求教python 编程问题，关于modbus-tk

用modbus-tk，来读取功率仪的704地址的数据，想实现每隔三秒读取一次，求教该怎么编写循环程序，目前已实现单次读取，程序如下：

import serial 
import modbus_tk
import modbus_tk.defines as cst
from modbus_tk import modbus_rtu
import time

def sleeptime(hour,min,sec):
    return hour*3600 + min*60 + sec
second = sleeptime(0,0,3)
time.sleep(second)
master = modbus_rtu.RtuMaster(
    serial.Serial('COM8', baudrate=19200, bytesize=8, parity='N', stopbits=1)
)
master.set_timeout(5.0)
data = master.execute(1, cst.READ_HOLDING_REGISTERS, 704, 1)
print (data)

master = modbus_rtu.RtuMaster(serial.Serial('COM8', baudrate=19200, bytesize=8, parity='N', stopbits=1))
master.set_timeout(5.0)
while (True):

    data = master.execute(1, cst.READ_HOLDING_REGISTERS, 704, 1)
    print (data)
    second = sleeptime(0,0,3);
    time.sleep(second);