---
title: "TTL to RS-232 interfacing circuit"
subject: "Instrumentation II"
chapter: 3
key: "semester_5/instrumentation_ii:ttl-rs232-interface"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/ttl-rs232-interface
site: BCT Engineering
---

# TTL to RS-232 interfacing circuit

Instrumentation II · Chapter 3: Serial Interfacing With Microprocessor Based System

- Asked in: 2079 Baisakh, 3 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/ttl-rs232-interface
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3

## Question

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

## 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.

## Answer

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)
```text
 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).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why can TTL not connect directly to RS-232?
   A. Same levels
   B. RS-232 uses 5 V only
   C. Levels differ and are inverted (RS-232 1 = -3 to -15 V)
   D. TTL is faster
   Answer: C. RS-232 uses bipolar inverted levels, so a driver/receiver level shifter is needed. (Key point: Why needed)

2. How does the MAX232 get about +/-10 V from +5 V?
   A. Two batteries
   B. Zener diode
   C. Transformer
   D. Internal charge pump with external capacitors
   Answer: D. A voltage doubler/inverter charge pump uses four capacitors. (Key point: Charge pump)

3. Which chips were the older driver and receiver pair?
   A. FT232 and CP2102
   B. MC1488 and MC1489
   C. 74373 and 74138
   D. MAX491 and MAX232
   Answer: B. MC1488 is the quad line driver (+/-12 V) and MC1489 the quad receiver. (Key point: Older chips)

## Related in Instrumentation II

- [USB to RS-232 adapter design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-rs232-adapter) — Instrumentation II, chapter 3
- [Connecting two DTE RS-232 devices](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adte-dte-problem) — Instrumentation II, chapter 3, asked 2×
- [Simplex, half and full duplex using RS-232](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232-simplex-half-full) — Instrumentation II, chapter 3, asked 2×
- [Interfacing 1408 DAC for 0 to 10 V](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adac1408-0-10v) — Instrumentation II, chapter 4
- [Importance and working of RS-232C](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232c-importance-signals) — Instrumentation II, chapter 3
- [Interfacing ADC0808 with 8085 using 8255A](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aadc0808-8255-interface) — Instrumentation II, chapter 4

## Related in other subjects

- [Short note: RS-232 standard](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ashort-note-rs232) — Microprocessor, chapter 4, asked 3×
- [RS-232 signals, handshaking, DTE-DCE](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ars232-signals-dte-dce) — Microprocessor, chapter 4, asked 3×
- [Define combinational logic circuit](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Acombinational-circuit-define) — Digital Logic, chapter 3
- [K-map Σ(0,2,3,4,7,8,10,13)+d, logic circuit](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Akmap-logic-circuit-0234) — Digital Logic, chapter 3