Answer/entry register and display driver
I have been progressing with the relay square root machine project. I have finished the schematic capture for the answer and entry register which also acts as a display driver and drives a bank of 8 nixie tubes. This circuit is a shift register which accepts the users input of a floating point number entered via the phone dial interface. It also acts as an answer register to display the final result. You can see a video of the first test of this working here.
The AE register circuit needs to store 8 digits of binary coded decimal information. It also decodes these to drive the 10 cathodes of the nixie tubes. There is a ripple blanking signal to suppress leading zeros and this is also affected by the decimal point position. It also has to include a multiply by 5 operation during transfer into the accumulator which requires a little extra logic. Also there is an extra set of signals which are generated for the input to the subtractor as part of forming the 9s complement of the number. There is also a test for AE=0 which is used by the central control logic. All the latches are built up using the basic D-type relay flip flop unit. The advantage of edge triggering is that fewer relays are used and only a single clock line is needed. The shift input to the AE reg LSB digit comes from the C register and dial counter.
I used EAGLE for the schematic capture but it does not support hierarchical schematic sheets. So therefore since I am not really using this to build circuit boards, I faked it by making components which look like the interfaces to other sheets. Really all I am only trying to do is generate a circuit diagram for documentation.