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Phase 3 PR 45 -- DL205 string byte-order quirk (low-byte-first ASCII packing). Adds ModbusStringByteOrder enum {HighByteFirst, LowByteFirst} + StringByteOrder field on ModbusTagDefinition (default HighByteFirst, the standard Modbus convention). DecodeRegister + EncodeRegister String branches now respect per-tag byte order. Under LowByteFirst each register packs the first char in the low byte instead of the high byte -- the AutomationDirect DirectLOGIC DL205/DL260/DL350 family's headline string quirk. Without the flag the driver decodes 'eHllo' garbage from HR[1040..1042] even though wire bytes are identical. Unit tests: String_LowByteFirst_decodes_DL205_packed_Hello (5 chars across 3 regs with nul pad), String_LowByteFirst_decode_truncates_at_first_nul, String_LowByteFirst_encode_round_trips_with_decode (asserts exact DL205-documented byte sequence {0x65,0x48,0x6C,0x6C,0x00,0x6F} + symmetric encode->decode), String_HighByteFirst_and_LowByteFirst_differ_on_same_wire (control: same wire, different flag => different decode). 56/56 Modbus.Tests pass. Integration test: DL205StringQuirkTests.DL205_string_low_byte_first_decodes_Hello_from_HR1040 against the dl205.json pymodbus profile; reads HR[1040..1042] with both flags on the same tag map and asserts LowByteFirst='Hello' + HighByteFirst!='Hello'. Gated on MODBUS_SIM_PROFILE=dl205 since the standard profile doesn't seed HR[1040..1042]. Verified 2/2 integration tests pass against running pymodbus dl205 simulator. Baseline for PR 46 (BCD decoder), PR 47 (V-memory octal helper), PR 48 (CDAB float order), PR 49 (FC03/FC16 per-device caps) -- each lands its own DL205_<behavior> test class in tests/.../DL205/.

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This commit is contained in:
Joseph Doherty
2026-04-18 21:43:32 -04:00
parent 5ee9acb255
commit cd19022d19
4 changed files with 170 additions and 6 deletions
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47
tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusDataTypeTests.cs
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@@ -172,4 +172,51 @@ public sealed class ModbusDataTypeTests
wire[1].ShouldBe((byte)'i');
for (var i = 2; i < 8; i++) wire[i].ShouldBe((byte)0);
}
// --- DL205 low-byte-first strings (AutomationDirect DirectLOGIC quirk) ---
[Fact]
public void String_LowByteFirst_decodes_DL205_packed_Hello()
{
// HR[1040] = 0x6548 (wire BE bytes [0x65, 0x48]) decodes first char from low byte = 'H',
// second from high byte = 'e'. HR[1041] = 0x6C6C → 'l','l'. HR[1042] = 0x006F → 'o', nul.
var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
StringLength: 5, StringByteOrder: ModbusStringByteOrder.LowByteFirst);
var wire = new byte[] { 0x65, 0x48, 0x6C, 0x6C, 0x00, 0x6F };
ModbusDriver.DecodeRegister(wire, tag).ShouldBe("Hello");
}
[Fact]
public void String_LowByteFirst_decode_truncates_at_first_nul()
{
// Low-byte-first with only 2 real chars in register 0 (lo='H', hi='i') and the rest nul.
var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
StringLength: 6, StringByteOrder: ModbusStringByteOrder.LowByteFirst);
var wire = new byte[] { 0x69, 0x48, 0x00, 0x00, 0x00, 0x00 };
ModbusDriver.DecodeRegister(wire, tag).ShouldBe("Hi");
}
[Fact]
public void String_LowByteFirst_encode_round_trips_with_decode()
{
var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
StringLength: 5, StringByteOrder: ModbusStringByteOrder.LowByteFirst);
var wire = ModbusDriver.EncodeRegister("Hello", tag);
// Expect exactly the DL205-documented byte sequence.
wire.ShouldBe(new byte[] { 0x65, 0x48, 0x6C, 0x6C, 0x00, 0x6F });
ModbusDriver.DecodeRegister(wire, tag).ShouldBe("Hello");
}
[Fact]
public void String_HighByteFirst_and_LowByteFirst_differ_on_same_wire()
{
// Same wire buffer, different byte order → first char switches 'H' vs 'e'.
var wire = new byte[] { 0x48, 0x65 };
var hi = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
StringLength: 2, StringByteOrder: ModbusStringByteOrder.HighByteFirst);
var lo = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
StringLength: 2, StringByteOrder: ModbusStringByteOrder.LowByteFirst);
ModbusDriver.DecodeRegister(wire, hi).ShouldBe("He");
ModbusDriver.DecodeRegister(wire, lo).ShouldBe("eH");
}
}