Vaisala PTB220 - Manual (Page 88)

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The next picture (below) shows a typical RS232C TX output signal
(upper signal) and TTL level serial TXD output signal (lower signal)
at baud rate 9600. The vertical scale is 5V/div for upper signal and
2V/div for lower signal. The ground level is shown with a small arrow
at the right. At the left, the signals are at rest.
The next picture (below) shows a typical RS232C TX output signal
(upper signal) and TTL level serial TXD inverted output signal (lower
signal) at baud rate 9600. The vertical scale is 5V/div for upper signal
and 2V/div for lower signal. The ground level is shown with a small
arrow at the right. At the left, the signals are at rest.