AVR-Chat

On Sat, Oct 02, 2004 at 01:03:48AM -0400, James Russo wrote:

> My situation is that with the terminators on, it doesn't keep the RO
> high. With them off it does. I have put pull-up/pull-down resistors
> and activated the terminators and all seems well.

This makes sense, actually.  The terminator tends to pull the A & B
lines together, with regard to their potential difference, and RS485
sensitivity is usally around 200mV difference between A & B.  Thus,
with the terminator off, that would be an open circuit, and to the
MAX487, that is a known state.  Howver, with the terminator on, but no
balancing resistors to help seperate A & B in the case of no active
transmitter, that would put the line in an unknown state, i.e., < 200
mV between A & B.

> Seems like according to the datasheet this wouldn't be necessary. Page
> 10 of the data sheet (the table on the right) shows that the RO should
> be "1" when the inputs are open. What would be the electrical definition
> of "inputs open" be?

I believe that would be with no terminator, or the transceiver
completely disconnected from the bus.

Note that for short runs, the terminator is probably not necessary.
For longer runs, you should be careful about the ground connections,
as well as using a twisted pair, and the balancing resistors.  In
general, the longer the run, the more care you need to take wiring the
bus.

> So, I would think that using these two boards together, I shouldn't
> need the pullup/down since it has this "fail-safe" feature. It's
> kind of a non-issue for now since I will just use the pull-up/pull
> down, but would just like to know why chip isn't working as
> advertised, even if only for my own enlightment.

I think the only time the pull-up/pull-down resistors aren't necessary
is when there is no terminator, and you might do that for short cable
lengths.  And even so, if your software is tolerant of a missing byte
or two at the beginning or end of a transmission, i.e., using a packet
protocol with a few bytes of leadin (like ROBIN - see
http://www.bdmicro.com/code/robin), you might not even notice these
"glitches".

-Brian
-- 
Brian Dean
BDMICRO - ATmega128 Based MAVRIC Controllers
http://www.bdmicro.com/