In my last post I started doing a more detailed look at the Identification Friend or Foe (IFF) system component shown in a recent video from Fran Blanche.
https://www.youtube.com/watch?v=dXrLSOReMFA
The information in this post is based on only what I could see in the video, so there things I cannot be sure of.
On board A3 there is an array of 12, 4-bit register ICs. Since the displays on the front panel only need 4 bit each, it’s safe to assume that these registers are holding the values for 12 of the digits being displayed on the front panel.
The A2 board has another set of four registers. These could be the remaining registers for the main part of the display, or they could the mode display on the left. There should still be four more registers somewhere so they may be on the one board that wasn’t shown in the video.
Here is one other interesting circuit I was able to identify, there are three instances of this on the boards I can see. The 7485 is a 4 bit comparator, it takes two 4-bit inputs and outputs one of three signals indicating if the one input is less then, greater then, or equal to the other. We can see that one set of inputs is connected to 7474 flip flops. I wasn’t able to see which of the three outputs is being used. One possibility is that these circuits are determining which data gets displayed on each row of the display. The desired display could be latched into the flipflops and when the corresponding data becomes available the “equals” output would latch in that data.
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