What is the intended usage of "WE" or "PE/OE" pins on 4517 shift register

Question

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?

Answer 1

The two most basic configurations for this circuit which I can think of are:

1. Variable length shift register (middle taps are used as outputs)
2. 64 bits serial memory element with x4 write speed (middle taps are used as inputs)

The above two are the most basic, and the middle taps are used for a single purpose in each of them. In 1, WE signal should not be asserted at all. In 2, the logic driving the middle taps should become a high impedance when WE gets deasserted.

We can find the application in which the middle taps will be used as both inputs and outputs: if you are using pins D, 16, 32 and 48 of both packaged registers (total of 8 pins) as inputs, and pins 16,32,48,64 of both packaged registers (total of 8 pins) as outputs - you get a 8-bits wide serial memory element. This memory can sample a Byte of data off the bus and put a Byte of data on the bus. If you are using a single bus for both reading and writing (which is usually the case), the fact that outputs go high Z when you assert WE is very handy - you can connect the bus directly to the inputs/outputs without any additional tri-states. Just make sure you are gating the clocks to both registers when you neither want to sample data off the bus, nor put the next sample on the bus.