Again about semaphore

[Andey]

Hello.
In my home I have FN485 devices. They are controlled via dedicated RS485 bus. FN485 can to operate as stand alone device - it has input and output, when input is closed output relay is closed too. But I decided that I need additional control. To turn light from application, to know what inputs are closed, to turn light when switch that is controlled by other device is toggled. So I see at least three events that can to access to RS485 bus to operate with FN485. And I decided that I need to manage access to serial bus. The only way I know is to use semaphore. In scripting tab/user library  I have created script you can to see below

Code:

require('sem')
require('serial')

function readSerialFN485()
  local buf = {}
  while true do
    local timeout = #buf > 0 and 0.5 or 15
    local char = port:read(1, timeout)
   
    if char then
      buf[ #buf + 1 ] = char:byte()
    else
      return buf
    end
  end
end


function FN485_write(param1, relay, value)

  local semaphore = sem.open("FN485_serial_access")
  if semaphore:trywait() == false then
    semaphore:wait()
  end
 
  if not port then
    port = serial.open('/dev/RS485-1', { baudrate = 9600, duplex = 'half' })
    port:flush()
  end
 
  data_addr = 100 + param1
  data_relay = 100 + relay
  data_value = 100
  if value then
    data_value = 100 + 1
  end
 
  qqq = string.format('%c%c%c%c%c%c%c%c%c%c', 99, data_addr, data_addr, 211, 211, data_relay, data_relay, data_value, data_value, 13)
  if port then
    port:write(qqq)
    port:drain()
  end
 
  semaphore:post()
 
end
-- function reads all input states
-- returns bitwise UINT8 value; bit 0 = state of input 0
function FN485_read_all_inputs(param1)
  local semaphore = sem.open("FN485_serial_access")
  if semaphore:trywait() == false then
    semaphore:wait()
  end
 
  if not port then
    port = serial.open('/dev/RS485-1', { baudrate = 9600, duplex = 'half' })
    port:flush()
  end
 
  data_addr = 100 + param1
 
  qqq = string.format('%c%c%c%c%c%c%c%c%c%c', 99, data_addr, data_addr, 109, 109, 100, 100, 100, 100, 13)
  if port then
    port:write(qqq)
    port:drain()
   
    res = readSerialFN485()
   
  end
 
 
  -- parse read result
  if res then
    -- check read parameters and if parameters are OK return something
    if (res[2] == data_addr and res[3] == data_addr and res[4] == 109 and res[5] == 109) then
      local retValue = bit.lshift( res[7] - 100, 4) + res[9] - 100
      return retValue
    else
      log(res)
    end
  end
end

From point of view code is correct. 
For test purposes I have started to call read all input function from resident script with log output each time after read_all_inputs function is called. And I see that log is properly created only several times. Then I need to open resident script and save it to get again only several logs. But had expected continuous logs with timeout I had specified for resident script.
 What I am doing wrong.