Digi One IA Rs232 coms are intermittent is it a grounding issue?

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I am using a Digi One IA to connect to a RS232 port on an Emerson PLC. I am observing intermittent behavior. It works with one PLC and not another. I am using a 3 wire Rs232 cable with software flow control. Does pin 5 on the Digi have to be connected to system ground in addition to connecting to the signal ground on the PLC? I am told the Digi ground is floating. Thanks.