I have an oscilloscope PPS10, it works very well on RS232. But on my laptop, no hardware RS232. I tried the adapter USB to COM, but it does not work. This is most likely due to the small current output TxD and RTS. This current is not sufficient to supply the opto-coupler.
I decided to try powered optocouplers from 5V USB. And apply out from the opto-coupler to the RxD PL2303 (TTL level).
In the Internet, I found a schematic diagram of optical isolation in the HPS40: http://www.hardlock.org.ua/hard/hps40_usb/index.html
At first, I tried this pattern of cable.
.--- 510R
.\ / T ---+ +-/\/\/-- +5V
.=== | |
| | R --------------------------+-------- RxD PL2303
.=== |
| | S ---+------------------------------- GND
-----
But it does not work.
I found in my oscilloscope diodes, protect optocoupler against reverse polarity.
These diodes have shifted the voltage at the open transistor at 2 V. This is a bit for RS232, but a lot for TTL.
After that I tried such a scheme.
+------------+-------- +5V
| >
| > 1,2К
| >
| +-------- to RxD PL2303
.--- | 510R |/
.\ / T --------------------------------/\/\/---| npn transistor
.=== | |\
| | R -----------------------------+ e|
.=== |
| | S ------------------------------------------+-------- GND
-----
and it works.