[HECnet] No connectivity to arsgea 4

[Dave McGuire]

On 1/23/21 6:40 PM, David Moylan wrote:
> Reminds me a little of the hardware development of the BBC Micro. The original design used an NS 81LS95 multiplexer for the memory, and for reasons that nobody could explain, only the NS component worked in their design - the same component from other vendors failed.
> 
> The core reason was due to the bus speed and how the original infrastructure design was done, but the choice of many components was critical to the stability of the machine.
> 
> I'm not implying NS is better - just saying that it's interesting to compare stability and long term failure rates in the field and how much they can vary.

   Far be it for me to disparage Nanny Beeb, but this type of dependency 
is indicative of poor design.  Part of the design process involves 
calculating out all margins and tolerances to arrive at worst-case 
figures for things like logic voltage levels, timing, etc.

   The datasheet for a component constitutes a sort of contract.  The 
manufacturer of the component guarantees that the component's 
specifications will match those listed in the datasheet...but nothing 
more.  All manufacturers can and do to change HOW a component meets 
those specifications, and subtle variations can and do occur as a result.

   Datasheets usually list minimum, typical, and maximum values for 
various parameters.  From one fab to the next, from one production batch 
to the next, and even from one chip to the next, the component's 
performance on a given parameter will vary within that range.  All 
designers know this (or at least they're supposed to!) and they must 
perform end-to-end calculations based on the min/max range of those 
parameters.

   A dependency on a particular manufacturer's implementation of a 
standard and common design pattern like the 81LS95 is a bug.  I sure 
hope they fixed it in a later revision.

            -Dave

-- 
Dave McGuire, AK4HZ
New Kensington, PA