Wire and Verify¶

Connect the flashed Radish node to the CT-485 data pair and confirm raw frames on MQTT. Complete Software Setup first — Wi‑Fi, MQTT, and basic device checks should already look good.

Preconditions¶

• ESP32 online in ESPHome / Home Assistant
• MQTT broker reachable (a test subscribe may still be empty until the bus is wired)
• MCU + RS-485 transceiver assembled per Hardware Setup
• AutoNet left off if you are on radish2.yaml (sniff-only until this step succeeds)

Wiring and safety¶

CT-485 is a two-wire differential data bus. Radish only needs that data pair — not thermostat power.

1. Pick an access point: thermostat backplate, air handler board, an existing splice, etc.
2. Turn off power to the HVAC equipment before connecting anything.
3. Connect RS-485 A/B only to the HVAC data pair (often labeled Data 1/Data 2 and/or colored green/yellow). Confirm against actual equipment connections, as wires may be mislabeled/miscolored.
4. Do not connect the RS-485 data leads to thermostat power rails (R/C) or other 24 VAC terminals — data pair only.
5. Double-check polarity and grounding before power-up. Prefer isolated RS-485 if unsure about grounds.
6. Power the ESP32/transceiver from USB or a dedicated 5V/12V supply for bring-up. R/C→DC is possible but can disturb the bus (a 24T1812 converter caused communication faults in testing); see Hardware Setup — Power.
7. Restore HVAC power. If the system reports communication faults after reconnecting, disconnect immediately and inspect for shorts, reversed polarity, ground loops, or a noisy R/C-derived supply.

Verify the link¶

With the node online and wired to the bus, subscribe to raw frames:

mosquitto_sub -h <broker> -u <user> -P <pass> -t radish/rs485/raw

You should see spaced uppercase hex arriving as the network talks. Traffic volume depends on the system; a quiet bus may only publish occasionally.

Good polarity¶

Valid CT-485 chunks are structured frames (addresses, message type, payload, checksum) — mixed bytes the Python decoder can parse, not long runs of nearly identical high values. Example:

FF 01 03 00 00 00 01 F7 00 12 01 03 00 00 09 01 02 03 04 05 01 6F A0 B9 53 CF AC 4D 82 6B 
01 FF 03 00 00 00 A5 00 80 11 00 00 00 09 19 02 0A 40 04 03 F7 DA E2 A2 FF 94 1A B4 E8 
FF 01 03 02 03 00 01 41 00 06 01 06 00 00 00 F0 55 B6 
01 FF 03 02 03 00 A5 41 80 11 06 00 00 09 19 02 0A 40 04 03 F7 DA E2 A2 FF 94 1A 73 DD 
01 FF 03 02 03 00 03 C1 00 73 01 06 00 00 00 F0 B6 53 45 54 55 50 32 B7 C2 E1 47 41 53 20 4C 45 41 4B 20 46 55 4E E0 4F 46 46 E8 4F 4E C3 C2 E1 4C 45 41 4B 20 46 41 4E 20 54 41 50 E0 4C 4C E8 4C E0 4D E0 48 C3 C2 E1 4C 45 41 4B 20 4F 50 20 54 45 53 54 E8 4F 46 46 E0 4F 4E C3 C2 E1 47 41 53 20 53 41 4D 50 4C 45 20 44 41 54 41 20 28 74 69 6D 65 73 29 E0 4C E8 4D E0 48 C3 A7 84 F3 
FF 01 03 02 03 00 01 C1 80 11 06 00 00 09 35 36 66 30 61 01 6F A0 B9 53 CF AC 4D 1A 83 
00 FF 03 00 00 00 A5 77 00 01 00 A7 8C 
02 FF 02 00 00 00 A5 00 80 11 00 00 00 09 19 02 0A 40 04 03 F7 DA E2 A2 FF 94 1A B2 EA 
FF 02 02 02 03 00 05 02 20 00 DD 47 

(53 45 54 55 50 32 / 47 41 53 20 4C 45 41 4B are ASCII like SETUP2 / GAS LEAK inside a menu payload — another sign the capture is real frames, not bit salad.)

Reversed A/B¶

If Data 1/2 polarity is reversed, captures look inverted/noisy: dense FE/FF runs mixed with other high bytes, and the decoder will not produce clean frames:

6A 34 7E FA FE 12 9A 02 7E FE FE 6A 98 FE 02 F2 FA EA FE FE FE FC FE FE …
00 00 7F 7E FF FF FD 13 81 F9 FD F5 FF FF FF FF FF FF FF FF FF FF …

Fix by swapping the physical Data 1 / Data 2 wires (or the RS-485 A/B leads), then power-cycle if the UART stays quiet after the swap.

Do not treat UART tx_pin / rx_pin swaps as a polarity fix. Those pins must match the transceiver (ESP RX ← RO, ESP TX → DI). Wiring them backwards can silence RX or lock up the UART until you restore the correct mapping and power-cycle.

Next steps¶

Once raw hex looks sane, run the Python decoder to pretty-print frames. Optional on-device join is covered in the ESPHome Component guide — keep AutoNet disabled until sniffing is solid.

Official Specification¶

See CT-485 Physical Specification for official electrical details.