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Not all RA I/O platforms operate identically, especially with regard to fault conditions. In general the modern stuff that's only connectible to a Logix family controller will be more complex and feature-rich and granular.
That is especially true of the embedded analog inputs in the CompactLogix 1769-L24ER-QBFC1B. They're a "universal" input module, so there are more input range / sensor types than you'll see in any other 1769 I/O module.
The specifications for the input values aren't easy to find in the User Manual; you have to go to the CompactLogix Controllers Specifications Technical Data, 1769-TD005. Even that has a couple of errors for the analog output data, corrected in Knowledgebase article 532996.
When you leave the input signal Data Format set for "Raw/Proportional", then all input signals (0-10 V, 0-20 mA, 4-20 mA, 1-5 V... anything) will be scaled -32767 to +32767. It's not "going to full downscale" when you have a wire break, that's just the value equivalent to 0 mA on the current loop.
When you're using a signal format that allows it, like 4-20 mA, the 1769-L24ER-QBFC1B analog input does let you choose the open-circuit response, including Upscale, Downscale, Zero, or Last State. When you choose Last State, you still have the Underrange bit to let you know the circuit is open.
Turning our attention to the 1794-IE12 high density analog input module, it uses the very mature 1794 FLEX data bus. I remember starting to use FLEX I/O for the first time in a sawmill in north Texas in 1994, with a PLC-5/40E and RIO networking. In general, FLEX products need to work with a very broad variety of PLC and Logix controllers and networks, so they are less sophisticated than modern stuff. There's a next-generation "FLEX 5000" platform that's very new, but that's not the topic of discussion.
The 1794-IE12 only supports three common analog signal ranges: 0-20 mA, 4-20 mA, and -10 to +10 V. And it has no scaling or open-circuit responses or filters at all.
The 1794-UM002 User Manual for FLEX I/O Analog Modules has an explicit low-level data map that actually shows the values in hexadecimal.
According to that chart, an input signal in the 0.0 to 21.00 mA range goes from 0000 to 7FF8 (+32760).
So that's the essential answer: the values returned on an open circuit state are different for the two modules because one of them scales data starting from 0, and the other starting from -32768.
You mentioned that with an open circuit the signal from the FLEX input channel goes to about 6, not 0. That's probably just because of the resolution of the A/D converters inside that module. The specifications in the 1794-IN106 Installation Instructions suggests that the input resolution is 16 bits, which is 0.641 uA/count. It makes sense that there might be enough very low level signal noise to show a low number in the event of an open-wire state.
