Error 227 Entering into the passive mode

hi,

   i developed one tool to download files from ftp. it's working fine. but sometimes i got the error 227 entering into the passive mode. what is the problem in this. why is this occured. i searched for solution's some one told this occured becasue of firewall, another one told this server error, i got diff anwsers from every one. what is the real cause of this. can any one tell this.