On 9/09/2011 11:02 AM, Dwayne Reid wrote:
> At 06:44 PM 9/8/2011, David Duffy (AVD) wrote:
>> I'd like to send 4 x RS485 signals over a CAT5 cable.  The device at
>> each end of the cable is completely isolated, so I can't get a common
>> mode reference from ground.
>>
>> Any ideas on sending 4 differential signals over a CAT5 ?  The cable
>> length can vary from 1 - 100 metres and the data rate is about 1mb/s.
>>
>> Magnetics like Ethernet springs to mind, but I was wondering about
>> somehow using the multiple pairs to generate a virtual common mode
>> reference.
> Because the end device is completely isolated, you *should* be able
> to dispense with the ground connection.
>
> However, I *might* be wrong.  In that case, split your termination
> resistors in two, each half the normal value and connected in
> series.  Then connect the mid-point of those termination resistors to
> your ground connection.
>
> In fact, you should need to do that for only one of the signals.  The
> rest might move around with repsect to that ground but they will
> still be well within the common-mode range of the differential receivers.

Hi Dwayne,
Ah, I'd heard something like that a while back, but couldn't recall the=20
specifics.  Thanks.  I might give that a go.
David...

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___________________________________________
David Duffy        Audio Visual Devices P/L
Unit 8, 10 Hook St, Capalaba 4157 Australia
Ph: +61 7 38235717      Fax: +61 7 38234717
Our Web Site: www.audiovisualdevices.com.au
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