On 2015-Mar-12, at 6:45 AM, Noel Chiappa wrote: That's not quite the circuit arrangement, but yes, that is the operating principle. The center-tap is driven by the address-selection to +supply, one end is driven by the read-pulse driver for one polarity of pulse, the other end by the write-pulse driver for the other polarity, the latter both going to GND. In a direct-coupled arrangement a regulated dual-polarity power supply is needed, rather than single-polarity, and there's going to be some asymmetry and additional complexity to driving the output drivers in either a complementary or stacked-pair arrangement. With the transformers, the +/? pulse drivers are identical in design and components - there is no asymmetry - there's some benefit to that in easily getting matching characteristics of the +/? address pulses. Even in that other design I was describing (from the late-70s), the transformers did the level-shifting to drive the output drivers as half are on the +supply and half on the ?supply, which then directly drove the address wires. For a single-polarity supply it could be done with an H-bridge-style arrangement, requiring more transistors. There's lots of ways of doing it, with changing economies and tradeoffs. It's a mid-60s calculator, and while the design issues are not entirely the same, it's from the same period when every cheap 6- or 7-transistor radio had two audio transformers to implement push-pull drive - which is to say little transformers were a functional and economic option for the time. We don't bother today of course, as a zillion transistors in an IC are better and cheaper. (Just for some additional context, it wasn't till the late-60s that solid-state audio really started focussing on direct-coupled designs and getting rid of the output transformer.)