Modbus Master Cannot Communicate with Slave

modbus_new_rtu('/dev/ttyUSB0', 9600, 'N', 8, 1);  # Wrong serial params

#include <modbus/modbus.h>
#include <stdio.h>
#include <errno.h>

int main() {
  modbus_t *ctx;
  uint16_t tab_reg[64];
  
  // RTU mode — match slave settings
  ctx = modbus_new_rtu('/dev/ttyUSB0', 115200, 'N', 8, 1);
  if (ctx == NULL) {
    fprintf(stderr, 'Unable to create RTU context\n');
    return -1;
  }
  
  modbus_set_slave(ctx, 1);  // Slave ID
  modbus_set_timeout(ctx, 1, 0);  // 1 second timeout
  
  if (modbus_connect(ctx) == -1) {
    fprintf(stderr, 'Connection failed: %s\n', modbus_strerror(errno));
    modbus_free(ctx);
    return -1;
  }
  
  // Read holding registers starting at 0, count 10
  int rc = modbus_read_registers(ctx, 0, 10, tab_reg);
  if (rc == -1) {
    fprintf(stderr, 'Read failed: %s\n', modbus_strerror(errno));
  } else {
    printf('Read %d registers\n', rc);
  }
  
  modbus_close(ctx);
  modbus_free(ctx);
  return 0;
}