A general term for a type of network device that protects serial equipment from voltage surges.
If two pieces of data terminal equipment (DTE) are connected by a long serial line, voltage differences with respect to ground between the devices can cause surges over the line that can damage the devices.
This can be a problem in a mainframe environment when you connect terminals to asynchronous mainframe hosts using long RS-232 cables.
The problem is especially troublesome when the cabling has to run outdoors between buildings or when nearby generators or other equipment induce voltages.
The solution to these problems is to insert a data isolator between the mainframe host and the terminal. This isolator provides electrical isolation between the two devices, somewhat like an opto isolator for fiber-optic cabling. Data isolators typically use transformers to electrically isolate the two connected circuits from voltage surges.
Data isolators can support high data transfer speeds, and they come with a variety of interfaces, such as RS-232, RS-422, and Time to Live (TTL) connections.
Check out this EN-30 Network Isolator from Amazon.