Resol VBus RevE

Isolated RESOL VBus interface

I designed this board to connect my RESOL DeltaSol CS Plus solar controller to ESPHome and Home Assistant, with hardware for both receiving and transmitting VBus messages.

The board takes a socketed LOLIN D1 mini or LOLIN S3 Mini and runs from 5 V. An isolated DC/DC converter and digital isolator keep the VBus side electrically separate from the ESP and its USB connection.

My assembled prototype connected to the solar controller. This is an earlier revision that needed a small wiring rework; the published design files are Rev E.

Get started

• Build a board: download the Rev E prototype release, then read the fabrication notes and assembly guide.
• Explore the design: open the KiCad 10 project or view the schematic PDF.
• Understand the circuit: see how it works.
• Check the parts: use the BOM for one board and SMT placement file.

Rev E at a glance

• Two layers, 64 x 50 mm, with SMD parts on the front.
• Isolated power, RX and TX, independent of the VBus supply level.
• TX stays off during startup and low isolated-supply voltage.
• Replaceable ESP module and accessible test points.
• JST XH B2B-XH-A, 2.50 mm connectors for both external power and VBus.
• TS391RIYLT receiver with hysteresis and input filtering.

J1 is the 5 V input; J2 is VBus. The connectors are identical, so label the cables. Use either USB power or external 5 V, never both at once. Keep GND and GND_ISO separate. This is a low-voltage interface, not a mains input.

Prototype status

The target controller is a DeltaSol CS Plus running firmware 1.11, with an approximately 20 cm VBus cable.

Rev E is a prototype: its CAD checks pass, but I have not yet assembled or bench-tested this revision. Other controllers, longer cables and the S3 Mini need their own tests. See validation for the checks and remaining limits. The Rev E layout below is a CAD view.

Firmware

VBus uses UART at 9600 baud, 8N1, non-inverted. TX must idle HIGH; driving it LOW enables the bus sink when the isolated supply is ready. Firmware must wait for a bus grant before transmitting.

I am developing the ESPHome TX support separately. This repository contains the hardware, not a ready-to-flash firmware image. It does not imply that the standard ESPHome VBus component already supports transmitting settings.

Rev E layout

Editing the design

Install KiCad 10 or newer with its standard libraries and open the .kicad_pro file. Project libraries use relative paths. The optional RECOM 3D model is not included; a missing model affects the 3D view only. See third-party notices.

The maintenance notes explain test-point exclusions and the checks to run after changes.

Background and license

My original design drew on the ESPHome VBus documentation and FatBeard's VBus PCB project. This is an independent project, without endorsement by RESOL, ESPHome or LOLIN. Their names and trademarks remain theirs.

Original hardware: CERN-OHL-P-2.0; documentation: CC-BY-4.0; scripts: MIT. KiCad library data retains CC-BY-SA-4.0 with the KiCad library exception. See LICENSE.md and third-party notices.