Using Nodemcu to drive B7971 nixie tubes via 74HC595 shift registers

init.lua

-- lookup table mapping characters to bit representations
lookup = {
  ["A"] = 0x0e9c,
  ["B"] = 0x8ee0,
  ["C"] = 0x00cc,
  ["D"] = 0x8ce0,
  ["E"] = 0x00dc,
  ["F"] = 0x009c,
  ["G"] = 0x06cc,
  ["H"] = 0x0e1c,
  ["I"] = 0x80e0,
  ["J"] = 0x0c48,
  ["K"] = 0x301c,
  ["L"] = 0x004c,
  ["M"] = 0x1c0e,
  ["N"] = 0x2c0e,
  ["O"] = 0x0ccc,
  ["P"] = 0x0a9c,
  ["Q"] = 0x2ccc,
  ["R"] = 0x2a9c,
  ["S"] = 0x06d4,
  ["T"] = 0x80a0,
  ["U"] = 0x0c4c,
  ["V"] = 0x100d,
  ["W"] = 0x2c0d,
  ["X"] = 0x3003,
  ["Y"] = 0x1003,
  ["Z"] = 0x10c1,
  ["0"] = 0x1ccd,
  ["1"] = 0x8020,
  ["2"] = 0x0ad8,
  ["3"] = 0x16c0,
  ["4"] = 0x0e14,
  ["5"] = 0x20d4,
  ["6"] = 0x06dc,
  ["7"] = 0x1081,
  ["8"] = 0x0edc,
  ["9"] = 0x0ed4,
  ["*"] = 0xb233,
  ["_"] = 0x4000,
  [" "] = 0x0000,
  ["!"] = 0xffff
}

latch_pin = 2

function show(flw)
  -- bail if invalid string
  if (flw == nil) or (flw:len() < 4) then
    print("String is nil or shorter than 4 characters")
    return
  end

  -- convert to upper case
  flw = flw:upper()

  print("Starting.")
  -- iterate over first 4 characters in string
  for i = 4,1,-1 do
    -- get current character
    c = flw:sub(i,i)
    uart.write(0,"Sending '" .. c .. "' ")
    -- look up the bit pattern for the character
    val = lookup[c]
    -- in the event of an invalid character, show underscore
    if val == nil then
      val = lookup["_"]
      uart.write(0,"subsituting '_' ")
    end
    uart.write(0, "as '0x" .. string.format('%04x', val) .. "'.\r\n")
    -- shift out high and low bytes
    spi.send(1, bit.rshift(val, 8), bit.band(val, 0xFF))
  end
  print("Latching.")
  -- latch the data
  gpio.write(latch_pin, gpio.HIGH)
  gpio.write(latch_pin, gpio.LOW)
  print("Done.")
  print()
end

gpio.mode(latch_pin, gpio.OUTPUT)
gpio.write(latch_pin, gpio.LOW)
-- bus width 16 bits
result = spi.setup(1, spi.MASTER, spi.CPOL_HIGH, spi.CPHA_LOW, spi.DATABITS_8, 0)
print("Result of SPI setup " .. result)
print("Ready.")