IBM 3705 Communications Controller

Up to three expansion frames can be attached for a theoretical capacity of 352 half-duplex lines and two independent channel adapters.

IBM intended it to be used in three ways: The storage word length is 16 bits.

Register zero is the program counter which gave the address of the next instruction to be executed; the other seven are accumulators.

Most are register-to-register or register-immediate instructions which execute in a single memory cycle.

Special hardware assists in the calculation of a cyclic redundancy check for detection of transmission errors.

An Amdahl 4705 has a maximum memory capacity of 1024 KB, all of which is contained within the base frame.

The first Communication Scanner and the first Channel Adapter occupied the same frame as the CCU.

All Amdahl 4705s were provided with an 8" floppy drive, which was used for diagnostics as well as for remote program loading.

In theory this would have allowed for rather imaginative uses such as Morse Code and connection to devices with unusual framing methods.

The Type II Communication Scanner performed functions similar to a USART.

The Type III Communication Scanner was a high performance device for attachment of Bisync and HDLC/SDLC lines.

For a Type II Communications Scanner, processing of a received or transmitted character might take fifty storage cycles (forty instructions).

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.

This would give a theoretical limit of a million cps (eighty 9.6 kbit/s half-duplex lines).

Data transfer was via cycle steal which made the overhead low if large buffers were used.

The Type 3 Adapter could be simultaneously operational (but not in mid-command) on two different channels which might be connected to different hosts.

It was restricted to 26 half duplex lines (assuming Type 2 Scanner).

The Amdahl 4745 was completely and in every respect a logical superset, or a physical subset of the IBM 3745.