The switched 24 V supply that the Modbus port gives to port-powered devices is shorted, overloaded, or cannot come up, so the controller is holding it off. Port-powered devices on the bus have lost power. Modbus communication keeps running, so externally powered devices keep working.
| At a glance |
|
| Code |
620 |
| Shown as |
Modbus Supply Fault |
| Severity |
Warning |
| Category |
6xx — Hardware |
| Clears |
Automatically, when the bus supply comes back up on a retry |
| LED |
Power light, amber slow blink; Health light shows the warning |
New in an upcoming firmware update. Earlier firmware does not raise this alarm.
- Watches the supply’s fault signal when it switches on and all the time it runs. A fault seen on two checks in a row, a second apart, counts.
- Retries three times, two seconds apart.
- Holds the supply off when those retries fail, and raises this alarm. The Power light shows amber slow blink — see What the LEDs Mean.
- Keeps talking on the bus. The data lines do not need the bus supply, so externally powered devices — such as a NextStep pump — carry on.
- Tries again every 30 seconds. When the supply comes up clean, the alarm clears by itself and port-powered devices start again.
Bus Power stays on; the controller does not change your setting.
Every port-powered device drops out, so expect a Device Disconnected (100) alarm for each, and control modes that use them go to their safe state.
| Cause |
What to look for |
| Short in the bus cable |
24 V and 0 V touching — a crushed cable, a stray strand at a terminal, water in a junction box. |
| Too many port-powered devices |
The bus supply is overloaded. Add up what the devices draw. |
| A failed device |
One device shorting the supply. |
| 24 V wired to the wrong terminals |
A device’s supply landed on the data or ground terminals. |
| Nothing drawing on the supply |
The supply reports a fault when it has nothing connected. If nothing is powered from the bus, turn Settings → Modbus → Bus Power off. |
Before you start: isolate power before working on the bus wiring. See Safety.
- Check the Power light. Red fast blink is a Power Fault (600) — fix input power first. Amber slow blink is this alarm.
- Disconnect the port-powered devices at the controller, then add them back one at a time. The device or cable section that brings the fault back is the problem. The controller retries every 30 seconds, so wait for it after each change.
- Inspect the cable for damage, and each device’s terminals for stray strands or 24 V on the wrong terminal.
- Check the load. If each device works alone but not all together, the bus supply is overloaded: power some devices from their own supply and set them as externally powered — see Port powered or externally powered.
- If the fault persists with nothing connected, contact support.
- Only while Bus Power is on. With Settings → Modbus → Bus Power off, or the source set to External, the controller does not switch the supply on, and this alarm is not raised.
- Not the same as a Power Fault (600). A power fault also switches the bus supply off, but because the controller’s own input is low. While input power is low, this alarm is not raised or cleared.