Wiser for KNX and modbus problem

[Kilogica]

Code:

require('luamodbus')
mb = luamodbus.rtu()

mb:open('/dev/RS485-1', 9600, 'N', 8, 1, 'H')
mb:connect()

buffer = {}

mb:setdebug(function(msg)
  buffer[ #buffer + 1 ] = msg
end)

mb:setslave(8)

res = { mb:readregisters(3109, 2) }
log(res)

mb:close()

log(table.concat(buffer))

I disabled RTU in mapper and then tried it, it suppose to log something about frequency (tried with reading test) unfortunately it keeps logging that

* string: [08][03][0C][25][00][02][D6][09]
ERROR Bad file descriptor

i tried reading for value, specify swap, specify datatype, try different registers, change to 8N2...

What I'm doing wrong?