RS-485 Troubleshooting

RahulS wrote:
> 
> 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.

This sounds as if the RS485 network is incorrectly terminated and
biased.

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On 25 Jan 2006 03:02:45 -0800, RahulS wrote:

> 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.

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

"Bob Stephens" <roberts@dcxchol.com> wrote in message 
news:hewtwp16d2b6.1r7fp0inum465$.dlg@40tude.net...
> On 25 Jan 2006 03:02:45 -0800, RahulS wrote:
>
>> 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.
>
> 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.

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

Steve at fivetrees wrote:
> "Bob Stephens" <roberts@dcxchol.com> wrote in message
> news:hewtwp16d2b6.1r7fp0inum465$.dlg@40tude.net...
> > On 25 Jan 2006 03:02:45 -0800, RahulS wrote:
> >
> >> 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.
> >
> > 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.
>
> 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


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

karel wrote:
> "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.


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" <rahuls@kpitcummins.com> wrote in message
news:1138357530.405792.162570@g49g2000cwa.googlegroups.com...
> 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

On Fri, 27 Jan 2006 11:47:41 +0100, Meindert Sprang wrote:

>> 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