The xx4517 (http://www.onsemi.com/pub_link/Collateral/MC14517B-D.PDF) is a 16-pin chip which contains two 64-bit shift register units, each of which in turn is divided into four 16-bit registers. Each of the shift units has a pin which may be called "WE" or "PE/OE" which selects between two modes of operation. In one mode, the bit 0 input is latched into the first register, and each of the other registers latches the output of its predecessor. The outputs of all four shift registers are sent to device pins. In the second mode, the drivers for all four outputs are disabled; the first three pins are repurposed so that the second through fourth shift registers will take data from them pins rather than from their predecessor shift registers. The final output pin is simply unused.
It's clear how the device could be useful when the mode pin is strapped low. I can also see how it would be useful to have a means of switching between either of the following pairs of modes:
Intermediate shift values are output and passed to the next stage Pins are floated but shift-register outputs are passed to the next stage anyway Pins are floated but shift-register outputs are passed to the next stage anyway Pins are floated, and shift registers take input from them
I'm curious in what way the mode-function as it actually sits can be practically utilized. In all the cases I can think of where I would sometimes want a device to load the middle taps from external pins, there would be times when I would want it not load middle taps from the external pins but not output anything to them either.
Since parts with the xx4517 pinout have been made for decades while some other kinds of shifter registers have come and gone, and since 14 pins seems to have been at least as common a size for discrete logic as 16 pins, it would seem curious if those pins were never used by any of the circuits where the part has been employed (especially since one can readily imagine other useful purposes to which such pins could have been put). Is there some clever means of using the 4517's WE pin that I'm unaware of? The only thing I can think of would be for applications which only want to use the mid-tap pins as inputs to drive them via 4.7K resistors, and ignore the fact that they'll often be back-driven, but that seems really really icky. Were there better uses?