Hardware implementation thumb|A Standard Modular System (SMS) card Most of the logic circuitry of the 1620 was a type of resistor–transistor logic (RTL) using "drift" transistors (a type of transistor invented by Herbert Kroemer in 1953) for their speed, that IBM referred to as Saturated Drift Transistor Resistor Logic (SDTRL). Other IBM circuit types used were referred to as: Alloy (some logic, but mostly various non-logic functions, named for the kind of transistors used), CTRL (another type of RTL, but slower than SDTRL), CTDL (a type of diode–transistor logic (DTL)), and DL (another type of RTL, named for the kind of transistor used, "drift" transistors). Typical logic levels of all these circuits (S Level) were high: 0 V to -0.5 V, low: -6 V to -12 V. Transmission line logic levels of SDTRL circuits (C Level) were high: 1 V, low: -1 V. Relay circuits used either of two logic levels (T Level) high: 51 V to 46 V, low: 16 V to 0 V or (W Level) high: 24 V, low: 0 V.
Hardware implementation thumb|A Standard Modular System (SMS) card Most of the logic circuitry of the 1620 was a type of resistor–transistor logic (RTL) using "drift" transistors (a type of transistor invented by Herbert Kroemer in 1953) for their speed that IBM referred to as Saturated Drift Transistor Resistor Logic (SDTRL) Other IBM circuit types used were referred to as Alloy (some logic but mostly various non-logic functions named for the kind of transistors used) CTRL (another type of RTL but slower than SDTRL) CTDL (a type of diode–transistor logic (DTL)) and DL (another type of RTL named for the kind of transistor used "drift" transistors) Typical logic levels of all these circuits (S Level) were high 0 V to -05 V low -6 V to -12 V Transmission line logic levels of SDTRL circuits (C Level) were high 1 V low -1 V Relay circuits used either of two logic levels (T Level) high 51 V to 46 V low 16 V to 0 V or (W Level) high 24 V low 0 V