Trouble understanding 4021 pinout

Forum 2005-2010 (read only) Hardware Interfacing
1

I'm looking into shift registers, here's the diagram I'm up against at the moment:

Datasheet: Intelligent Power and Sensing Technologies | onsemi

From what I understand, I need to find:

  1. The 8 parallel inputs (easy, pins 1, 4, 5, 6, 7, 13. 14, 15)
  2. The parallel in/serial out latch (looks like pin 9)
  3. The clock (pin 10)

So I have these, but I'm confused as to what the following pins do:

pins 2, 3, 12 (Q6, Q8, Q7 "BUF OUT"):
Are these buffers for the last 3 digits being shifted out serially? A kind of look-ahead?

pin 11 (SER IN):
I suppose this is a serial input, but I don't see why it'd ever be needed, unless you were using the chip to act as a sort of 8 bit serial register. Personally I'm using this to read 8 buttons simultaneously, so I don't think I'll use it, but it'd be good to know what it does.

2

The thought just occured to me, perhaps SER IN is for daisy-chaining a bunch of these together?

EDIT: Looked over the shiftIn tutorial one more time, and I realized that there was no serial out. I get it now, Q6-8 are the serial outputs, with Q8 being the bit at the front of the chain. However, I'm still wondering what SER IN is for.

3

However, I'm still wondering what SER IN is for.

You got it:-

SER IN is for daisy-chaining a bunch of these together