Ras Cas Multiplexing Timing Diagram

# RAS/CAS Multiplexing — the Waveform That Saved Half the Pins

Mostek's MK4096, and the addressing trick every later DRAM generation inherited

THE SAME SIX PINS, USED TWICE PER ACCESS RAS̄ latches row address Address bus (6 pins) ROW address bus settles to COLUMN value CAS̄ latches column address COLUMN address same pins RAS̄ falls CAS̄ falls 12 address bits, carried on only 6 physical pins — strictly sequential, never simultaneous two vendors, same multiplexing concept Mostek MK4096 — 16-pin DIP, set the reference pinout TI TMS4030 — competing part, second-sourced to Mostek's pinout independent convergence on RAS/CAS confirms the fix was necessary, not just one vendor's preference genuine price and design-win competition at 4K density

1K DRAM worked, but systems wanted roughly four times the bits per chip — and unmultiplexed, that density needs about 12 dedicated address pins, too many for a standard package.

Pin count grows with the square root of array size under multiplexing, not linearly with it — which is why this scheme, not just this generation's bit count, became the DRAM interface standard.

WHY 4K COULDN'T SHIP WITHOUT THIS TRICK address pins needed, unmultiplexed vs. multiplexed pins density generation → unmultiplexed pins = log₂(density) multiplexed (RAS/CAS) half the pins, same density 4K: 12 pins → 6 pins 1K 4K 16K 64K 256K every later generation inherited this scheme rather than re-solving the pin-count problem the durable contribution was the interface convention, not the bit count itself

closes the gap between the original 1K implementation and the already-live 64K generation

Take ras cas multiplexing timing diagram further

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