Well, levels are now OK.
Here are the adaptations I made to the spy circuit :
- lower the input voltage with resistor dividers. I chose 68K and 22K, giving me a factor of 22/(68+22) ~= 0.25. So input level of 3V gives 0.75V and 12V gives 3V, which is perfectly in the working range of the MAX circuit - see data sheet on
http://pdfserv.maxim-ic.com/en/ds/MAX3222-MAX3241.pdf
- tie pin 1 of each serial port with already connected pins 4 and 6, following the recommandation of the first schema on
http://www.airborn.com.au/rs232.html
- fixed the errors in the wiring of the second serial port
These changes are included in the schema below.
*** EDIT : Schema removed again. Signal needs to be inverted !
Working circuit available in
post 197 ***
Now I have a "perfect" signal. The scope captures below are from pins 11 (T1IN) and 14 (T1OUT) of the MAX3232, respectively. So obviously, I'm in the correct range...
However, on the PC, I cannot get anything in Hyperterminal or MTTTY (sample MSDN application - see
http://msdn.microsoft.com/library/de...sdn_serial.asp )
I see two possible reasons :
- Wrong wiring of serial port
- Loopback handshake is not supported by these apps.
Can someone with experience on serial communications (Putput ?) comment on this ?
Thanks - the mission continues...