USB to RS-232 adapter design

A USB to RS-232 adapter converts USB packets into a serial UART stream and then changes the logic levels to RS-232 voltages. It has three parts: a USB-UART bridge chip, an RS-232 level translator and the connectors.

Block / circuit diagram

 PC USB-A                                     DB-9 male (DTE)
 VBUS  -----+--> 3V3 regulator (inside chip)
 D+    -------> USB-UART bridge   TXD ----> MAX3243 T1IN -> T1OUT -> TXD (3)
 D-    -------> (FT232R / CP2102  RTS ----> T2IN -> T2OUT -> RTS (7)
 GND   -----+   / PL2303)         DTR ----> T3IN -> T3OUT -> DTR (4)
                                  RXD <---- R1OUT <- R1IN <- RXD (2)
                                  CTS <---- R2OUT <- R2IN <- CTS (8)
                                  DSR <---- R3OUT <- R3IN <- DSR (6)
 MAX3243: VCC 3.3 V, C1+/C1-, C2+/C2-, V+/V- with 0.1 uF capacitors
 (A MAX3232 has only 2 drivers and 2 receivers, so it is enough for TXD, RXD, RTS and CTS only.
  The MAX3243 has 3 drivers and 5 receivers, which also covers DTR, DSR, DCD and RI.)

Components: ferrite bead and 4.7 uF/0.1 uF bypass capacitors on VBUS; 27 ohm series resistors on D+/D- (if not internal); a 12 MHz crystal only for bridge chips without an internal oscillator (for example PL2303HX), because the FT232R and CP2102 have an internal oscillator and need no crystal; EEPROM for VID/PID (optional); TX/RX LEDs.

Voltage translation

• USB: differential D+/D- of 0-3.3 V.
• Bridge UART side: TTL/CMOS levels, 0 V = logic 0 and 3.3 V = logic 1.
• RS-232: logic 1 = -3 to -15 V, logic 0 = +3 to +15 V (inverted). The MAX3232/MAX3243 has a charge pump (four capacitors) that makes about +/-6 V from 3.3 V, and it inverts the signal.

Control signals