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" schreef in bericht news: snipped-for-privacy@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?
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
How do you measure that? Not by clipping the probe on RS485+ and the ground clip on RS485- I hope?
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" schreef in bericht news: snipped-for-privacy@g47g2000cwa.googlegroups.com...
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.
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
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
all of these had already been suggested
this one seems more and more obvious
RS-485 drivers are differential but not floating.
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
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
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)
Three connections were tried as follows:
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)
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
Three connections were tried as follows:
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)
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
Good. If you can see data in hyperterminal, everything seems to be ok.
Typo? Do you mean 485+ or 485-?
Logical. You short-circuit 485- to ground through the ground of the CRO.
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.
Which is bad, because the common mode voltage on RS485 may not exceed 7V. Always use a ground connection too.
Meindert
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
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