layoutx/plc.h

#ifndef __PLC_H__
#define __PLC_H__

#include <stdint.h>

#define PLC_AREA_I  0x81
#define PLC_AREA_Q  0x82
#define PLC_AREA_M  0x83
#define PLC_AREA_DB 0x84

#define PLC_MAX_BLOCK 256

typedef struct {
    char ip[64];
    int  port;
    int  area;          /* PLC_AREA_* */
    int  db;            /* used when area == PLC_AREA_DB */
    int  read_start;    /* first byte of the polled block (block-relative) */
    int  read_len;      /* bytes per poll, <= PLC_MAX_BLOCK */
    int  poll_ms;
    int  reconnect_ms;
    int  tsap_local;    /* e.g. 0x0100 */
    int  tsap_remote;   /* e.g. 0x0102 (rack0 slot2), 0x0001 for S7-200 SMART */
} PlcConfig;

/* Start background polling thread. 0 on success, -1 on error. */
int plc_init(const PlcConfig *cfg);

/* Stop thread and close connection. */
void plc_shutdown(void);

/* 1 when the last poll succeeded, 0 otherwise. */
int plc_is_connected(void);

/* Read a single bit of the cached block. byte_idx is relative to read_start.
 * Returns 0/1 on success, -1 if the block is not (yet) available. */
int plc_get_bit(int byte_idx, int bit_idx);

/* Copy one cached byte. Returns 0 on success, -1 if unavailable. */
int plc_get_byte(int byte_idx, uint8_t *out);

/* Human readable last error (static buffer). */
const char *plc_last_error(void);

#endif