There are unidirectional security gateways that only allow data to flow in a single direction. The legacy hardware creates a file, and copies it to a directory

There are unidirectional security gateways that only allow data to flow in a single direction. The legacy hardware creates a file, and copies it to a directory managed by the gateway. The gateway then copies it to some data dump on the other side, onto an updated and centrally managed part of the network. Barring some vulnerability in the gateway, passing data in the other direction is impossible.