RS422/RS485 Shield

The RS422/RS485 Shield gives the OpenMV Cam a long-distance differential serial link suited to industrial buses, with wide-input power, surge protection, and ADC/digital I/O.

RS422/RS485 Shield

For full datasheet, photos, and ordering see the RS422/RS485 Shield product page.

Highlights

  • 10 Mb/s RS-422 or RS-485 with on-board termination

  • 6-36 V input, reverse-voltage tolerant

  • 0-5 V ADC input with ±36 V overvoltage protection

  • 0-5 V digital I/O for camera-sync triggers, short-circuit protected

Pinout

RS422/RS485 Shield Pinout

Pin reference

Pin

Function

P4

RS-422 / RS-485 TX → drives the differential line out

P5

RS-422 / RS-485 RX ← receives differential line in

P6

Level-shifted AIN readback (0–3.3 V on P6)

P10

SYN — open-drain digital I/O on the terminal block

PWR in

6–36 V wide input on the terminal block (reverse-voltage tolerant)

AIN in

Analog input on the terminal block

VIN out

5.4 V at up to 600 mA from the on-board regulator

3.3V rail

Powers the shield’s on-board electronics

GND rail

Common ground

Note

AIN is overvoltage-protected up to ±36 V and defaults to a 0–5 V voltage input, scaled down to 0–3.3 V on P6. Bridge the 4–20 mA mode shunt on the front of the shield to switch AIN to a 4–20 mA current-loop input.

Note

SYN is an open-drain digital line, pulled up to 3.3 V on the camera side and 5 V on the SYN terminal side. By default it’s an input — the shield level-shifts 0–5 V on SYN down to 0–3.3 V on P10. Change the on-board solder jumper to flip P10 into an output, level-shifting 0–3.3 V on P10 up to 0–5 V on SYN.

Note

Each of P4, P5, P6, and P10 is connected to the camera by default through a solder jumper — open the jumper on any pin you want to reclaim for unrelated use. P6’s jumper is on the back of the shield; P4, P5, and P10 are on the front.

Note

The on-board termination resistors are connected by default — open the corresponding back-side solder jumpers to disconnect them. Two cover the RS-422 A/B pair and two cover the RS-422 Y/Z pair (which doubles as the RS-485 A/B termination), four jumpers in total.

About RS-422 and RS-485

Both standards send serial data as a balanced (differential) signal over twisted pairs for long-distance, noise-tolerant links:

  • RS-422 is full-duplex over four wires. A driver transmits on a dedicated TX pair labeled Y/Z, and the peer transmits back on a separate RX pair labeled A/B. One transmitter and up to ten receivers per pair.

  • RS-485 is typically half-duplex over two wires. Transmit and receive share a single pair, called A/B in RS-485 terminology but physically the same Y/Z lines on this shield. Up to thirty-two nodes can share the bus and any of them can drive it.

How the shield supports both

The shield carries two THVD1426 transceivers, each able to handle either standard:

  • The first transceiver drives the Y/Z pair (which doubles as the RS-485 A/B pair). It is the only one with its driver hooked up, so all outbound traffic from the camera goes out this pair regardless of mode.

  • The second transceiver drives the A/B pair. Its driver is tied off — this transceiver is receive-only and only matters in 4-wire RS-422 mode.

Both transceivers’ receivers are always enabled, and their RX outputs are AND’d together onto a single receive line back to the camera:

  • In 2-wire RS-485 mode, only the first transceiver is active. Wire the bus to Y/Z; the A/B side sits idle and the AND gate just passes the first transceiver’s RX through.

  • In 4-wire RS-422 mode, the peer transmits to the camera on the A/B pair (picked up by the second transceiver) while the camera transmits on Y/Z (with the first transceiver’s own receiver echoing its outgoing data back). The AND gate combines them — whichever pair sees a low pulse (start bit, data) reaches the camera.

The terminal-block labels reflect the dual mapping:

  • RS-422 (4-wire) — TX out on Y/Z, RX in on A/B.

  • RS-485 (2-wire) — TX/RX share the Y/Z pair (= A/B in RS-485 nomenclature). Leave the A/B terminals on the shield unconnected.

Usage

Talk to a differential serial peer on P4 (TX) / P5 (RX):

from machine import UART

uart = UART(3, baudrate=115200)
uart.write("hello\n")
print(uart.read())

Read the AIN terminal-block input through the level-shifted P6 pin:

from machine import ADC
import time

ain = ADC("P6")

while True:
    v = ain.read_u16() * 3.3 / 65535
    print("AIN:", v * (5.0 / 3.3), "V")
    time.sleep_ms(100)

React to a falling edge on the SYN line — for example, to sync the camera with another device pulling SYN low:

from machine import Pin

def on_sync(pin):
    print("SYN falling edge")

syn = Pin("P10", Pin.IN)
syn.irq(on_sync, Pin.IRQ_FALLING)