals are not correct

Answers like this offer little hope of progress.
Since you seem to need a bit of help in answering,
please answer yes/no to each of the following:

-) are you using a dual-beam (two-channel) oscilloscope?

-) did you attach one channel to RS485+ ?

-) did you attach the other channel to RS485-?

If "yes" to all three, please describe the waveforms observed,
and explain what you think incorrect about them.

"RahulS" <rahuls@kpitcummins.com> wrote in message
news:1138189568.656826.165680@g47g2000cwa.googlegroups.com...
> I have already done that, after which i was able to conclude that
> differential signals are not correct

To whom are you replying now? Please leave a piece of the original text you
are replying to, so everyone reading this thread, knows what you are talking
about.

If you are replying to me: what exactly was the difference between both
signals?
Can you make a simpla ASCII (character) drawing of that?

Meindert

RahulS wrote:

> The port works fine for RS232 signals but for RS485 signals, the
> differential output is not proper.I think there is some problem with
> the differential voltages.

RS485 is almost idiot proof. If you aren't getting the right voltages at 
the outputs you've:

- loaded the output with something that stops it working
- stopped loading the output but buggered the chip
- forgotten to turn the thing on (enable)
- got the scope ground clipped to somewhere whappy
- don't know what to look for or how to look for it.

Paul Burke

"Paul Burke" <paul@scazon.com> schreef in bericht 
news:43pbdiF1ou81sU1@individual.net...
> RahulS wrote:
>
>> The port works fine for RS232 signals but for RS485 signals, the
>> differential output is not proper.I think there is some problem with
>> the differential voltages.
>
> RS485 is almost idiot proof. If you aren't getting the right voltages at 
> the outputs you've:
>
> - loaded the output with something that stops it working
> - stopped loading the output but buggered the chip
> - forgotten to turn the thing on (enable)
> - got the scope ground clipped to somewhere whappy

all of these had already been suggested

> - don't know what to look for or how to look for it.

this one seems more and more obvious

On 2006-01-25, Meindert Sprang <mhsprang@NOcustomSPAMware.nl> wrote:

>> When i monitor the waveform between RS485- and GND, the waveform is
>> correct.But when  i monitor the waveform between RS485+ and RS485-, the
>> waveform is incorrect.

RS-485 drivers are differential but not floating.

> How do you measure that? Not by clipping the probe on RS485+ and the ground
> clip on RS485- I hope?
>
>> Could this be the case that the RS485 driver IC is not functioning
>
> The only proper way of measuring this is by using a dual
> channel CRO, one channel on each RS485 pin and checking if the
> waveforms are eachothers' opposite.

Or using a scope with a floating ground.

Or using a differential scope probe.  If you do a lot of RS-485
work, it can be a handy (albiet expensive) thing to have.

-- 
Grant