On Jun  3,  7:09pm, Jonathan Stone wrote:
> 
> >I have an unproven hypothesis that it might be unnecessary to
> >'slow-down' NCR 53C94 clock rate when PMAZ-A is attached with
> >slowly-clocked (12.5MHz) TURBOchannel option slots found in 3MIN or
> >MAXINE.  A crude try with my MAXINE shows PMAZ-A still works at 25MHz
> >clock rate, but I'm not sure it's globally true.
> 
> I haven't seen them, but I understand that the DEC drivers (both OSF/1
> and Ultrix) set a slow 53c94 clock on half-speed TC busses.  The
> Mach/SPrite code in 4.4BSD/pmax did so, too I think we should
> understand why they did that, and be sure it's safe not to, before
> changing.
> 
> I'm guessing, but could it be poissble for 2 or 3fast-clocked 53c94
> PMAZ-As to overrun a 12.5MHz TC bus (or the host memory system)
> causing DMA overrun and data loss?
> 
> So even if it works in practice, I'd be reluctant to commit this
> unless it's sysctlable and defaults to off. Make sense?


  The PMAZ-A card gets it clock from the TurboChannel clock.  [I've
measured the clock input on the 53C94 on several different
configurations to verify the 53C94 clock configuration was getting set
correctly.]  Since the TC clock on the Maxine and 3MIN is 12.5Mhz,
configuring the 53C94 for a 25Mhz clock actually means you will be
slowing down the 53C94 (i.e. when the 53C94 counts clock cycles for a
250ms timeout, the 53c94 the timeout will actually be 500ms).

Michael

-- 
Michael L. Hitch			mhitch@montana.edu
Computer Consultant
Information Technology Center
Montana State University	Bozeman, MT	USA