On Mon, 28 Feb 2000, Edson Brusque wrote:

> Hello,
>
>     I'm using a DS75176 to transform between RS485 to TTL, but now I want to
> optoisolate it.

At what point do you wish to apply the isolation barrier? I'm assuming
you'll want to place the barrier in the TTL interface -right? Otherwise,
if you place it in the 485 interface then you no longer have 485! You only
need three optocouplers in the ttl interface 1 for the receive, 1 for the
tx and 1 for the tx enable. The other issue is powering the isolated
RS485 drivers. But the answers depend on your system. For example, if your
nodes are relatively close, then you may wish to run a 4-wire cable - 2
for power and 2 for the differential data bus. One isolated supply can
provide power for the whole bus (and also provide a convenient location
for the termination). If your nodes are far apart, then you'll most
probably want to run just the differential bus and have separate supplies
for each node (or perhaps clusters of nodes).

There are dozens of optocouplers to choose from. The exact choice depends
on your requirements. If you need fast data rates, then you'll need fast
optocouplers... You may want to place the TX and TXen into a single dip.
But the TX EN doesn't have to be as fast as the TX (unless perhaps you
implement the arbitration scheme BAJ and I were discussing the other day -
then the TX EN can be used to generate a precisely timed arbitration
edge).

>     Someone here have some ideas of how can I make it?

Not without details. What are you trying to accomplish. How many nodes?
How far apart? What data rates? Cost? etc...

Scott