R, S and B Series logic probes Tools for classic Flip-Chip logic Part of the PDP-8 Hardware Documentation by Douglas W. Jones THE UNIVERSITY OF IOWA Department of Computer Science

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:

• A 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.
• A 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 and -15.
• A 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.