> Have a project that the client wants to replace a RS232 with RS485, prett=
y
> simple other than these two issues:
>=20
> 1. direction control.  Since the RS485 is 3 wire differential, the
> transceivers I have seen have direction control pins.  However what happe=
ns if
> you don't want this...ie...several of the converters Ive seen do not see =
to
> require this for a point to point setup.  I'm looking at one from FTDI an=
d it
> uses a Exar SP481 and the two for direction control are floating.
>=20

1. Use the TX signal to activate the driver in such a way that the driver i=
s on=20
only when the TX is not in its idle state. When TX is in its idle or off st=
ate,=20
the outputs have to be clamped with resistors. Disadvantage: The line is on=
ly=20
actively driven when the TX line is in its active state which can lead to i=
t=20
being more susceptible to noise in the passive state.

2. Use a timer on the passive to active flank of the TX (high to low if no=
=20
RS232 driver) that holds the driver active for one byte time. This has a co=
uple=20
of disadvantages: Works best with fixed baudrate and the driver may not shu=
t=20
off exactly after the last stop bit since there can (and most likely will) =
be=20
more flanks than from the start bit.

3. Use the micro that transmits to control the driver enable signal with a=
=20
dedicated output. Some controllers can actually use one pin for both RX/TX =
and=20
another for driver enable if used for half duplex (Freescale 9S12, for=20
example). Tis is easy for a micro like a PIC where you have full control of=
 the=20
transmitter but it can be hard to do from a PC running Windows (or other no=
=20
real time OS) since there is no way to certainly know when the last bit of =
the=20
last byte has left the transmitter chip.

/Ruben
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

--=20
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