Modbus TCP Connection Refused — Complete Guide

modbus_new_tcp('192.168.1.100', 502);  # No connect call shown

#include <modbus/modbus.h>

int connect_modbus_tcp() {
  modbus_t *ctx = modbus_new_tcp('192.168.1.100', 502);
  if (ctx == NULL) {
    fprintf(stderr, 'Unable to allocate context\n');
    return -1;
  }
  
  // Set timeout
  modbus_set_timeout(ctx, 5, 0);  // 5 seconds
  
  // Connect
  if (modbus_connect(ctx) == -1) {
    fprintf(stderr, 'TCP connection failed: %s\n',
            modbus_strerror(errno));
    modbus_free(ctx);
    return -1;
  }
  
  printf('Connected to Modbus TCP slave\n');
  
  // Read data
  uint16_t regs[10];
  int rc = modbus_read_registers(ctx, 0, 10, regs);
  if (rc > 0) {
    printf('Read %d registers\n', rc);
  }
  
  modbus_close(ctx);
  modbus_free(ctx);
  return 0;
}

// Test with: nc -zv 192.168.1.100 502