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Why bother with such a device? This adds additional circuitry that provides no long term real value, only additional un-necessary components to fail. Both HAAS and Fanuc have standardized RS232 pin outs.
Is it likely to damage RS232 components? No. But it is unlikely to provide any protection from other causes that could damage RS232 circuits.
See my my web site www.beta-a2.com on electrical isolation.
Note: the "Smart Cable SC880 DB-9 to DB-25" ad states that the 25 pin connector is female. Since HAAS and Fanuc usually have a 25 pin Female connector on the machine this means you need a male to male adapter which they supply. Now you have a long cantilevered mass hanging off of the machine RS232 connector.