building an RS-485<->RS422 converter

From: Dwight K. Elvey <dwight.elvey_at_amd.com>
Date: Thu May 27 11:29:54 2004

>From: "Bert Thomas" <bert_at_brothom.nl>
>
>Ethan,
>
>Ethan Dicks wrote:
>>
>> It is a point-to-point RS-485 link, in this particular installation, not
>> multi-drop, fortunately.
>>
>> I presume I would just tie the enable pins to "true"
>>
>> The RS-485 device in question is a simple 4-wire transmit/receive pair.
>>
>
>I don't understand why you need a converter anyway. RS422 and RS485 use
>the same voltage levels AFAIK. RS422 uses two pairs of wires: one for
>sending and one for receiving. Devices are daisy-chained. RS485 uses one
>pair of wires, for both sending and receiving. Some RS485 devices have
>seperate terminals for the sending and receiving wires, so they are in
>fact RS422 devices. Normally those wires are connected in parallel, so
>that the devices also receives what it sends.
>
>There is basically one situation that has to be avoided and that is two
>transmitters connected at the same time. If you say that you RS485
>device has 4 wires, I'd guess it has 2 for sending and 2 for receiving,
>so it would simply connect easy to the RS422 device.
>
>BTW, a common mistake is that people think that since RS422/RS485 are
>differential busses, the ground does not have to be connected. I once
>red a very good article on this topic that explains that the grounds of
>all devices should _always_ be connected.
>
>Correct me if I'm wrong.

Hi
 Yes, the ground/commons do need to be connected. The receivers
only have so much common range that they work over. These
are more tolerant of ground noise but they have limits just
as well as any solid state device does. You absolutely need
to have a reasonably good common ground. As I recall, they
are spec'ed as about 7 volts common noise and have a maximum
of around 15 volts common voltage. With high power devices
or circuits that do are not connected to a common ground,
these levels can be easily exceeded.
 It is best to have a star type ground system to avoid
ground loops.
 If you need true isolation, one should use optical isolators
or transformers.
Dwight

>
>--
>One thing I just wondered: in what time zone do you people at the
>southpole live?
>
>Bert
>
Received on Thu May 27 2004 - 11:29:54 BST