melsec.h

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// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
/**
 * @file melsec.h
 * @brief Mitsubishi MELSEC MC protocol (binary 3E frame) codec (DETWS_ENABLE_MELSEC) -
 *        zero-heap batch-read request builder + response parser for MELSEC PLCs over TCP/UDP.
 *
 * The QnA-compatible binary 3E request frame (all multi-octet fields LITTLE-endian, unlike
 * the big-endian PLC protocols):
 * @code
 *   50 00            subheader (request)
 *   00               network number
 *   FF               PC (station) number
 *   FF 03            request destination module I/O number (0x03FF)
 *   00               request destination multidrop station
 *   LL LL            request data length (the octets from the monitoring timer on)
 *   TT TT            monitoring timer
 *   01 04            command (0x0401 batch read)
 *   00 00            subcommand (0x0000 word units)
 *   dd dd dd         head device number (3 octets)
 *   CC               device code (D = 0xA8, M = 0x90, ...)
 *   pp pp            number of device points
 * @endcode
 * The response subheader is `D0 00`, followed by the same routing, a 2-octet length, a
 * 2-octet end code (0x0000 = success), then the read data (2 octets per word point).
 *
 * Frame layout + device codes verified against a third-party MC-protocol implementation.
 * This is the codec; the TCP/UDP send is the application's.
 *
 * @author  Douglas Quigg (dstroy0)
 * @date    2026
 */
 
#ifndef DETERMINISTICESPASYNCWEBSERVER_MELSEC_H
#define DETERMINISTICESPASYNCWEBSERVER_MELSEC_H
 
#include "ServerConfig.h"
 
#if DETWS_ENABLE_MELSEC
 
#include <stddef.h>
#include <stdint.h>
 
#define MELSEC_3E_REQ_SUBHEADER0 0x50 ///< request subheader (sent 0x50 then 0x00)
#define MELSEC_3E_REQ_SUBHEADER1 0x00
#define MELSEC_3E_RES_SUBHEADER0 0xD0 ///< response subheader (0xD0 0x00)
#define MELSEC_3E_RES_SUBHEADER1 0x00
 
#define MELSEC_NETWORK_DEFAULT 0x00   ///< local network
#define MELSEC_PC_DEFAULT 0xFF        ///< host station
#define MELSEC_DEST_IO_DEFAULT 0x03FF ///< own station CPU
#define MELSEC_DEST_MULTIDROP_DEFAULT 0x00
 
#define MELSEC_CMD_BATCH_READ 0x0401  ///< batch read
#define MELSEC_CMD_BATCH_WRITE 0x1401 ///< batch write
#define MELSEC_SUBCMD_WORD 0x0000     ///< word units
#define MELSEC_SUBCMD_BIT 0x0001      ///< bit units
 
// Device (area) codes for the binary 3E frame.
#define MELSEC_DEV_D 0xA8  ///< data register (D)
#define MELSEC_DEV_R 0xAF  ///< file/extension register (R)
#define MELSEC_DEV_M 0x90  ///< auxiliary relay (M)
#define MELSEC_DEV_S 0x98  ///< state (S)
#define MELSEC_DEV_X 0x9C  ///< input (X)
#define MELSEC_DEV_Y 0x9D  ///< output (Y)
#define MELSEC_DEV_TN 0xC2 ///< timer current value (TN)
#define MELSEC_DEV_TS 0xC1 ///< timer contact (TS)
#define MELSEC_DEV_CN 0xC5 ///< counter current value (CN)
#define MELSEC_DEV_CS 0xC4 ///< counter contact (CS)
 
#define MELSEC_ENDCODE_OK 0x0000 ///< response end code: success
 
// Binary 3E frame geometry (octet offsets and fixed lengths).
#define MELSEC_3E_READ_REQ_LEN 21      ///< total octets in a batch-read request frame
#define MELSEC_3E_READ_REQ_DATA_LEN 12 ///< batch-read request data-length field: timer..points
#define MELSEC_3E_RES_MIN_LEN 11       ///< shortest valid response: subheader through end code
#define MELSEC_3E_RES_LEN_OFFSET 7     ///< offset of the response data-length field
#define MELSEC_3E_RES_DATALEN_BASE 9   ///< offset where the data-length-counted region (end code) begins
#define MELSEC_3E_RES_DATA_OFFSET 11   ///< offset of the response read data (after the end code)
#define MELSEC_ENDCODE_LEN 2           ///< end-code octets (counted inside the data length)
 
/**
 * @brief Build a binary 3E batch-read (word units) request.
 * @param device_code      MELSEC_DEV_* (or a raw device code).
 * @param head_device      starting device number (24-bit).
 * @param points           number of word points to read.
 * @param monitoring_timer the CPU monitoring timer (units of 250 ms; 0 = wait indefinitely).
 * @return total octets written (21), or 0 on overflow / bad input.
 */
size_t melsec_build_read(uint8_t *buf, size_t cap, uint8_t device_code, uint32_t head_device, uint16_t points,
                         uint16_t monitoring_timer);
 
/** @brief A parsed 3E response. @ref data points INTO the source buffer (LE word values). */
struct MelsecResponse
{
    uint16_t end_code;   ///< 0x0000 on success
    const uint8_t *data; ///< response payload (empty on error)
    size_t data_len;
};
 
/** @brief Parse + validate a binary 3E response (subheader 0xD0 0x00, length, end code, data). */
bool melsec_parse_response(const uint8_t *buf, size_t len, MelsecResponse *out);
 
#endif // DETWS_ENABLE_MELSEC
 
#endif // DETERMINISTICESPASYNCWEBSERVER_MELSEC_H