Coleco Rom Carts

[Mike Harris]

Someone knows....
Opening and cleaning some old cartridges you can have 1 rom, 2 roms and 3 roms.

I get it 8, 16, 24.....

None of these original carts/roms have any extra chips or capacitors.
However when I see peoples new designs like the ones I bought online have a chip.  A SN74HC21N Dual 4 input Gate

Why is this necessary for a single 32k chip?

I am just trying to understand.
Let me ask a child would and say why can't you just connect the address lines directly through the card edge connector like the originals.
 

[ChildOfCv]

The Colecovision provides 4 separate select lines that represent 4 different slices of the address space.  You need those select lines to know when to actually dump data onto the bus.  You can't just wait for a recognizable address because the CPU may be in transition, or it may simply not be ready.  Only when one of the select lines go low do you have permission to output.  That 4-input AND gate collects the 4 select lines into a single one that the chip can use to activate.

 

The old cartridges used multiple 8K ROMs, and each one had its own select line.

Edited November 2, 2019 by ChildOfCv

[Mike Harris]

That explanation should have been so obvious if I just looked at the pin outs of the eeproms.