snp.h

Go to the documentation of this file.

// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
/**
 * @file snp.h
 * @brief GE Fanuc SNP (Series Ninety Protocol) serial frame codec (DETWS_ENABLE_SNP).
 *
 * SNP is the GE Fanuc Series 90 (90-30 / 90-70) master-slave serial protocol over RS-485. A message is
 * a BCC-checked frame delimited by control characters:
 *
 *     [SOH-or-other-control][data...][checksum]
 *
 * SNP frames the payload with an ASCII/binary control byte, a length, the data, and an arithmetic-sum
 * BCC (the low byte of the sum of every framed byte). This builds/validates that frame so a device can
 * read/write registers on a Series 90 PLC; the RS-485 UART transport (and the SNP-X session setup) is
 * the remaining device step. Pure, zero heap, no stdlib, host-testable.
 */
 
#ifndef DETERMINISTICESPASYNCWEBSERVER_SNP_H
#define DETERMINISTICESPASYNCWEBSERVER_SNP_H
 
#include "ServerConfig.h"
#include <stddef.h>
#include <stdint.h>
 
#if DETWS_ENABLE_SNP
 
/** @brief SNP control bytes (subset). */
// SNP control bytes: wire values compared/emitted, so integer constants in a namespacing struct.
struct Snp
{
    static constexpr uint8_t SNP_ENQ = 0x05; ///< enquiry / attach.
    static constexpr uint8_t SNP_ACK = 0x06; ///< acknowledge.
    static constexpr uint8_t SNP_NAK = 0x15; ///< negative acknowledge.
    static constexpr uint8_t SNP_SOH = 0x01; ///< start of header (a request/response frame).
    static constexpr uint8_t SNP_EOT = 0x04; ///< end of transmission.
};
 
/** @brief Arithmetic-sum BCC: the low 8 bits of the sum of @p len bytes. */
uint8_t detws_snp_bcc(const uint8_t *bytes, size_t len);
 
/**
 * @brief Build an SNP frame: [control][length][data...][BCC]. length is the data byte count.
 * @return the frame length (2 + data_len + 1), or 0 on overflow / bad args (data_len > 255).
 */
size_t detws_snp_build(uint8_t control, const uint8_t *data, size_t data_len, uint8_t *out, size_t cap);
 
/** @brief A parsed SNP frame (data points into the input). */
struct SnpFrame
{
    uint8_t control;
    const uint8_t *data;
    size_t data_len;
};
 
/** @brief Validate the BCC and parse an SNP frame. @return true if the BCC matches and it is well-formed. */
bool detws_snp_parse(const uint8_t *frame, size_t len, SnpFrame *out);
 
#endif // DETWS_ENABLE_SNP
#endif // DETERMINISTICESPASYNCWEBSERVER_SNP_H