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Saw wave on my RS232??

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blapcb

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Saw wave on RS232 Tx ?

I am using a MAX3232 for RS232 line driver and basically it works. But, when examining the Tx line with a scope, it is observed that it has the shape of a saw wave with amplitude of about 250-300mV. This happens both at the positive as well as the negative voltages. Voltage to the MAX3232 is 3.3v and all caps are 0.1uF (as per spec).

Why is this? What could be wrong?

Attached is scope image

Thanks
 

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    RS232 Tx line saw.JPG
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hi blapcb

If you are using MAX3232 device it is a spected behaviour.
You can use MAX3233 instead to fix that problem.

+++
 

No

The MAX233 is an advanced version that fix some problems related to MAX232.
One of that is the non negligible ripple at internally generated bias voltage.

+++
 

RS232 interfaces powered by single supply (usually 3.3V or 5V) use charge-pump DC-DC converter to proper supply Tx outputs (output swing no less than +/-5V).
Ripples you seen just correspond to charge-pump operation. To decrease current consumption in MAX3232 the charge-pump operate in "skip" mode.
"Skip" mode means that the charge-pump is in idle mode and output voltages decrease slowly until output voltages drop below defined threshold.
When it happens the charge-pump starts to work and charges output voltages quickly to other higher threshold.

So there is nothing wrong with these ripples. It's a feature of this product which is required in order to decrease current consumption.
It's absolutely normal and does not prevent a proper operation of RS232 interface.

Meanwhile if you use and load more Tx channels it will change a frequency of ripples.
 

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