VisualHMI Modbus one master and multiple slaves: start_read polling, cycle scheduling, and read/write slaves

Modbus One Master Multiple Slaves: The HMI serves as the Modbus master, polling the registers of multiple slaves through the start_read function in the background. The core functions include start_read to create tasks, get_xxx to read from the cache, and on_cmd_resp to receive results - without blocking the script.

1. start_read parameters

start_read(index, vtype, addr, quantity, cycle, cycle_run, mode)

2. Basic Usage

function on_init()
  -- 每周期都读从机0的 4x1000~1009（10个寄存器）
  start_read(0, VT_4x, 0x1000, 10)
  -- 每 3 个周期读一次从机1的 4x2000~2009，且在周期内第3次执行
  start_read(1, VT_4x, 0x2000, 10, 3, 2)
end

function on_run(screen)
  -- 读数据直接拿缓存，零通信开销
  local v = get_uint16(VT_4x, 0x1000)
  local f = get_float(VT_4x, 0x2000)
end

start_read only creates tasks, actual data is read from local cache through get_xxx - the same mechanism as in the register operation section. To receive a read completion notification, use the on_cmd_resp callback (requires create_resp_que).

3. Multi-Slave Polling Scheduling

-- 8 台从机，每台读 20 个寄存器，错开周期避免总线拥堵
function on_init()
  for i = 0, 7 do
    start_read(i, VT_4x, 0x0000, 20, 8, i)  -- 8周期轮一遍，每周期读一台
  end
end

function on_cmd_resp(slave, vtype, addr, count, ret, wr)
  if ret == 0 then
    -- 第 slave 台从机读取成功，处理数据
  else
    -- 通信异常，标记从机离线
  end
end

With multiple slaves, bus bandwidth is limited, so use cycle/cycle_run for off-peak reading; exceptions are determined by checking if ret is non-zero in on_cmd_resp, and marked offline.

Compiled from Guangzhou Dacai Technology VisualHMI development documentation (hmi-doc.gz-dc.com) LUA tutorial "Modbus One Master Multiple Slaves", copyright reserved by Dacai Technology.