24 May
2000
24 May
'00
7:45 a.m.
select a bunch of memories, and one to select a bunch
of '138's that
selected the memories. Back when small (1K-bit) memories were the most
common sort, there were lots of chips expecting to see that select strobe,
and the worst case load made the prop-delay long.
Compared to the typical 500-650ns prop delay of common memories back in
1976 teh prop delay of the '138 was relatively short.
configuration might have been a bank of 72 2102's
(they liked parity back
then) and about eight 1702's. One 138 would drive each of the rows of RAMs,
I found it to be rare except in mainframes and then it was usually 4kx1
parts which were common and available.
layout I suggested. 2102's back then had a
typical access time of 600 ns
and 1702's were somewhere between 750 and 1000 nanoseconds. I don't think
one could claim that 50 ns is negligible with a 2 MHz 8080 under those
conditions.
It was. The 2102s were fast enough to run with no waits but 1702s were so
slow another 50ns was nothing as waits were added in 500ns increments.
By 1977 the 1702 was long gone and nearly history for anything other than
hobby and existing production and the 2508/2716 were moving in fast
with a 450ns Tacc.
Allison