Monitoring TX/RX pins without trouble

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upand_at_them

Active Member

• Feb 8, 2015

• #1

I'm building a simple serial monitor (sniffer?) that's comprised of a PIC and an OLED display to monitor the TX or RX line of another project. Do I need to buffer the signal, or will feeding it directly to the appropriate PIC input not cause any trouble?

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Feb 8, 2015

• #2

It shouldn't cause any trouble, it's simply a high impedance CMOS input - a series resistor to limit the voltage (with the internal diodes) is all that's needed.

I've had a number of simple RS232 'monitors' over the years - they connected between the end of the RS232 cable and one of the items, and crudely stick LED's directly across the data and handshakes lines - so apply a HEAVY load to the lines, but always worked perfectly well with them in-circuit.

 

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upand_at_them

Active Member

• Feb 8, 2015

• #3

Oh, I should have pointed out that I'm monitoring the TX/RX lines of another uC project. I was worried about loading those lines, since they're probably very low current.

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Feb 9, 2015

• #4

upand_at_them said:

Oh, I should have pointed out that I'm monitoring the TX/RX lines of another uC project. I was worried about loading those lines, since they're probably very low current.

Click to expand...

Like I said, the input of a PIC is VERY high impedance, and the output of a PIC is quite a LOW impedance - feeding two PIC inputs from one PIC output is of no consequence whatsoever, you could feed thousands.

 

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Pommie

Well-Known Member

Most Helpful Member

• Feb 9, 2015

• #5

If you want to monitor both then you need a pic with two uarts.

Mike.

 

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upand_at_them

Active Member

• Feb 9, 2015

• #6

Great, thanks, Nigel.

Pommie, I'm making two monitors. Displays nowadays are cheap!

 

Mosaic

Well-Known Member

• Feb 11, 2015

• #7

would u be decoding the ascii?

 

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upand_at_them

Active Member

• Feb 13, 2015

• #8

Mosaic said:

would u be decoding the ascii?

Click to expand...

Yes. And putting it on the display.

 

Mosaic

Well-Known Member

• Feb 16, 2015

• #9

Would this be shared?
I have use for such an item.