In 1990s, When I worked with process control systems, primarily DCS, for petrochemicals, beverages, and other chemical plants, we had phone modems connected to our systems. Only precaution was that modems were not connected to phone port unless someone needed remote access and were disconnected after use.
Actually, we used to computer simulate operations of facility to test our DCS systems against.
Actually, we used to computer simulate operations of facility to test our DCS systems against.