Manufacturer:  Systron Donner Model:  1037-5 Year:  1967 Schematic:  RE

Max. Rate:  50 MHz Display:  NIXIE Digits:  8 Logic:  discrete transistor, TTL

From the outside this counter appears to be a fairly standard higher-end NIXIE-display model of the period. Examination of the internals reveals it to be a tour of the unusual semiconductor devices that were developed in the 1960s.

2 unijunction transistors are used, one to generate the display delay, the other as an oscillator to trigger display updates.

Silicon-Controlled-Switches are used in each decade as a combination latch and driver for the biquinary NIXIE tubes. This achieved the storage function without requiring a full set of flip-flops to duplicate the counter flip-flops.

Tunnel diodes are used in the first decade to achieve the high (50MHz) count frequency. The decade begins with a 5-stage Johnson counter. Each stage uses a tunnel diode and discrete transistor to make a 'flip-flop'. This counter is double-clocked (somehow) by each input pulse so only 5 of the 10 states are effectively used. The Johnson counter drives a conventional two-transistor flip-flop to complete the 10 states for the decade. The 5 outputs from the Johnson counter and the output from the flip-flop go on to drive a decoder and Silicon-Controlled-Switch latch and thence on to the NIXIE tube.

Curiously, the timebase divider string is made from seven 7490 TTL decade counters, date-coded from 1966 and 1967. The SSI 7400 series had been introduced a couple of years earlier but this is quite early for these MSI devices.

From top, the first board contains an input amplifier and the 50MHz tunnel diode ring counter. Next is a 250KC decade counter/decoder/driver. At bottom is the timebase divider string containing the early 7490 ICs.

A closer view of the 50MHz tunnel diode ring counter of the first decade, with the components arranged in a .. ring!