"CORRTEX" is the magic word for finding details; thank you for that reply. (CORRTEX = COntinuous Reflectometry for Radius vs Time EXperiment.)
I found an article explaining CORRTEX at https://permalink.lanl.gov/object/tr?what=info:lanl-repo/lar...
It's based on a portable Motorola 6800 microprocessor, so the PDP-11 connection is unclear. A coaxial cable is placed up to the detonation point, and as the shock front progresses the cable is crushed, reflecting the TDR pulse back from that point. They call it "Radar on a wire". The interval between pulses is between 20 and 90 microseconds, so this is much faster than the alleged PDP-11 network card. The original system was developed in 1975 and evolved to the 6800-based system. More information in the paper http://dx.doi.org/10.1063/1.1136283
An earlier system was SLIFER (Shorted Location Indicator by Frequency of Electrical Resonance). It used the same idea of a coaxial cable progressively shorted by the blast wave. However, instead of using reflectometry, it used a resonant Colpitts oscillator whose frequency was 1/4 wavelength. So as the cable was crushed, the frequency increased. Plotting the frequency vs time showed the progression of the blast wave.
The oscillator itself was a small circuit board in the blast hole, while data was recorded in a separate trailer. To quote the paper: "In the event the recording facility must be located near surface zero, shock mitigation may be required to assure survival of equipment." The paper includes PCB layouts in case you want to make your own SLIFER system :-)
I found an article explaining CORRTEX at https://permalink.lanl.gov/object/tr?what=info:lanl-repo/lar... It's based on a portable Motorola 6800 microprocessor, so the PDP-11 connection is unclear. A coaxial cable is placed up to the detonation point, and as the shock front progresses the cable is crushed, reflecting the TDR pulse back from that point. They call it "Radar on a wire". The interval between pulses is between 20 and 90 microseconds, so this is much faster than the alleged PDP-11 network card. The original system was developed in 1975 and evolved to the 6800-based system. More information in the paper http://dx.doi.org/10.1063/1.1136283