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Old 17 July 2016, 21:18   #42
Join Date: Jun 2010
Location: out in the wild
Posts: 1,245
Originally Posted by SpeedGeek View Post
The DKB2632 originally used 74F244s for it's address buffers which place a full TTL load on the address bus. The problem is the Address bus was already overloaded (especially after the FCC guys switched the 1K pull downs to pull ups)... and there is much much more!

It's truly amazing that with all the design bugs this combination worked as well as it did!
Address is not a problem, as the address bus is valid a full cycle before /AS (during /ECS), so there is plenty of time for all lines to settle, and for logic to trickle to a stable state before /AS triggers.

1K pulldown on the data lines is a little more challenging, but the LCX16245 drivers that I'm using are strong enough to drive the load - just not really fast due to the high capacitive load with FPU and all the RAMs unbuffered on the bus. My prototype has some trouble doing a -1-1-1 burst, but since the first access works fine, I can always revert to a -2-2-2 burst. I'd like to try fiddling with shifting clocks first, and only if that's not successful, I'll slow down.

The 30MHz limit is only given if you use the 2M or 4M on the 2630. If you can live without that, it should be safe to clock a lot higher. Haven't checked the 68000-access state machine yet, so I don't know if that gives a limit. Did you ever try running at higher speeds without local memory? I know it doesn't make sense without a mem expansion and it will only work properly without DMA devices.

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