feat(modbus-rtu): add ModbusCrc CRC-16/MODBUS helper + golden vectors

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW
2026-07-24 13:30:05 -04:00
parent e787aa572b
commit 8d0f60ec51
2 changed files with 75 additions and 0 deletions
@@ -0,0 +1,27 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+public sealed class ModbusCrcTests
+{
+    // Known CRC-16/MODBUS vectors (init 0xFFFF, poly 0xA001). CRC returned as ushort;
+    // on the wire it is appended low-byte-first.
+    [Theory]
+    // FC03 read HR: unit 1, addr 0, qty 1  -> CRC 0x0A84  (bytes 84 0A on the wire)
+    [InlineData(new byte[] { 0x01, 0x03, 0x00, 0x00, 0x00, 0x01 }, (ushort)0x0A84)]
+    // "123456789" canonical check value for CRC-16/MODBUS = 0x4B37
+    [InlineData(new byte[] { 0x31,0x32,0x33,0x34,0x35,0x36,0x37,0x38,0x39 }, (ushort)0x4B37)]
+    public void Compute_matches_known_vectors(byte[] frame, ushort expected)
+        => ModbusCrc.Compute(frame).ShouldBe(expected);
+
+    [Fact]
+    public void AppendLowByteFirst_writes_lo_then_hi()
+    {
+        var body = new byte[] { 0x01, 0x03, 0x00, 0x00, 0x00, 0x01 };
+        var withCrc = ModbusCrc.Append(body);
+        withCrc.Length.ShouldBe(body.Length + 2);
+        withCrc[^2].ShouldBe((byte)0x84); // CRC low byte
+        withCrc[^1].ShouldBe((byte)0x0A); // CRC high byte
+    }
+}