Add memory location support (%MX, %MW, %MD, %ML) to Modbus slave plugin by devin-ai-integration[bot] · Pull Request #63 · Autonomy-Logic/openplc-runtime

devin-ai-integration · 2026-01-06T05:16:36Z

Add memory location support (%MX, %MW, %MD, %ML) to Modbus slave plugin

Summary

This PR adds support for IEC 61131-3 memory locations (%MX, %MW, %MD, %ML) to the Modbus slave plugin, matching the functionality available in OpenPLC v3. The changes include:

Runtime infrastructure:

Added bool_memory[BUFFER_SIZE][8] buffer to image_tables.c/h for %MX support
Updated plugin_driver.c/h to pass bool_memory to plugins
Added setBufferPointers_v4 symbol lookup with bool_memory parameter (falls back to v1 for older programs)
Added -DOPENPLC_V4 flag to compile.sh for conditional compilation in glueVars.c

Modbus plugin enhancements:

New OpenPLCSegmentedCoilsDataBlock class supporting %QX (0 to qx_bits-1) and %MX (qx_bits to qx_bits+mx_bits-1)
New OpenPLCSegmentedHoldingRegistersDataBlock class with address segmentation matching v3:
- 0-1023: %QW (int_output)
- 1024-2047: %MW (int_memory)
- 2048-4095: %MD (dint_memory, 2 registers per 32-bit value)
- 4096-8191: %ML (lint_memory, 4 registers per 64-bit value)
Word-order handling for 32-bit and 64-bit values (big-endian/high-word-first by default)
parse_buffer_mapping_config() for backward-compatible JSON config parsing

Python struct updates:

Added bool_memory field to PluginRuntimeArgs ctypes struct
Added read_bool_memory/write_bool_memory methods to SafeBufferAccess

Updates since last revision

Copilot review fixes:

Added try/finally blocks around mutex acquire/release to ensure mutex is always released even if an exception occurs
Fixed function signature cast for setBufferPointers_v4 lookup
Improved error message clarity in %MX handling

Review & Testing Checklist for Human

Verify C/Python struct alignment: The bool_memory field in plugin_runtime_args.py must be in the exact same position as in plugin_driver.h. Field order must match: lint_memory -> bool_memory -> mutex_take. Misalignment causes memory corruption.
Rebuild runtime before testing: The C runtime must be rebuilt after pulling these changes. The plugin and runtime are distributed together and must match.
Test Modbus holding register segmentation: Connect a Modbus client and verify that reading/writing to addresses 0-1023 affects %QW, 1024-2047 affects %MW, 2048-4095 affects %MD (32-bit), and 4096-8191 affects %ML (64-bit).
Test word order for 32-bit/64-bit values: Write a known 32-bit value (e.g., 0x12345678) to %MD via Modbus and verify the high word (0x1234) is at the lower address and low word (0x5678) is at the higher address.

Recommended test plan:

Rebuild the runtime with the latest changes
Compile a PLC program with %QW, %MW, %MD, %ML, and %MX variables
Enable Modbus slave plugin with segmented config
Use a Modbus client (e.g., ModbusPoll, pymodbus) to read/write each address range
Verify values match between Modbus and PLC program

Notes

This PR provides the runtime infrastructure for %MX support. A separate PR for xml2st is required to update the glueVars.c.j2 template to actually glue %MX variables using #ifdef OPENPLC_V4. Without that change, %MX addresses will work in Modbus but won't be connected to PLC located variables.

Link to Devin run: https://app.devin.ai/sessions/a39db7c299fb417481ba9800b03995c3
Requested by: Thiago Alves (thiago.alves@autonomylogic.com) / @thiagoralves

- Add bool_memory buffer to C runtime (image_tables.c/h) for %MX support - Update plugin_driver.c/h to pass bool_memory to plugins - Update Python PluginRuntimeArgs struct to match C struct alignment - Add bool_memory support to buffer_types.py and buffer_accessor.py - Add read_bool_memory/write_bool_memory methods to SafeBufferAccess - Implement OpenPLCSegmentedCoilsDataBlock for %QX and %MX address segmentation - Implement OpenPLCSegmentedHoldingRegistersDataBlock for %QW, %MW, %MD, %ML - Add word-order handling (big-endian by default) for 32-bit and 64-bit values - Add parse_buffer_mapping_config() for backward-compatible JSON config parsing - Update init() to read buffer_mapping from JSON and use segmented data blocks - Add -DOPENPLC_V4 flag to compile.sh for conditional compilation - Add setBufferPointers_v4 symbol lookup with bool_memory support JSON config format now supports: - Legacy format: max_coils, max_discrete_inputs, max_holding_registers, max_input_registers - New segmented format: holding_registers.qw_count/mw_count/md_count/ml_count, coils.qx_bits/mx_bits, discrete_inputs.ix_bits, input_registers.iw_count Address mapping (matching v3): - Holding registers: 0-1023 (%QW), 1024-2047 (%MW), 2048-4095 (%MD), 4096-8191 (%ML) - Coils: 0 to qx_bits-1 (%QX), qx_bits to qx_bits+mx_bits-1 (%MX) The runtime now tries setBufferPointers_v4 first (with bool_memory), falling back to setBufferPointers for older programs compiled without -DOPENPLC_V4. Co-Authored-By: Thiago Alves <thiagoralves@gmail.com>

devin-ai-integration · 2026-01-06T05:16:39Z

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Pull request overview

This PR adds support for IEC 61131-3 memory locations (%MX, %MW, %MD, %ML) to the Modbus slave plugin, providing feature parity with OpenPLC v3. The changes introduce a new bool_memory buffer for %MX support across the runtime and enhance the Modbus plugin with segmented data blocks for all memory location types.

Key Changes:

Added runtime infrastructure for bool_memory[BUFFER_SIZE][8] buffer with v4 symbol lookup and backward compatibility
Implemented segmented Modbus data blocks supporting %QX/%MX coils and %QW/%MW/%MD/%ML holding registers with configurable word ordering
Updated Python plugin infrastructure to expose bool_memory through SafeBufferAccess API

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
scripts/compile.sh	Added `-DOPENPLC_V4` flag for conditional compilation of memory location support
core/src/plc_app/image_tables.h	Declared `bool_memory` buffer and `ext_setBufferPointers_v4` function pointer
core/src/plc_app/image_tables.c	Implemented bool_memory buffer initialization and v4/v1 fallback logic
core/src/drivers/plugins/python/shared/safe_buffer_access_refactored.py	Added read_bool_memory/write_bool_memory methods
core/src/drivers/plugins/python/shared/plugin_runtime_args.py	Added bool_memory field to PluginRuntimeArgs ctypes structure
core/src/drivers/plugins/python/shared/buffer_types.py	Added bool_memory mapping to buffer types registry
core/src/drivers/plugins/python/shared/buffer_accessor.py	Added bool_memory field mapping in buffer pointer resolution
core/src/drivers/plugins/python/modbus_slave/simple_modbus.py	Implemented segmented coils and holding registers data blocks with parse_buffer_mapping_config
core/src/drivers/plugin_driver.h	Added bool_memory field to plugin_runtime_args struct
core/src/drivers/plugin_driver.c	Added bool_memory buffer initialization in structured args generation

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Copilot · 2026-01-06T11:54:47Z


+    // Try to load v4 version with bool_memory support (optional - only present in v4 programs)
+    *(void **)(&ext_setBufferPointers_v4) =
+        plugin_manager_get_func(pm, void (*)(unsigned long), "setBufferPointers_v4");


The function signature cast void (*)(unsigned long) is incorrect for setBufferPointers_v4. This function takes 14 pointer parameters, not an unsigned long. This type mismatch could lead to undefined behavior. The cast should match the actual function signature declared in image_tables.h or use a generic function pointer type.

Suggested change

plugin_manager_get_func(pm, void (*)(unsigned long), "setBufferPointers_v4");

plugin_manager_get_func(pm, void (*)(void), "setBufferPointers_v4");

Copilot · 2026-01-06T11:54:47Z

+                if error_msg == "Success":
+                    values.append(1 if value else 0)
+                else:
+                    print(f"[MODBUS] Error reading coil %MX{coil_addr - self.qx_bits}: {error_msg}")


The error message incorrectly references %MX{coil_addr - self.qx_bits} when it should reference the actual MX address, which is mx_offset (or coil_addr - self.qx_bits). The variable coil_addr in the message calculation should consistently use the offset value to match the address space being accessed. Consider using mx_offset directly for clarity.

Copilot · 2026-01-06T11:54:48Z

+                    buffer_idx, bit_idx, bool(value), thread_safe=False
+                )
+                if error_msg != "Success":
+                    print(f"[MODBUS] Error setting coil %MX{coil_addr - self.qx_bits}: {error_msg}")


Same issue as Comment 3 - the error message calculation coil_addr - self.qx_bits should use the mx_offset variable that was already calculated in _get_segment_info for consistency and clarity.

Copilot · 2026-01-06T11:54:48Z

+                "qw_count": min(
+                    hr_config.get("qw_count", DEFAULT_HOLDING_REG_CONFIG["qw_count"]), BUFFER_SIZE
+                ),
+                "mw_count": min(
+                    hr_config.get("mw_count", DEFAULT_HOLDING_REG_CONFIG["mw_count"]), BUFFER_SIZE
+                ),
+                "md_count": min(
+                    hr_config.get("md_count", DEFAULT_HOLDING_REG_CONFIG["md_count"]), BUFFER_SIZE
+                ),
+                "ml_count": min(
+                    hr_config.get("ml_count", DEFAULT_HOLDING_REG_CONFIG["ml_count"]), BUFFER_SIZE
+                ),


The repetitive pattern of min(hr_config.get(...), BUFFER_SIZE) could be extracted into a helper function to reduce duplication and improve maintainability. This same pattern appears multiple times in the function for different configuration sections.

…x function signature cast Co-Authored-By: Thiago Alves <thiagoralves@gmail.com>

devin-ai-integration Bot mentioned this pull request Jan 6, 2026

Add %MX (bool memory) support for OpenPLC v4 Autonomy-Logic/xml2st#23

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thiagoralves requested a review from Copilot January 6, 2026 11:53

Copilot AI reviewed Jan 6, 2026

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devin-ai-integration Bot and others added 2 commits January 6, 2026 12:12

Address Copilot review comments: add try/finally for mutex safety, fi…

90fa360

…x function signature cast Co-Authored-By: Thiago Alves <thiagoralves@gmail.com>

Update image_tables to match function signature

7deda6a

devin-ai-integration Bot force-pushed the devin/1767674901-modbus-slave-memory-support branch from fee1732 to 7deda6a Compare January 6, 2026 14:40

thiagoralves merged commit 822c2ac into development Jan 6, 2026
2 checks passed

thiagoralves deleted the devin/1767674901-modbus-slave-memory-support branch January 6, 2026 16:44

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Add memory location support (%MX, %MW, %MD, %ML) to Modbus slave plugin#63

Add memory location support (%MX, %MW, %MD, %ML) to Modbus slave plugin#63
thiagoralves merged 3 commits into
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	plugin_manager_get_func(pm, void (*)(unsigned long), "setBufferPointers_v4");
	plugin_manager_get_func(pm, void (*)(void), "setBufferPointers_v4");

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Add memory location support (%MX, %MW, %MD, %ML) to Modbus slave plugin

Summary

Updates since last revision

Review & Testing Checklist for Human

Notes

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devin-ai-integration Bot commented Jan 6, 2026 •

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