USB cable for PPS10


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:
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.


I’m really interested in this cable modify.
i’d like to make it but I’m not sure about the left side of the circuit you drew…
is it the 3.5mm jack?
what do the letters mean?
510R is a 510 ohm resistor?
sorry for the questions but I’m afraid of burning my pps 10!
i’d like to keep the original cable as well as the usb to rs 232 cable I bought, i want to insert this circuit between them
thanks a lot


I know this thread is quite old, but I actually built the circuit proposed by Good Loki today, but had no success.

I used a BC547 transistor. I don’t know if it is appropriate for such a circuit, but since there was no information given about which transistor Good Loki used (except that it was a npn), I was left guessing.

If anyone has any further suggestions on how to get the PPS10 PC software to read from a USB-to-Serial converter, I would be very grateful.



Apart from differences in voltage level, there can also be differences in timing when devices use more then just communication (like directly “playing” with the control lines). This is also a reason why in some special cases a serial to USB-converter does not work.
The BC547 should do normally as it is one of the most popular NPN transistors in Europe.