Jim Ruxton
2018-03-10 17:28:19 UTC
Just curious how others might handle this scenario. I have 2 RS-485
signals. I want to route one of the 2 to a UART pin on a PIC. I am
looking at using 2 MAX485 receivers or something similar. These have RX
enable pins that either output the signal or put the output pin into a
high impedance state. I am wondering about tieing the 2 outputs together
and be sure I only have 1 of the 2 outputs enabled. I am just concerned
that on power up the outputs may not be in a high impedance state and
will fight each other until my PIC is fired up completely. My other
option is to join the 2 outputs with series resistors. Since I am going
to have a single optical isolator after the MAX485s I could tie the
outputs together through current limiting resistors going to the
isolator. Or maybe someone has a better suggestion or knows another
rs-485 IC that does this ie. allows the selection of 1 of multiple
inputs. I did a quick mockup of the 2 scenarios only showing the
relevant bits. I am considering here.
https://drive.google.com/open?id=1im2HMS2sjCmgZup1q0AzfC9YSY-Z1ew2
I would appreciate any thoughts or opinions.
Jim
signals. I want to route one of the 2 to a UART pin on a PIC. I am
looking at using 2 MAX485 receivers or something similar. These have RX
enable pins that either output the signal or put the output pin into a
high impedance state. I am wondering about tieing the 2 outputs together
and be sure I only have 1 of the 2 outputs enabled. I am just concerned
that on power up the outputs may not be in a high impedance state and
will fight each other until my PIC is fired up completely. My other
option is to join the 2 outputs with series resistors. Since I am going
to have a single optical isolator after the MAX485s I could tie the
outputs together through current limiting resistors going to the
isolator. Or maybe someone has a better suggestion or knows another
rs-485 IC that does this ie. allows the selection of 1 of multiple
inputs. I did a quick mockup of the 2 scenarios only showing the
relevant bits. I am considering here.
https://drive.google.com/open?id=1im2HMS2sjCmgZup1q0AzfC9YSY-Z1ew2
I would appreciate any thoughts or opinions.
Jim
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