This circuit is a synchronous binary decade counter using relays. It counts from 0-9 and back to 0 again. Its based on my design as demonstrated in this video. It is used to count the pulses that come from the telephone dial numeric input device. It also counts up during the square root process in order to work out each digit of the square root as calculations take place. It acts as an input to the AE register LSB and gets shifted in as necessary.
Designing a synchronous decade counter which only uses 12 relays for 4 bits was a bit tricky. K-maps gave me terms which were not very easy to cram into the limited number of contacts. Therefore I took the approach of computing a change, no-change signal for each digit which gets propgated down the chain. Then there is some logic to disable this and reset the whole counter when the count gets clocked beyond 9. The circuit also generates an =9 signal for determining when to move on during the calculation of square roots and also generates the subtractor signals SUB1X2 and SUB123 which are needed for 9s complement formation.
Once again this circuit makes use of the edge triggered D-type flip flop I posted about previously. The diodes are used mostly for OR gates, but some are protecting the circuit from the internal operations of the flip flops (isolation diodes).
Also included in this circuit is the telephone dial pulse inverter, and a monostable circuit which generates a short clock pulse to load the dialed number into the AE reg during data entry. This takes advantage of the dial rotation contacts on the phone dial which are closed for the whole rotation of the dial and open to indicate the dial returned to rest.