正点原子阿波罗开发板libmodbus无法通信

#include "modbus_rtu_test.h"
#include "modbus.h"
#include "stdio.h"
#include "string.h"
#include "pcf8574.h"
//#define RS485_RE GET_PIN(G, 8)
#define RS485_RE    6 //(0-7)
#define I2C_BUS    "i2c2"
static void test_thread(void *param)
{
    uint16_t tab_reg[64] = {0};
    modbus_t *ctx = RT_NULL;
    int rc; /* 存放modbus函数返回值 */
    /*初始化pcf8574*/
    pcf8574_device_t dev = RT_NULL;
    dev = pcf8574_init(I2C_BUS, RT_NULL);
    if (dev == RT_NULL)
    {
        rt_kprintf("pcf8574 init fail\n");
    }
    ctx = modbus_new_rtu("/dev/uart2", 115200, 'N', 8, 1);
    if (ctx == NULL) {
            rt_kprintf("failed to create modbus context");
        }
    modbus_rtu_set_serial_mode(ctx, MODBUS_RTU_RS485);
    pcf8574_pin_write(dev, RS485_RE, 0);//设置RS485_RE为接收模式
//    modbus_rtu_set_rts(ctx, RS485_RE, MODBUS_RTU_RTS_UP);
    modbus_set_slave(ctx, 3);
    rc=modbus_connect(ctx);
    if (rc == -1) {
        rt_kprintf("failed to connect to server");
    }
    modbus_set_response_timeout(ctx, 0, 1000000);
    int num = 0;
    while (1)
    {
        memset(tab_reg, 0, 64 * 2);
        int regs = modbus_read_registers(ctx, 0, 20, tab_reg);
        printf("-------------------------------------------\n");
        printf("[%4d][read num = %d]", num, regs);
        num++;
        int i;
        for (i = 0; i < 20; i++)
        {
            printf("<%#x>", tab_reg[i]);
        }
        printf("\n");
        printf("-------------------------------------------\n");
        rt_thread_mdelay(5000);
    }
    //7-关闭modbus端口
    modbus_close(ctx);
    //8-释放modbus资源
    modbus_free(ctx);
}
static int rtu_test_init(void)
{
//    rt_pin_mode(RS485_RE, PIN_MODE_OUTPUT);
    rt_thread_t tid;
    tid = rt_thread_create("test",
                           test_thread, RT_NULL,
                           2048,
                           12, 10);
    if (tid != RT_NULL)
        rt_thread_startup(tid);
    return RT_EOK;
}
INIT_APP_EXPORT(rtu_test_init);