--- title: "USB to RS-422 cable design" subject: "Instrumentation II" chapter: 3 key: "semester_5/instrumentation_ii:usb-rs422-cable" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/usb-rs422-cable site: BCT Engineering --- # USB to RS-422 cable design Instrumentation II · Chapter 3: Serial Interfacing With Microprocessor Based System - Asked in: 2071 Chaitra, 6 marks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/usb-rs422-cable - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3 ## Question Design a cable that has a USB connector at one end and an RS-422 connector at the other end. Assume the USB is connected to a laptop and the RS-422 connector is attached to a printer. Your design should include the following: (i) Technical names of the pins and wires involved in the design. (ii) Intermediate chips to maintain voltage uniformity between the two standards. (iii) Neat and labeled sketch of the wiring between the two standards. ## Short answer A converter in the USB hood uses an FT232R USB-UART bridge and MAX491 full-duplex RS-422 transceiver. USB pins: VBUS, D-, D+, GND. RS-422 DB-9: TX+, TX-, RX+, RX-, GND. The transceiver converts TTL to differential pairs; 120 Ω termination across RX+/RX- ensures signal integrity. ## Answer #### Approach A laptop USB port cannot drive RS-422 directly, so the cable contains a small converter board (USB-UART bridge + RS-422 line driver/receiver), hidden in the USB connector hood. Example design uses an **FT232R** (USB to UART) and a **MAX491 / SN75ALS180** full-duplex RS-422 transceiver. #### (i) Pins and wires | USB-A pin | Name | Wire colour | Use | |---|---|---|---| | 1 | VBUS (+5 V) | Red | Power for converter | | 2 | D- | White | Data - | | 3 | D+ | Green | Data + | | 4 | GND | Black | Ground | | RS-422 DB-9 pin (assumed) | Signal | Direction (cable as DTE) | |---|---|---| | 1 | GND | - | | 2 | TX+ (Y, non-inverting out) | Out to printer RX+ | | 3 | TX- (Z, inverting out) | Out to printer RX- | | 4 | RX+ (A, non-inverting in) | In from printer | | 5 | RX- (B, inverting in) | In from printer | | 9 | Shield | Chassis | RS-422 uses two twisted pairs (one for each direction) and a common ground. #### (ii) Intermediate chips 1. **FT232R USB-UART bridge:** handles the USB protocol and converts it to TTL TXD/RXD (and RTS/CTS); it has an integrated 3.3 V regulator, oscillator and USB resistors. 2. **RS-422 transceiver (MAX491 or SN75174 driver + SN75175 receiver):** converts TTL TXD to a differential pair (TX+/TX-) and the differential RX+/RX- to TTL RXD, with +/-2 to +/-6 V differential levels. For the MAX491 the driver enable DE is tied to VCC and the receiver enable RE' is tied to GND, so the driver and receiver are always on. 3. 120 ohm termination between RX+ and RX- at the receiving end, 0.1 uF bypass capacitors, and a ferrite bead on VBUS. #### (iii) Wiring sketch ```text Laptop Converter inside USB hood Printer USB-A (RS-422 in) VBUS (1) -------> FT232R VCC / MAX491 VCC D- (2) --------> USBDM TXD ---> DI Y (DO+) --- TX+ ---> RX+ D+ (3) --------> USBDP RXD <--- RO Z (DO-) --- TX- ---> RX- GND (4) --------> GND A (RI+) <--- RX+ <--- TX+ B (RI-) <--- RX- <--- TX- DE = VCC, RE' = GND 120 ohm across A/B GND -------------------------- DB-9 pin 1 ------------ GND ``` The cable is a four-wire USB side and a two-twisted-pair plus shield at the printer side. The transceiver is powered from VBUS (5 V) and the FT232R I/O is set to 3.3 V/5 V as required, keeping voltage levels uniform between the two standards. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why can the laptop USB port not drive the RS-422 printer directly? A. Different signalling and protocol, so a USB-UART bridge plus RS-422 transceiver is needed B. RS-422 uses higher voltage than USB can supply only C. The printer has no connector D. USB has no ground Answer: A. USB protocol must be converted to UART by the FT232R and then to differential RS-422 levels by a transceiver. (Key point: Need for converter) 2. What does the FT232R do in the cable? A. Handles USB protocol and converts to TTL TXD/RXD B. Converts TTL to differential pair C. Provides 120 ohm termination D. Supplies shield ground Answer: A. The FT232R is a USB-UART bridge giving TTL serial lines. (Key point: Role of FT232R) 3. How many wires/pairs does RS-422 use for full-duplex data? A. Two twisted pairs, one per direction, plus ground B. One wire with common ground C. Four single-ended wires only D. A single twisted pair for both Answer: A. RS-422 uses a differential pair for transmit and another for receive, plus common ground. (Key point: RS-422 wiring) 4. Where is the 120 ohm resistor placed? A. Across VBUS and GND B. Across RI+ and RI- at the receiving end C. Between TXD and RXD D. Across D+ and D- Answer: B. The termination goes across the differential receiver inputs to match the line. (Key point: Termination) ## 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 - [USB protocols followed in USB design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-design-protocols) — Instrumentation II, chapter 3 - [Technical dilemmas in circuit design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adesign-dilemmas) — Instrumentation II, chapter 7 - [8255 mode 1 keyboard and printer with decoding](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-keyboard-printer-mode1-fig) — Instrumentation II, chapter 2 - [RS-232A criteria for simplex/half/full duplex](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232-simplex-half-full-criteria) — Instrumentation II, chapter 3 - [Keyboard and printer interface in 8255 mode 1](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Akeyboard-printer-mode1) — Instrumentation II, chapter 2 ## Related in other subjects - [Euler's formula: one end fixed, other hinged](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Aeuler-one-fixed-one-hinged) — Strength of Materials, chapter 8, asked 2× - [Intermediate beam design for BM and deflection](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aintermediate-beam-design-for-bm-and) — Design of RCC Structure, chapter 4 - [Lossless one-port properties; Foster II and Cauer II](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Alossless-one-port-foster-cauer-ii) — Filter Design, chapter 4 - [Lossless one-port properties; Cauer I and Foster II](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Alossless-one-port-cauer-foster-ii) — Filter Design, chapter 4