--- title: "USB to RS-232 adapter design" subject: "Instrumentation II" chapter: 3 key: "semester_5/instrumentation_ii:usb-rs232-adapter" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/usb-rs232-adapter site: BCT Engineering --- # USB to RS-232 adapter design Instrumentation II · Chapter 3: Serial Interfacing With Microprocessor Based System - Asked in: 2074 Asoj, 6 marks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/usb-rs232-adapter - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3 ## Question Explain the design of a USB to RS-232 adapter with the aid of a neat circuit diagram, appropriate voltage translation chips, and necessary handshake/control signals. ## Short answer A USB to RS-232 adapter uses an FT232R USB-UART bridge chip to convert USB packets to TTL UART, then a MAX3243 level translator with charge pump to generate ±6 V RS-232 signals and invert them. Handshake signals (RTS, CTS, DTR, DSR) are supported via the bridge and driver chips. ## Answer 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 ```text 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 | Signal | Direction (adapter as DTE) | Use | |---|---|---| | TXD, RXD | out, in | Data | | RTS, CTS | out, in | Hardware flow control | | DTR, DSR | out, in | Device ready | | DCD, RI | in | Modem status | | GND | - | Return | The bridge chip handles baud rate, data bits, parity and stop bits set by the host driver (virtual COM port), and buffers data in FIFOs. The host sends bulk OUT data to TXD and receives bulk IN data from RXD; modem control lines are set through control transfers. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What are the main parts of a USB to RS-232 adapter? A. USB-UART bridge chip, RS-232 level translator, connectors B. Only a cable C. A CPU and monitor D. Two modems Answer: A. The bridge converts USB to UART levels, and the translator converts to RS-232 voltages. (Key point: Parts) 2. What does the MAX3232 do? A. Makes +/-6 V from 3.3 V with a charge pump and inverts the signal B. Converts USB packets C. Stores data D. Generates the baud clock Answer: A. A charge pump with capacitors gives RS-232 levels and inversion. (Key point: Level translator) 3. What are RS-232 logic levels? A. Logic 1 = +3 to +15 V B. Logic 1 = -3 to -15 V, logic 0 = +3 to +15 V C. 0 and 5 V D. 0 and 3.3 V Answer: B. RS-232 is inverted: 1 is negative, 0 is positive. (Key point: RS-232 logic levels) 4. How does the host send data through the adapter? A. Via bulk OUT to TXD; modem lines via control transfers B. Via isochronous to RTS C. Via SOF D. Directly on D+ Answer: A. Bulk OUT carries TXD data, bulk IN returns RXD data, and control lines are set by control transfers. (Key point: Data path) ## Related in Instrumentation II - [TTL to RS-232 interfacing circuit](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Attl-rs232-interface) — Instrumentation II, chapter 3 - [USB to RS-422 cable design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-rs422-cable) — Instrumentation II, chapter 3 - [USB enumeration; device vs host interface chips](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-enumeration) — Instrumentation II, chapter 3 - [USB interface chips: device vs host](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-interface-chips) — 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× ## Related in other subjects - [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× - [Short note: RS-232 standard](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ashort-note-rs232) — Microprocessor, chapter 4, asked 3× - [Design: 250 m head plant, 66 kV double circuit](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2071-250m-0-92mh) — Power Plant Design, chapter 4, asked 13× - [Types of signals in digital control](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Asignal-types-in-dcs) — Digital Control System, chapter 1