teener@apple.com (Michael Teener) (09/08/89)
In article <PAISLEY.89Sep7122146@lobotomy.cme.nist.gov> paisley@cme.nist.gov (Scott Paisley) writes: > Does anyone know if the NuBus slot are still at 10Mhz? It would seem > to me that faster slots would be a great selling point. Especially if > apple is planning on make the IIci there mainstream machine. The answer is no, not ever ... and yes, maybe. To explain ... 1) NuBus must always have a fundamental clock rate of 10 MHz or existing cards will fail. It is also *extremely* difficult to get a synchronous bus to run much faster without drastic increases in system cost *for all cards*. The major difficulty is getting the arbitration and bus turn-around (going from writing the address to reading the data) to run faster. Note that Next only runs their Nubus-similar system at 12.5 Mhz, which is not enough of an improvement for us to obsolete everything in sight. 2) HOWEVER, it still may be possible to run data bursts faster than the current 10 MHz. This is what Next does to get the 25 MHz burst rate ... effectively sending two words for each 12.5 MHz clock. There are currently two proposals before the IEEE P1196-R (NuBus Revision) working group that do just that ... effectively getting a 20 MHz data burst rate. These proposals are coming from Supermac and E-Machines people, and are both quite compatible with existing cards. The big change will have to come in Mac CPU's: a) The existing unused (and shorted together) -5.2V lines will have to be nuked (You weren't using them, were you?) and reallocated to other functions. b) At least two of these recycled signals are used for a new clock line and a special mode indicator. The first detailed discussion of these proposals will be done at the next 1196 meeting in the Boston area next week. The working group is trying to get a draft of the revision finished by Christmas. ****There is absolutely no commitment by Apple to do anything about any of these proposals.**** ---- Michael Teener -- 408-974-3521 -- IEEE P1196-R Secretary -------+ ---- Internet teener@apple.com, AppleLink TEENER1 | ---- Apple may know my opinions, but *I* am responsible for them | ---------------------------------------------------------------------+ Transportation by Cheetah N9900U, a loyal beast for the past 6 years.