On Fri, 28 Jan 2000, Nicolas Welte wrote: > Did CBM have a patent for the GCR technology or maybe the speed zone > implementation with the variable bit clock? Why did others always use > variable spindle speeds instead? It's probably easier to design the data > recovery circuit for a fixed clock, but a variable speed drive also > isn't that trivial... GCR predated Commodore's use of it. IBM used it on some of their 8" drive systems. Some of the IBM type controllers have built in GCR translation. > Also, it's a while since I tried to understand what the GCR ROM actually > does in a 8250: I think it looked like being able to convert a whole > byte in one direction, but only half a byte into the other one. Can > someone please confirm if this is correct or not? As I recall, the GCR translation in the 8050/8250/4040/3040/2040 series takes place one nybble at a time, 4 bits ASCII and 5 bits GCR. However, the parallel/serial data conversion takes place in an 8 bit shift register. I don't recall the details of where the 4/8 bit split is made. The use of the ROM allows the ASCII/GCR conversion to take place as the data is read/written from/to the disk without additional processing time required. - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi.
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