Communication Scanners
Three different communication scanners were offered:
The Type 1 Communication Scanner was an entry level device which presented an interrupt on every received bit. Transmission also required an interrupt for every bit. In theory this would have allowed for rather imaginative uses such as Morse Code and connection to devices with unusual framing methods. A maximum of 64 half-duplex lines could be attached. The aggregate bandwidth was restricted due to the heavy processing requirements.
The Type II Communication Scanner performed functions similar to a USART. There was an interrupt for every transmitted or received character. Six different asynchronous character formats, two 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 9600 bit/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 9600 bit/s was still present. Restricting scanner size to 16 lines allowed line speeds of up to 30,000 bit/s. A scanner size of 8 lines allowed speed of about 60,000 bit/s.
Read more about this topic: IBM 3705 Communications Controller