luamodbus documentation

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You can enable debug mode to see what data is sent/received:

Code:

require('luamodbus')

buffer = {}

function logger(c)
table.insert(buffer, c)
end

mb = luamodbus.rtu()
mb:open('/dev/RS485-1', 9600, 'E', 8, 1, 'H')
mb:connect()
mb:setslave(1)
mb:setdebug(logger)

-- read/write here

log(table.concat(buffer))

You can also add mb:flush() after sleep to discard any invalid data that could be received.

Code:

mb:writereadregisters(write_reg_start, write_reg_value_1, write_reg_value_2, write_reg_value_3..., read_reg_start, read_reg_count)