- 2071 Chaitra · 6 marks
USB to RS-422 cable design
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.
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.
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
- 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.
- 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.
- 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
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.
✓ Answer checked by Claude Sonnet 5.5
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