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TTL to RS-232 interfacing circuit

Short answer

A MAX232 IC converts TTL (0V/5V) to RS-232 levels (space +8V, mark -8V) using an internal charge pump to generate ±10V from a +5V supply. The driver inverts the signal; a line receiver converts back to TTL.

Show the interfacing circuit of TTL logic with RS 232, appropriate line drivers and line receivers.

TTL levels (0 V = 0, +5 V = 1) are not compatible with RS-232C levels (logic 1 = -3 to -15 V, logic 0 = +3 to +15 V, inverted). A line driver converts TTL to RS-232 and a line receiver converts RS-232 back to TTL.

Using MAX232 (single +5 V supply)

 TTL side                  MAX232                 RS-232 side
 TXD (UART/uP) ---------> T1IN --[driver]-> T1OUT ------> TXD (to PC pin 3)
 RTS ------------------> T2IN --[driver]-> T2OUT ------> RTS
 RXD <------------------- R1OUT <-[receiver]- R1IN <------ RXD
 CTS <------------------- R2OUT <-[receiver]- R2IN <------ CTS
          +5V -> VCC           C1 (1 uF) between C1+ and C1-
          GND                  C2 (1 uF) between C2+ and C2-
                               C3 from V+ to Vcc (or GND), C4 from V- to GND
  • An internal charge pump (voltage doubler and inverter, 4 capacitors) makes about +10 V and -10 V from +5 V.
  • The driver inverts and level shifts: 0 V (TTL 0) -> +8 V (RS-232 space), 5 V (TTL 1) -> -8 V (mark).
  • The receiver has a 5 kohm input resistance with hysteresis and gives TTL levels.

Older arrangement: MC1488 quad line driver (needs +12 V and -12 V supply) and MC1489 quad line receiver (+5 V).

✓ Answer checked by Claude Sonnet 5.5

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