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I don't see 'at' in your example. I need to understand the context here if I am to attempt to answer. POS sometimes stands for Point of Sale - or cash register - which is a kind of computer terminal. Is the context a retail outlet?
In your example 'on' is used in connection with 'terminal' and not because of 'open'. That is 'on a computer (terminal)'.
Open AT or Open ON? 
Linear Mode
