forms of Bisync and HDLC/SDLC were supported. A single scanner could attach up to 96 (64 for the first scanner) half-duplex lines. This is the basis of the theoretical maximum capacity of 352 lines. In practice the limit was lower as a scanner with more than 48 half duplex lines could not support any 9.6 kbit/s lines. The Type III Communication Scanner was a high performance device for attachment of Bisync and HDLC/SDLC lines. It operated on entire frames. DMA was used to fetch and store the bytes of a frame. In theory the line attachment capacity was the same as for Type II Communication Scanner (352 line limit). The need to restrict scanner size to 48 lines to support 9.6 kbit/s was still present. Restricting scanner size to 16 lines allowed line speeds of up to 30 kbit/s. A scanner size of 8 lines allowed speed of about 60 kbit/s. Maximum communication line connection estimates With modem supplied clocking, a single line was limited to 56 kbit/s; with internal clocking the limit was 2.4 kbit/s. Aggregate line connection ability was limited by processing speed. For a Type II Communications Scanner, processing of a received or transmitted character might take fifty storage cycles (forty instructions). In a 3705-II this gives an aggregate capacity of 20 kB/s. This would allow about 160 half-duplex lines running at a mean speed of 120 cycles per second (cps). Alternatively sixteen half-duplex lines running at 1200 cps (9.6 kbit/s) would be the theoretical limit. IPSANET experience was that six full-duplex 9.6 kbit/s lines carrying a heavy load was the limit. IBM software may have had superior performance. For a Type III Communications Scanner cycle steal processing of a single character was fairly inexpensive—a single storage cycle was required. This would give a theoretical limit of a million cps (eighty 9.6 kbit/s half-duplex lines). In practice the limit was probably lower as some processing would be required for end of frame. Also if the frame contents were moved about in storage this would require 3.5 storage cycles per byte. IBM communications scanners were limited to 48 lines on the first communications scanner and 96 lines on communications scanners two through four. Amdahl communications scanners were theoretically capable of 128 lines per scanner, but this was artificially limited to 48 and 96 for marketing reasons. Channel adapters Two types of channel adapter were offered: Type 1 and