On Mon, 29 Dec 1997 23:10:59 +0100 Sami Khawam
<a9501901@UNET.UNIVIE.AC.AT> writes:
>Hello,
>How can I detect, in an open-drain line,
>a High signal from a device while the PIC
>is outputing a Low signal?

This is not a conventional application.  Usually every device except one
outputs a high signal, which is essentially an open circuit.  The device
which is currently allowed to send may output a low signal, which all
devices on the line interpret as a low.  The bus acts as a wired AND
gate: the line reads high only if *all* connected devices are outputting
high.  So the PIC is to detect a Low signal from a device while the PIC
is sending a High signal.

If you really need to know if all *other* devices are outputting high
except one, a possible method is to sense current flow from the bus to
the OC pin with an external discrete transistor:

    +5V----|
      |   Rp
     Rp    *-------PIC Sense In
      |   |/c
Bus---*---| NPN
      |   |\e
      |-R1--*------PIC OC In/Out

R1 provides a path to guarantee that OC In reads low when the bus is low.
 It should be about the same resistance as Rp.

If some other device is outputting a low (holding the bus at ground
potential) the transistor will never turn on.  Sense In will read high.
If no other device is outputting a low except the PIC, the transistor
turns on, and Sense In will read low.  Sense In will read high when the
PIC is outputting high in either case.  While the PIC is outputting high,
the OC In/Out pin can be read to determine the level on the bus.

This is a special technique.  Nearly all OC busses run with a protocol
that avoids the need to do this.  This circuit degrades the 'low' voltage
the PIC can send to the bus by the Vbe of the transistor, which may cause
problems.

>Can a pin be read when the TRIS is set to
>output?

Yes.  The value read is always determined by the voltage present at the
pin, regardless of whether the pin is set as input or output.