At 03:16 AM 9/15/99 +0800, you wrote:
>If I'm not wrong, RS485 uses current loop for signaling.
>

RS 485 is a voltage based standard.  It uses differential pairs to transmit
data making it quite resistant to noise.  Its length limitations are
primarily based on the RC time constant of the line.  At 2400 baud you
could go really far.

The differential pair typically go to a comparator, and since any external
RFI will affect them both the same way, the comparator will still see the
proper signal.  I believe the 488 part gives you +/- 9V so you can do your
noise immunity based on that.

-Erik Reikes

>So it should be quite immune to interference and
>a twisted-pair cable will not give as much improvement
>as with voltage signaling.
>
>I believe UTP cable should be good enough.
>
>Regards,
>Peter Tiang
>
>
>> Hi all,
>>
>> I want to interface my PIC project with the outer world using a low
>> baud rate RS485 interface (MAX488 full duplex, slew rate limited).
>> The maximum cable length is approx. 100 meters (333 feet) and could
>> be routed near other equipment which may cause interference on the
>> cable.
>>
>> My question is, is it absolutely nessary to use a Shielded TP cable
>> instead of the more common UTP cable (CAT5)?
>> I was wondering this since RS485 is already pretty solid especially
>> combined with low data rates (2400 bps).
>>
>> Thanks!
>>
>> Bye,
>>
>> Daniel...
>>