VisualHMI Timer: Complete case study on start_timer/on_timer and countdown

VisualHMI supports32 software timers(indexed from 0 to 31). Upon startup, timeout triggers theon_timer(timer_id)callback, performing periodic tasks, delayed operations, and status polling. ⚠️ This is theapplication-level software timer, whose accuracy is affected by system load.is not suitable for hard real-time control.

. 1. API Summary Table

countdown parameterOnly affects internal timing display/debugging, does not change the triggering behavior of on_timer - sequential timing triggers when timeout is reached, countdown timing triggers when timeout is reduced to 0, and the callback is the same for both.

2. Basic usage

function on_init()
  start_timer(0, 1000, 0, 0)   -- 1秒周期，无限重复
  start_timer(1, 5000, 1, 1)   -- 5秒倒计时，只触发1次
end

function on_timer(timer_id)
  if timer_id == 0 then
    -- 每秒做一次：状态轮询
  elseif timer_id == 1 then
    -- 5秒后执行一次
  end
end

3. Application: Send instructions to PLC after countdown is completed

Requirement: Set a countdown timer for hours and minutes, with a reset button at the end of the countdown to send instructions to the mainboard. Control layout: Bit status indicator LW1002 (toggle switch) controls start and stop; scroll wheel control LW1000 (hours, 0~23) and LW1001 (minutes) set the duration; text control LW1010 displays the countdown timer.

_TIMER_REPEAT_ = 0     -- 总时长（秒）
_TIMER_CNT_ = 0        -- 已触发次数

function on_update(slave, vtype, addr)
  if vtype == VT_LW and addr == 0x1002 then
    if get_uint16(VT_LW, 0x1002) == 1 and _TIMER_REPEAT_ > 0 then
      stop_timer(0)
      start_timer(0, 1000, 0, _TIMER_REPEAT_)  -- 每秒一次，共 repeat 次
    else
      stop_timer(0)
    end
  end
end

function on_timer(timer_id)
  if timer_id == 0 then
    _TIMER_CNT_ = _TIMER_CNT_ + 1
    local remain = _TIMER_REPEAT_ - _TIMER_CNT_
    -- 显示剩余时间 mm:ss
    set_string(VT_LW, 0x1010,
      string.format("%02d:%02d", math.floor(remain / 60), remain % 60))
    if remain == 0 then
      set_uint16(VT_LW, 0x1002, 0x00)   -- 复位启停按钮
      set_uint16(VT_LW, 0x1003, 0x01)   -- 发指令给主板
    end
  end
end

Key points: _TIMER_REPEAT_ is calculated when the user sets the duration (hour*3600 + min*60); start_timer passes the total number of repeats, and on_timer increments _TIMER_CNT_ each time, ending when they are equal. set_notify(0/1) controls whether to send notification instructions during the timer period, set to 1 when needed.

Compiled from Guangzhou Dacai Technology VisualHMI development documentation (hmi-doc.gz-dc.com) LUA tutorial "Timer", copyright owned by Dacai Technology.