ModbusLite — Modbus TCP for .NET

ModbusLite is a single-class Modbus TCP (MBAP) client for .NET 8, built for HMI tooling, SCADA glue code and test benches — where a compact, auditable implementation beats a 2 MB framework. It is the C# companion to my PLC/Ladder background: the software side of talking to real hardware.

What it does

• FC 1/2/3/4/5/6 — read coils, discrete inputs, holding and input registers; write single coil/register.
• Strict framing — MBAP protocol-id and transaction-id verification, unit-id echo checks, plausibility limits on lengths.
• Real error handling — slave exception frames (FC | 0x80) surface as a typed ModbusException with spec codes decoded (illegal address, illegal value, device busy…).
• Any Stream — NetworkStream in production, MemoryStream in tests; the client never knows the difference.

using var client = new ModbusClient("192.168.1.50");

ushort[] registers = client.ReadHoldingRegisters(start: 107, count: 3);
bool[]   inputs    = client.ReadDiscreteInputs(start: 0, count: 8);

client.WriteSingleCoil(address: 20, on: true);

Engineering notes

Every request/response cycle in the test suite runs against an in-memory slave: a fake Stream that parses client frames exactly like a real device and scripts spec-correct answers. Big-endian everywhere via BinaryPrimitives, coils unpacked LSB-first per the spec, and count limits (1..125 registers, 1..2000 bits) validated before anything hits the wire. 7 xUnit tests, zero network. MIT licensed.