function ipaddress = prologix_discovery()
%% PROLOGIX_DISCOVERY Find your prologix GPIB device.
%   ipaddress = PROLOGIX_DISCOVERY() finds prologix on your 
%   local network. When your prologix device is detected
%   string with ip address is returned. 
%   
%   Function uses persistent ipaddress: when ipaddress is 
%   changed use CLEAR ALL to find the current ipaddress.
%   See also CLEAR

%% setup correct variables.
% if location of prologix is known: just return stored ip address
persistent stored_ipaddress;
if ~isempty(stored_ipaddress)
    ipaddress = stored_ipaddress;
    return
end
local_port = randi([49152 65535]);
remote_port = 3040;

%% setup dsp to send and recieve udp packets.
hudpr = dsp.UDPReceiver('LocalIPPort',local_port);
hudps = dsp.UDPSender('RemoteIPAddress','255.255.255.255','RemoteIPPort',remote_port,'LocalIPPortSource','Property','LocalIPPort',local_port);
% start recieving udp packets
setup(hudpr);

%% Discover prologix.
% magic string to request ipaddress from prologix: ['5a' '00' '5b' 'db' 'ff' 'ff' 'ff' 'ff' 'ff' 'ff' '00' '00']
magic_msg = uint8([90 0 91 219 255 255 255 255 255 255 00 00]);
% Sending packet.
step(hudps,magic_msg);
% Recieving packet or resend.
for i = 1:100
    pause(0.1);
    msg = step(hudpr);
    if numel(msg)>0
        break;
    elseif mod(i,10) == 0
        %waiting dot and resending of string
        fprintf('.')
        step(hudps,magic_msg);
    end
end
if isempty(msg)
    error('No prologix found on network');
end
%ipaddress string is extracted from recieved UDP message. array position 21
%till 24.
ipaddress = num2str(msg(21:24)','%d.%d.%d.%d');
stored_ipaddress = ipaddress;
%% releasing connection.
release(hudps);
release(hudpr);