IBM 3705: Контроллер связи для систем С/360 и С/370

The IBM 3705 Communications Controller is a simple computer which attaches to an IBM System/360 or System/370. Its purpose is to connect communication lines to the mainframe channel. It was a first communications controller of the popular IBM 37xx series. It was announced in March 1972. Designed for semiconductor memory which was not ready at the time of announcement, the 3705 I had to use 1.2 microsecond core storage; the later 3705 II uses 1.0 microsecond SRAM. Solid Logic Technology components, similar to those in S/370, were used. The 3705 normally occupies a single frame two feet wide and three feet deep. Up to three expansion frames can be attached for a theoretical capacity of 352 half duplex lines and two independent channel adapters. The 3704 is an entry level version of the 3705 with limited features.

Three types of peripherals were available: communications scanners, channel adapters and 8" floppy disk (only available in models without a channel adapter). The first Communication Scanner and the first Channel Adapter occupied the same frame as the CCU. Extra peripherals required extra frames. All Amdahl 4705s were provided with an 8" floppy drive, which was used for diagnostics as well as for remote program loading. This floppy drive and all Channel Adapters were contained in the Base Frame, although additional S/370 channel I/O interfaces could require an Expansion Frame. That is, more than four I/O interfaces required an Expansion Frame, which accommodated four additional I/O interfaces.

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.

The Amdahl 4705 was compatible with and ran about 2.5 times as fast as a comparable 3705 II. The IBM 3704 was an entry level version of the 3705. It was restricted to 26 half duplex lines (assuming Type 2 Scanner). DMA devices were not supported on the 3704. Performance was about the same as a 3705 I. A 4704 (a 3704 compatible device) was planned, but was never introduced. The Amdahl 4745 was completely and in every respect a logical superset, or a physical subset of the IBM 3745. Whereas the IBM 3745 offered one or two physical processors, and one or two physical buses, both apparently for redundancy, the Amdahl offered one very highly reliable physical processor and one very highly reliable physical bus (but two logical buses), with no apparent need for redundancy. Whereas the IBM 3745 offered two buses, the Amdahl 4745 offered a single bus which throughput capacity exceeded the aggregate throughput capacity of the two buses of the IBM 3745. The design of the Amdahl 4745 was such that the control program (NCP) could not tell whether it was operating in an IBM 3745 or in an Amdahl 4745 (or, for that matter, in an IBM 3725 or in an Amdahl 4725). Amdahl's logical and physical simulation of the IBM 3745 (IBM 3725) was that complete. Later, a very high speed scanner was offered with the Amdahl 4745 (and also with the Amdahl 4725, for which IBM had no such corresponding 3725 offering) which offered increased redundancy over the IBM 3745 very high speed scanner (1.544 Mbit/s, T1, or 2.048 Mbit/s, E1) by having the line switchable to a backup 4725/4745 under operator console control.

The 3705 was eventually replaced by the 3720 and 3725. The 3745 was similar but not entirely compatible. The 3705 was withdrawn from marketing in December 1985. Hardware maintenance service was withdrawn in January 1999.