On Mon, 14 Feb 2005, Brent Brown wrote:

> OK, well said. But the common mode voltage on the MAX485 (a very typical
> RS485 transceiver chip) is limited, -7 to +12V. Not sure what the actual
> RS485 spec requires, but suspect it is something similar. Not a big range to
> work with. For two wire "floating" network (where there is no common GND
> at all) you would IDEALLY have some biasing resistors to the local power
> supplies on each node to keep the signal within this range - right? Without
> that, common mode voltage could drift anywhere - possibly outside of the
> specs of the transceiver chips thereby creating potential for communications
> errors (at least) and/or chip damage.

None of the dc coupled protocols can take too much common mode voltage. 
POTS is one of the more robust (you can boost RS232 to POTS voltage 
swing levels and go in excess of 10km at limited baud rate, but 
electrical interference will still bother you a lot, i.e. storms, nearby 
power lines, factory interference etc. The far end of such a loop is 
usually insulated optically or otherwise. Remember how nervous the POTS 
operators are about users connecting *anything* to their POTS system. 
The initial 52V of the POTS system can drop to 1V at the far end!

You can send RS422 and RS485 through pulse transformers is you use bit 
stuffing to prevent dc bias. That fixes the common mode problem.

hope this helps,
Peter
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