Altair parts substitutions

The propagation delays cited for the three popular bipolar technologies in which the '138 was offered seem to vary pretty widely. My experience has been that these spec's reflect worst-case conditions: conditions more frequently encountered then than now, since it was likely a designer in '76 or so would be using two levels of these decoders on a memory board, one to 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. When there's only a single 2764 to drive, the difference between the three available technologies is much smaller. A fairly typical memory 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, while another drove each of the 8 1702's. That's lots of capacitance, not to mention a fair DC load. I'd say the difference between STTL and standard TTL would have shown up there in the form of a difference of maybe 15-25 ns. Due to the size of memory devices back then, the decoding tree would have been pretty long, therefore combining the delay through several decoders. This would probably have been a depth of two in the case of the memory 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. Dick ----- Original Message ----- From: Tony Duell <ard(a)p850ug1.demon.co.uk> To: <classiccmp(a)classiccmp.org> Sent: Tuesday, May 23, 2000 7:19 PM Subject: Re: Re[2]: Altair parts substitutions