Vermes
Advanced Member level 4
Presented converter is based on UCB → RS232 module for Prolific, system model PL-2303HX.
To meet the RS232 standard according to the voltage levels -12V, +12V available in Pcs, 2 max232 systems were used in this device. All the lines except the RING line are converted by two chips and derived on V.24 port. In addition, all input/output lines in TTL standard (the same as the system PL-2303HX gives by default) were derived to the TTL port. The board is also equipped with USB output, from which you can supply the connected device. Prolific module was taken from a phone cable (Siemens SX1).
Scheme is very simple, but you need to use some jumpers on the board. The board was designed in Eagle and made in thermal transfer method.
Link to original thread (useful attachment) – Konwerter USB -> RS232 V.24/TTL