Isolating serial noise

C

Chris

Guest
Can i isolate my pic from my pc's noise by using 2 optocouplers and
powering the serial comms from the serial port as in the schematic?

http://chrispol.ath.cx:8080/serial.jpg

Thanks for any input / pointers
 
Chris wrote:
Can i isolate my pic from my pc's noise by using 2 optocouplers and
powering the serial comms from the serial port as in the schematic?

http://chrispol.ath.cx:8080/serial.jpg

Thanks for any input / pointers
Click to expand...
You can't draw enough current from a PC serial port to power a MAX232 -
typical MAX232 power consumption is 8mA.

MAX232 datasheet:
http://info.hobbyengineering.com/specs/TI-MAX232.pdf

Good article about taking power from the serial port:
http://www.hut.fi/Misc/Electronics/circuits/rspower.html

Why don't you forget about the opto-isolators and just power the MAX232
from the same supply as the PIC?

The PIC should not be disturbed by noise from the PC; and any
noise-sensitive circuitry will likely pickup noise from the PIC too!
 
Andrew Holme wrote:

Chris wrote:

Can i isolate my pic from my pc's noise by using 2 optocouplers and
powering the serial comms from the serial port as in the schematic?

http://chrispol.ath.cx:8080/serial.jpg

Thanks for any input / pointers


You can't draw enough current from a PC serial port to power a MAX232 -
typical MAX232 power consumption is 8mA.

MAX232 datasheet:
http://info.hobbyengineering.com/specs/TI-MAX232.pdf

Good article about taking power from the serial port:
http://www.hut.fi/Misc/Electronics/circuits/rspower.html

Why don't you forget about the opto-isolators and just power the MAX232
from the same supply as the PIC?

The PIC should not be disturbed by noise from the PC; and any
noise-sensitive circuitry will likely pickup noise from the PIC too!

yep thats the article i followd, the pic a/d does pickup noise when
Click to expand...
plugged into the serial port, verified by readings and a scope, all ic's
have filtering caps ad is also filtered, if i unplug the serial gnd from
the pic board the readings are clean as soon as i plug it back in i see
a wave......

unless is there a way to split off my pic power supply for that circuit
instead of the serail bu that would not cause noise??
 
Chris wrote:
Andrew Holme wrote:

Chris wrote:

Can i isolate my pic from my pc's noise by using 2 optocouplers and
powering the serial comms from the serial port as in the schematic?

http://chrispol.ath.cx:8080/serial.jpg

Thanks for any input / pointers


You can't draw enough current from a PC serial port to power a
MAX232 -
typical MAX232 power consumption is 8mA.

MAX232 datasheet:
http://info.hobbyengineering.com/specs/TI-MAX232.pdf

Good article about taking power from the serial port:
http://www.hut.fi/Misc/Electronics/circuits/rspower.html

Why don't you forget about the opto-isolators and just power the
MAX232
from the same supply as the PIC?

The PIC should not be disturbed by noise from the PC; and any
noise-sensitive circuitry will likely pickup noise from the PIC
too!

yep thats the article i followd, the pic a/d does pickup noise when
plugged into the serial port, verified by readings and a scope, all
ic's
have filtering caps ad is also filtered, if i unplug the serial gnd
from
the pic board the readings are clean as soon as i plug it back in i
see
a wave......
Click to expand...
How many milli-volts are we talking about? I wonder if you have a
ground-loop problem? Check your earthing.

unless is there a way to split off my pic power supply for that
circuit
instead of the serail bu that would not cause noise??
Click to expand...
Could you post a schematic and/or photo of your layout?

You might be able to do this without the MAX232: drive the opto LED,
via a series current-limiting resistor, directly from the PC serial
port. Going the other way, if you can independently control DTR and
RTS, you could do this:

DTR (+12V)------[1K]------[opto-transistor]-----RTS (-12V)
Connect RxD to the junction of the 1k resistor and the transistor.
 
yep i do have a ground loop problem, this was the solution i was
thinking about to solve it.....

the max setup is currently like

http://www.ac.ugal.ro/staff/ckiku/software/max232.gif

is there another way to get rid of the ground loop?

Chris


Andrew Holme wrote:

Chris wrote:

Andrew Holme wrote:


Chris wrote:


Can i isolate my pic from my pc's noise by using 2 optocouplers and
powering the serial comms from the serial port as in the schematic?

http://chrispol.ath.cx:8080/serial.jpg

Thanks for any input / pointers


You can't draw enough current from a PC serial port to power a

MAX232 -

typical MAX232 power consumption is 8mA.

MAX232 datasheet:
http://info.hobbyengineering.com/specs/TI-MAX232.pdf

Good article about taking power from the serial port:
http://www.hut.fi/Misc/Electronics/circuits/rspower.html

Why don't you forget about the opto-isolators and just power the

MAX232

from the same supply as the PIC?

The PIC should not be disturbed by noise from the PC; and any
noise-sensitive circuitry will likely pickup noise from the PIC

too!

yep thats the article i followd, the pic a/d does pickup noise when
plugged into the serial port, verified by readings and a scope, all

ic's

have filtering caps ad is also filtered, if i unplug the serial gnd

from

the pic board the readings are clean as soon as i plug it back in i

see

a wave......


How many milli-volts are we talking about? I wonder if you have a
ground-loop problem? Check your earthing.


unless is there a way to split off my pic power supply for that

circuit

instead of the serail bu that would not cause noise??


Could you post a schematic and/or photo of your layout?

You might be able to do this without the MAX232: drive the opto LED,
via a series current-limiting resistor, directly from the PC serial
port. Going the other way, if you can independently control DTR and
RTS, you could do this:

DTR (+12V)------[1K]------[opto-transistor]-----RTS (-12V)
Connect RxD to the junction of the 1k resistor and the transistor.
Click to expand...
 
Chris wrote:
yep i do have a ground loop problem, this was the solution i was
thinking about to solve it.....

the max setup is currently like

http://www.ac.ugal.ro/staff/ckiku/software/max232.gif

is there another way to get rid of the ground loop?
Click to expand...
Google for "ground loop."

You could possibly place a "star point earth" at the PIC GND terminal.

Do you get noise if you only connect the GND wire and leave the other
RS232 signals disconnected?

You mentioned them in your original post, but you don't show any power
supply decoupling caps in that circuit.

A photo might be more helpful since a schematic doesn't show if you
have a layout problem.
What is the amplitude of the noise at the PIC A/D input?
 
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