[abc80] Super Smartaid Cartridge

[H. Peter Anvin]

Here is a run-down of the 7400 series chips:

74LS245 = octal bidirectional buffer
74LS126 = quad buffer with 3-state outputs
          each buffer has an individual gate (1 = active)
74LS260 = dual 5-input NOR gate
74LS393 = dual 4-stage binary counter
74LS74  = dual D-type flip flop with preset and clear
74LS155 = dual 1-of-4 decoder
74LS00  = quad 2-input NAND gate
74LS32  = quad 2-input OR gate
74LS08  = quad 2-input AND gate

Based on these chips I'm going to make a couple of guesses:

I wasn't too far off with guessing how boot mode was implemented, except
that they look at the address *after* the RST# instead of *before*. The
counter and the wide NOR gate chips makes sense for that.

74LS155 pretty obviously decodes bits 1:0 from the address PROM. 74LS155
is interesting in that both decoders get the same input, but each one
has two enable inputs - one is low-low and one is low-high. One can be
fed XMEMFL# and one XMEMWR#, and one gets read and write flags for each
unit, which is great for both the CMOS and the write-to-ROM-to-switch
pages hack.

There seem, however, to be a lot more flip flops than ought to be
necessary, unless they have done something like implement a shift
register in 74LS74s. There are four chips -- six flip flops -- on the
board, plus the counter. Two are obvious: boot mode and page; and one
can guess that one is used to turn on the search-for-boot-mode circuit.
There are still too many...

	-hpa