RS-485 Troubleshooting
RahulS
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.
karel
"RahulS" <rah...@kpitcummins.com> schreef in bericht
news:1138186965.5...@o13g2000cwo.googlegroups.com...
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?
RahulS
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
Meindert Sprang
news:1138188073.1...@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
RahulS
differential signals are not correct
karel
>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.
Meindert Sprang
> 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
Paul Burke
> 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
karel
"Paul Burke" <pa...@scazon.com> schreef in bericht
news:43pbdiF...@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
Grant Edwards
>> 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
Bob Stephens
Is it 2-wire or 4-wire RS485? If it's 2-wire (half duplex) make sure you
are switching the transceiver at the proper time from transmit to receive.
Bob
Steve at fivetrees
news:hewtwp16d2b6.1r7fp0inum465$.dlg@40tude.net...
And, as I've said a few times, don't forget the ground reference (i.e. those
2 or 4 signals are measured relative to a common 0V reference - miss this
out and fun will ensue. Not).
Steve
(who is therefore in the habit of referring to 3- and 5-wire RS485
architectures)
http://www.fivetrees.com
CBFalconer
>
> 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
> RS485+ and RS485-, the waveform is incorrect.
This sounds as if the RS485 network is incorrectly terminated and
biased.
Always include adequate context in your replies. Even if using
broken Google usenet interface, when you can follow the
instructions below.
--
"If you want to post a followup via groups.google.com, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers." - Keith Thompson
More details at: <http://cfaj.freeshell.org/google/>
RahulS
Three connections were tried as follows:
1. CRO channel 1,
its signal pin connected to 485+
its ground pin connected to ground
we can see continuous rectangular waveforms ( on -off- on -off ) as i
am sending data 1 and 0 alternately but continuously
data can be seen on hyperterminal ( with 485-232 convertor in between)
2. CRO channel 1,
its signal pin connected to 485+
its ground pin connected to ground
similar ouput.
3.CRO channel 1,
its signal pin connected to 485+
its ground pin connected to 485-
no waveform observed ( coincides with reference line on CRO)
4.CRO channel 1,
its signal pin connected to 485+
its ground pin connected to 485-
its ground pin touched to 485+ ( indirectly it becomes case 1 )
rectangular waveform observed, data can be seen on hyperterminal ( with
485-232 convertor in between)
please note that I am only using differential output and not ground for
communication of two devices
RahulS
Meindert Sprang
> Three connections were tried as follows:
> 1. CRO channel 1,
> its signal pin connected to 485+
> its ground pin connected to ground
>
> we can see continuous rectangular waveforms ( on -off- on -off ) as i
> am sending data 1 and 0 alternately but continuously
> data can be seen on hyperterminal ( with 485-232 convertor in between)
Good. If you can see data in hyperterminal, everything seems to be ok.
> 2. CRO channel 1,
> its signal pin connected to 485+
> its ground pin connected to ground
>
> similar ouput.
Typo? Do you mean 485+ or 485-?
>
> 3.CRO channel 1,
> its signal pin connected to 485+
> its ground pin connected to 485-
>
> no waveform observed ( coincides with reference line on CRO)
Logical. You short-circuit 485- to ground through the ground of the CRO.
> 4.CRO channel 1,
> its signal pin connected to 485+
> its ground pin connected to 485-
> its ground pin touched to 485+ ( indirectly it becomes case 1 )
I have no clou as to what you are doing now????
The only proper way to measure this is to connect CRO channel 1 to 485+,
ground to ground and CRO channel 2 to 485- and ground to ground. Observe
both waveforms simultaneaously on the screen, they should be in opposite
phase and all the flanks matching.
> rectangular waveform observed, data can be seen on hyperterminal ( with
> 485-232 convertor in between)
>
> please note that I am only using differential output and not ground for
> communication of two devices
Which is bad, because the common mode voltage on RS485 may not exceed 7V.
Always use a ground connection too.
Meindert
Bob Stephens
>> we can see continuous rectangular waveforms ( on -off- on -off ) as i
>> am sending data 1 and 0 alternately but continuously
>> data can be seen on hyperterminal ( with 485-232 convertor in between)
>
> Good. If you can see data in hyperterminal, everything seems to be ok.
I agree with Meindert. If you're reading the data with HT, then you should
be good to go. Having said that however, I recently had some reliability
problems with a PCMCIA RS485 card, and needed to add 620 ohm biasing
resistors from RS485+ to VCC and RS485- to ground in addition to a 120 ohm
terminating resistor across the pair.
Bob