There are a number of ways to do this.
If you use the 5250 screens (green Screen) then you could use scanners that have 5250 interfaces. This is a real time application so if the AS/400 is down the scanners are down.
Another method would be to use a client server type approach where orders for example are downloaded to a different system and the scanning is done to that system. This system would then send the received data to the AS/400 as it gets it. If the AS/400 is down, the system stores the data until the as/400 is available when it will then send the data to the AS/400.