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The 1756-RIO is a scanner module for the old proprietary Allen-Bradley Universal I/O network. There are several A-B platforms that used that protocol and wiring, and dozens of third-party devices that integrated adapters and plugs that used it, including weigh scales.
But it's not the same as RS-232. You would need a programmable device (I can think of a handful, none of them modern, cheap or powerful) to bridge between the RIO network and the RS-232 port.
There are many EtherNet/IP based gateway devices that would be less expensive, faster, and easier ways to connect between a ControlLogix and a weigh scale with an RS-232 port.