Emitter-coupled Logic - History

History

ECL was invented in August 1956 at IBM by Hannon S. Yourke. Originally called current-steering logic, it was used in the Stretch, IBM 7090, and IBM 7094 computers. The logic was also called a current mode circuit.

Yourke's current switch, also known as ECL, was a differential amplifier, and the input logic levels were different from the output logic levels. "In current mode operation, however, the output signal consists of voltage levels which vary about a reference level different from the input reference level." In Yourke's design, the two logic reference levels differed by 3 volts. Consequently, two complementary versions were used: an NPN version and a PNP version. The NPN output could drive PNP inputs, and vice-versa. "The disadvantages are that more different power supply voltages are needed, and both pnp and npn transistors are required."

Instead of alternating NPN and PNP stages, another coupling method employed zener diodes and resistors to shift the output logic levels to be the same as the input logic levels.

ECL circuits in the mid-1960s through the 1990s consisted of a differential amplifier input stage to perform logic, followed by an emitter follower to drive outputs and shift the output voltages so they will be compatible with the inputs.

Motorola introduced their first digital monolithic integrated circuit line, MECL I, in 1962. Motorola developed several improved series, with MECL II in 1966, MECL III in 1968 with 1 nanosecond gate propagation time and 300 MHz flip-flop toggle rates, and the 10,000 series (with lower power consumption and controlled edge speeds) in 1971.

The high power consumption associated with ECL has meant that it has been used mainly when high speed is a vital requirement. Older high-end mainframe computers, such as the Enterprise System/9000 members of IBM's ESA/390 computer family, used ECL as did the Cray-1; and first generation Amdahl mainframes. Current IBM mainframes use CMOS.