AVR-Chat

Hi James,

On Fri, Oct 01, 2004 at 04:54:37PM -0400, James Russo wrote:

> Thanks for the reply. Right now I am stuck with 481 on the board, but it 
> seems like it will meet my requirements.

Just to clarify - the MAVRIC-II that you have uses the MAX487
transceiver, even though the silkscreen says MAX481 (they are pin
compatible).  As the MAXIM datasheets says about the MAX487:

  "The MAX487 feature reduced slew-rate drivers that minimize EMI and
  reduce reflections caused by improperly terminated cables, thus
  allowing error-free data transmissions up to 250 kbps."

I have run them at double that speed without problems, though over
short distances.  And the chip seems pretty tolerant regarding
termination.  Note that for short RS485 runs using these chips, I've
found that the terminator is generally unnecessary.

> I've been reading some more 
> about this throughout the day and it seems that having to bias the bus 
> (using a pullup and pulldown) configuration is quite commonly done.

The datasheet also states:

  "The receiver input has a fail-safe feature that guarantees a
  logic-high output if the input is open circuit."

Thus, when no driver is driving the bus, the state should not
fluctuate.  Even so, supplying the pull-up / pull-down resistors on
the A/B lines might be a good idea and is pretty common in RS485
implementations, especially since you might not be able to guarantee
that every transceiver on the bus has this feature.

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