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Man Pages
MODBUS_READ_REGISTER(3) libmodbus Manual MODBUS_READ_REGISTER(3)

modbus_read_registers - read many registers

int modbus_read_registers(modbus_t *ctx, int addr, int nb, uint16_t *dest);

The modbus_read_registers() function shall read the content of the nb holding registers to the address addr of the remote device. The result of reading is stored in dest array as word values (16 bits).
You must take care to allocate enough memory to store the results in dest (at least nb * sizeof(uint16_t)).
The function uses the Modbus function code 0x03 (read holding registers).

The function shall return the number of read registers if successful. Otherwise it shall return -1 and set errno.

EMBMDATA
Too many registers requested

modbus_t *ctx;
uint16_t tab_reg[64];
int rc;
int i;
ctx = modbus_new_tcp("127.0.0.1", 1502); if (modbus_connect(ctx) == -1) { fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno)); modbus_free(ctx); return -1; }
rc = modbus_read_registers(ctx, 0, 10, tab_reg); if (rc == -1) { fprintf(stderr, "%s\n", modbus_strerror(errno)); return -1; }
for (i=0; i < rc; i++) { printf("reg[%d]=%d (0x%X)\n", i, tab_reg[i], tab_reg[i]); }
modbus_close(ctx); modbus_free(ctx);

modbus_write_register(3) modbus_write_registers(3)

The libmodbus documentation was written by Stéphane Raimbault <stephane.raimbault@gmail.com>
12/20/2018 libmodbus v3.1.4

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