HIGHVAL LOWVAL

function OnParam(class, sec)
  if class == "INDX" and sec == "RTSSTD" then
     local h = tonumber(getParamEx("INDX",  "RTSSTD", "HIGHVAL").param_value);
     local l = tonumber(getParamEx("INDX",  "RTSSTD", "LOWVAL").param_value);

     ToLog("H = "..tostring(h));
     ToLog("L = "..tostring(l));
  end;  
end;

В лог выводит:

27-7-2017 12:12:32.567 H = 0
27-7-2017 12:12:32.567 L = 0
27-7-2017 12:12:33.556 H = 0
27-7-2017 12:12:33.557 L = 0
27-7-2017 12:12:34.428 H = 0
27-7-2017 12:12:34.428 L = 0
27-7-2017 12:12:36.9 H = 0
27-7-2017 12:12:36.9 L = 0
27-7-2017 12:12:39.794 H = 0
27-7-2017 12:12:39.794 L = 0
27-7-2017 12:12:45.805 H = 0
27-7-2017 12:12:45.805 L = 0
27-7-2017 12:12:46.909 H = 0
27-7-2017 12:12:46.909 L = 0
27-7-2017 12:12:48.79 H = 0

timber,

для INDX параметры называются MAX и MIN
т.е. нужно писать так:

local h = tonumber(getParamEx("INDX", "RTSSTD", "MAX").param_value);
local l = tonumber(getParamEx("INDX", "RTSSTD", "MIN").param_value);