Can't get modbus working on Flexboss21

[michaelahess]

I've wired my rs485 to usb adapter which is plugged into a windows 11 computer.

I have A, B, and Gnd wired up. Here's the topography:

FlexBoss21 Meter port (A+/B-/GND)
│
▼
USB-to-RS485 adapter (Prolific PL2303GC, "ATEN Serial Bridge", VID:PID 067b:23a3)
120Ω termination enabled on the adapter's TX pair
│ USB
▼
Windows PC ("garagesolar", physically near the inverter)

• Adapter enumerates as COM3
• Shared over the network via usbipd-win (VID:PID 067b:23a3, busid 1-4)
  │ USB/IP over LAN (TCP 3240)
  ▼
  Stargate (Ubuntu host running KVM/libvirt, separate physical machine)
• Linux usbip client (vhci-hcd) attaches the shared device,
  making it a local USB device on this host
• libvirt USB hostdev passthrough (virsh attach-device) hotplugs
  that same USB device directly into a VM
  │ USB passthrough
  ▼
  "hass" VM (KVM guest running Home Assistant OS)
• Device appears inside HAOS as /dev/ttyUSB2 (by-id:
  usb-Prolific_Technology_Inc._USB-Serial_Controller_CPATb11CN12-if00-port0)
• eg4_web_monitor integration configured: Local Device → Serial (USB/RS485),
  port = that device, baud 19200 / none / 8 / 1, unit ID 1

I have tested from the windows machine directly, and can't get much of anything from the inverter. Here's a summary of what I've gotten out of it:

FlexBoss21: Measurement = CT (what I expect to use long-term, no physical meter, GridBoss handles real CT metering):

Single query (addr 1, func 03/holding registers, 19200 8N1): timeout, zero bytes returned
Same query with DTR/RTS explicitly asserted on the serial port: timeout, zero bytes
Passive listen, 20 seconds, sending nothing at all: zero bytes received
Full sweep — addresses 1 through 20 × function codes 03 and 04 (40 combinations total): zero responses to every single one
Measurement = 0:Meter (Meter Type set to Eastron, no physical meter actually connected):

Passive listen, sending nothing, the inverter transmitted this on its own, repeatedly, cycling through registers:

01 04 00 18 00 02 F1 CC
01 04 00 0C 00 02 B1 C8
01 04 00 1E 00 02 11 CD
01 04 00 46 00 02 90 1E
01 04 00 00 00 02 71 CB
01 04 00 06 00 02 91 CA
01 04 00 12 00 02 D1 CE
01 04 00 48 00 02 F1 DD
01 04 00 4A 00 02 50 1D

Each of those is a valid Modbus RTU frame with a correct CRC — unit address 1, function 04 (read input registers), cycling through different register addresses, 2 registers at a time. This is the inverter acting as a master, itself polling for its configured Eastron meter — not answering as a slave to anyone.

I've tried with inverter set to ct and meter, and both meter brands.

Anybody have any insight? Help would be greatly appreciated!