DEC R, S and B series logic use voltages that seem alien to those whose
hardware experience began after 1970. The supply voltages are +10 and -15,
and the logic levels are 0 (active pull-up) and -3 (passive pull-down).
In working with this logic, I have developed several logic probes:
simple 2-lead probe,
Glows on 0V, off on -3V, made with just a Zener diode and an LED with integral
current limiter. Powerd by a single lead to -15.
slow pulse detector,
Blinks on either the positive or negative edge (switch selected),
but unable to reliably
detect pulses shorter than about 400 nanoseconds. Powered by leads to ground
fast pulse detector and logic probe, with 3 LEDs, green at 0V, orange for
pulses and red for -3V, with a switch to set the edge it detects.