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| RS-485 optoisolated is not requiered ? |
 Posted: Thu Dec 08, 2005 7:25 pm |
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I hardly found some rs-485 silicon device (transceiver)
with opto-insolated buses, it's generally not mandatory ?
I', tring to connect a PC with a 18f42( 40 metter each
to other) in a industrial area.
Best Regards |
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kender
Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley
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| Posted: Thu Dec 08, 2005 9:01 pm |
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Formally, galvanic isolation of the RS-485 bus is not required (unless you are making a medical instrument with electrodes directly attached to the patient's body). However, since you are going to run the bus in the industrial area, isolation would be recommended. If you environment has a lot of EMI, optoisolation can save you some debugging time and possibly prevent hardware from getting fried.
Here are some other RS-485 transceivers for your reference:
- Analog devices: ADM485, ADM2483 (with magnetic isolation)
- Maxim: MAX485, MAX1480 (optical isolation, see also this app note http://www.maxim-ic.com/appnotes.cfm/appnote_number/2116)
- Linear: LT1785, LTC1535 (capacitive isolation, built-in isolating DC-DC converter, pretty cool)
- And there are probably tons of others. Pretty much any major chip manufacturer makes RS-485 transceivers (National Semiconductor, Texas Instruments etc.). |
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Humberto
Joined: 08 Sep 2003
Posts: 1215
Location: Buenos Aires, La Reina del Plata
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| Posted: Thu Dec 08, 2005 9:21 pm |
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| Quote: |
I hardly found some rs-485 silicon device (transceiver)
with opto-insolated buses, it's generally not mandatory ?
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Yes, it is not mandatory. Unless if there are considerable ground shifting between both ends. To ovoid this, usually the transceivers are powered by a galvanically isolated DC-DC Converter power supply, this way the transmition line Gnd is isolated from the Logic Gnd.
To cover the distance you are talking about, taking care of using a good cable should be enough to get a reliable link. Care should be taken when selecting the cable, because a not suitable cable in an industrial enviroment can cause intermittent problems wich are very difficult to troubleshoot.
While the RS-485 specification does not specify protocol nor cabling, RS-422 does for the physical layer and as both are differential, by extension these recommendations should be used for RS-485 systems as well.
The RS-422 specification recommends 24AWG twisted pair cable with a shunt capacitance of 16 pF per foot and 100 ohm characteristic impedance. Shielded cable is only slightly more expensive than unshielded, so to not get any risk go for the shielded and life will smile you.
Humberto |
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SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
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| Posted: Fri Dec 09, 2005 8:42 am |
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The RS485 spec allows up to +/- 7V (or maybe +/- 12V, I am not sure) between the grounds of the transmitter and receiver in a non-isolated system. In many appliactions this is fine, but isolation is often cheap insurance. Adding isolation adds cost, bulk, power consumption, and often reduces speed, so it is something to consider.
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