The main reason for RTL (and then DTL) at the time was conserving transistors. Discrete ones were quite expensive. In the mid-1960s roughly speaking, in moder

The main reason for RTL (and then DTL) at the time was conserving transistors. Discrete ones were quite expensive. In the mid-1960s roughly speaking, in modern dollars, a discrete transistor was about $10, while a semiconductor diode was about $1 and a resistor was, like today, about $0.05. The cost of the transistors dwarfed concerns about the cost of the boards or other components. This is a flip-flop circuit from a PDP-8. [1] It's diode-transistor logic, primarily. It's for 3 bits. There are only two transistors used per flip-flop to store the bits. A modern designer would use more transistors to route the inputs/outputs and for addressing the flip-flop. [2] The PDP-8 designers used capacitors and diode and resistor networks, to selectively pass pulses, using transistors only when truly necessary to invert or boost a signal. Several dozen diodes, caps and resistors, plus pulse logic (which complicates the whole design) were still cheaper than a few more transistors. With early integrated circuits, the yield was terrible. Most of the devices didn't work. It's much easier to build up a working resistor than a working diode on an IC, and it's easier to build a diode than it is a transistor. Also, with a very large process size, there's little concern about the heat of resistors. When we start increasing speed and shrinking the die, the power dissipation becomes considerable. Even by the late 1960s the heat of RTL at high speed limited how densely you could pack in it an IC. This is the same reason why, much later, CMOS would displace NMOS, despite requiring 2x as many components on chip. It runs cooler so it can be packed much more densely and run at a higher clock. (If you absolutely needed speed, from the 1950s right into the 1990s, you would use ECL, a kind of logic that doesn't switch on/off so the transistors never enter full saturation; like a differential amplifier, the current flow is constant.) [1] https://homepage.cs.uiowa.edu/~dwjones/pdp8/UI-8/map/R211schematic1965.GIF https://homepage.cs.uiowa.edu/~dwjones/pdp8/UI-8/map/R211sch... [2] https://ars.els-cdn.com/content/image/1-s2.0-S0026269216301574-gr2.jpg https://ars.els-cdn.com/content/image/1-s2.0-S00262692163015...