Years ago, I accomplished double rings by connecting a single pole slave relay that has its coil driven by the interrupter's "LF" output. The contacts on this relay would then switch the "RN" output. The LF and RN cadences were aligned so that it provided a perfect double ring. I just used this switched output to feed the appropriate line cards. It worked well.

A means to pass CO double rings to the stations was to strap the 400D cards for steady ring. There is no ring generator used in this scenario. The ring side of each CO line is strapped to the CA terminal for its associated card slot. The RG and RB terminals on the 551's 66 block then became the common audible output to the stations. In this case, the true ring cadence from the CO is passed straight through to the ringers. Of course, this was ringing to ground, so the CO must be a 'real' one in order for it to work. It also required that the ringing plant in the CO to be synchronized of all hell would break loose when multiple calls would arrive.

Ed Vaughn, MBSWWYPBX