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.

.. image:: ../rs422-rs485-shield-hero.jpg
    :alt: RS422/RS485 Shield
    :width: 400px
    :align: center

For full datasheet, photos, and ordering see the
`RS422/RS485 Shield product page <https://openmv.io/products/rs422-rs485-shield>`_.

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
------

.. image:: ../pinout-rs422-rs485-shield.png
    :alt: RS422/RS485 Shield Pinout
    :width: 700px

Pin reference
-------------

.. csv-table::
   :header: "Pin", "Function"
   :widths: 20, 80

   "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.

.. rubric:: 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.

.. rubric:: 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
-----

.. note::

   The ``UART(3)`` peripheral number below follows the STM32 mapping.
   On another processor the bus wired to these pins may be different
   — check your board's reference.

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)