Correct.  The PDP-1 (18 bits) was DEC's 1's complement machine.  Its direct successors the 4/7/9/15 had both 1's and 2's complement arithmetic.  The 12-bit 5/8/12 machines had only 2's complement, but retained the PDP-4 mnemonic TAD (Two's-complement Add).  By the time the 36-bit 6/10/20 line was designed, it was clear that 1's complement was history, and the mnemonic was changed to ADD.  

(The PDP-3 was a PDP-1 with a 36-bit data path, and only one ever went into production; the PDP-2 was to be a 24-bit machine, perhaps a compromise between 6-bit and 8-bit byte systems, but was never even designed.)

John Cowan          http://vrici.lojban.org/~cowan        cowan@ccil.org
I marvel at the creature: so secret and so sly as he is, to come sporting
in the pool before our very window.  Does he think that Men sleep without
watch all night?    --Faramir