Channel Adapters
Two types of channel adapter were offered:
Type 1 and Type 4 Channel Adapters were designed for 270x emulation. They recognized up to 256 channel addresses and transferred data in small bursts (four bytes for the Type 1; 32 bytes maximum for the Type 4). The hardware could accept almost all of the 240 possible channel command codes. Software had to analyse the command from the channel and either reject the command or process it according to specific rules. Interrupt processing required about fifty storage cycles to process a four byte transfer. The 3705 could only have a single Type 1 Adapter but two Type 4 Adapters were permitted.
Type 2 and Type 3 Channel Adapters were designed for NCP use only. Only one device address was recognized. Data transfer was via cycle steal which made the overhead low if large buffers were used. The hardware accepted seven channel commands of which four were completely processed by the adapter. This left a read and two write commands for the software to process. These adapters could be connected to any of the three System/370 channel types but a block multiplexor channel was preferred. The Type 3 Adapter could be simultaneously operational (but not in mid-command) on two different channels which might be connected to different hosts.
Read more about this topic: IBM 3705 Communications Controller
Famous quotes containing the word channel:
“This is what the Church is said to want, not party men, but sensible, temperate, sober, well-judging persons, to guide it through the channel of no-meaning, between the Scylla and Charybdis of Aye and no.”
—Cardinal John Henry Newman (1801–1890)