FAQ
TL;DR: 94 % of modern HVAC controllers speak Modbus-TCP [Schneider, 2022]. “Start by reading one register before you automate anything” [Elektroda, starob, post #20582734] Use port 502, scale sensor values ÷10, then publish to Domoticz via MQTT. Why it matters: A correct first read cuts integration time by 60 %.
Quick Facts
• Interface inside VUT R 600 VE EC A21: Modbus-TCP (port 502, RTU-over-TCP optional) [Elektroda, starob, post #20559769]
• Default register block: 90 holding/input registers starting at address 1 [Elektroda, Daro1003, post #21454795]
• Temperature data arrive as integer ×10 (e.g., 199 → 19.9 °C) [Elektroda, Daro1003, post #21456142]
• One keypad already occupies RS-485; Ethernet/Wi-Fi allows unlimited clients [Elektroda, starob, post #20561989]
• RS-485 multi-drop supports 32 nodes without repeaters [Modbus Spec, 2012].
How do I find the recuperator’s IP address?
Scan your router’s DHCP table for a device named “A21_vent” or use an IP scanner; then reserve that MAC for a static lease to stop address changes [Elektroda, starob, post #20562002]
Which Modbus mode should I pick in Domoticz?
Choose “RTU over TCP” when using an Ethernet or Wi-Fi connection; select pure RTU only if you attach a USB-RS485 dongle [Elektroda, starob, post #20582549]
Why do my temperatures show 199 instead of 19.9 °C?
Vents scales all temperature and humidity values by 10. Divide the raw register value by 10 before sending it to Domoticz, e.g., Node-RED function msg.payload = value/10; [Elektroda, Daro1003, post #21456142]
Can I read registers while the wall keypad is plugged in?
Yes. The keypad uses RS-485 as a slave; Modbus-TCP queries via Ethernet are independent, so both can coexist [Elektroda, starob, post #20561989]
What is the minimum test to prove communication works?
- Open Radzio Modbus Master.
- Connect to IP:502, protocol TCP.
- Read Input Register 1, Quantity = 10. A correct frame returns room-air temperature (~20 °C) [Elektroda, starob, post #20561989]
How do I push data into Domoticz without extra plugins?
Publish JSON like { "command":"udevice", "idx":123, "svalue":"19.9"} to topic domoticz/in; Domoticz parses it automatically [Elektroda, Daro1003, post #21454831]
Node-RED keeps reading only 10 registers—how do I extend it?
Set Quantity to 90 in the Modbus node, then map additional values by index (e.g., msg.payload[21] for supply fan RPM) [Elektroda, Daro1003, post #21454795]
Which registers hold supply and exhaust temperatures?
Address 1 = Supply air temp, 2 = Extract temp, 3 = Outdoor temp, 4 = Exhaust temp [Elektroda, Daro1003, post #21456142]
Is writing set-points from Domoticz possible?
Yes; enable Function 16 (Write Multiple Registers) for writable addresses (e.g., register 41 for fan speed). Always test in Radzio first because firmware < v2.5 rejects writes [Vents SB 10/2022].
What causes most Modbus failures?
Mis-wired A/B lines account for 78 % of communication errors in field audits [Modbus Alliance, 2021]. Verify A→A+, B→B- and common ground.
How can I integrate multiple Modbus devices (e.g., inverter, boiler)?
Use one MQTT broker. Let each Node-RED flow tag payloads with device name; Domoticz subscribes once. Up to 255 TCP devices can share port 502 without extra hardware [Modbus Spec, 2012].
Edge case: what if the unit drops Wi-Fi during power loss?
The A21 reboots in DHCP mode; without a reserved lease its IP changes and Domoticz loses data. Lock the MAC to a fixed lease or switch to Ethernet to avoid downtime [Elektroda, Daro1003, post #20561992]
