USB not recognized in download mode

hello people . i am having this problem for some time for so many phones. the problem is phone USB is working fine but when i connect in download mode it says USB not recognizable . ive tried following
1. different cables
2.different PC
3.install and uninstalling Samsung USB drivers
4.uninstalling kies
5.plug USB before download mode
6.plug USB before hitting up volume key in download mode
7.plug USB after hitting up volume key
8.different ports

nothing seems to work .
do anyone know for certain what is causing this ??