datasheets for 82S21 - Signetics 32x2 SRAM?

[Tony Duell]

> The mentioned 7480 is maybe hard to locate even as a datasheet (good 
> idea to keep old  TI TTL databook, IIRC

Why do you think I keep (and actively grab) all old databooks :-)

> also at bitsavers), and even more difficult to locate the chip itself, 
> but this is a function that would easily fit into a GAL.
> A real problem are species what can't be simply replaced by some GAL, 
> namely open collector components or "extendable"
> gates like the 7450. Replacing a single chip of such a kind usually 

I hate to tell you this, but the 7480 will not fit into a GAL. The A* and 
B* pins are effectively open-collector I/O with intenral (6k) pull-ups. 
You can extend the input gating via that pin, or you can monitor the 
output of the A1/A2 NAND gate there.


> means replacing its environment as well, e.g. that 7450 would
> then require to pull the extension gate (was it 7460, my memory is 
> failing) as well.

Great fun if it happens to be on another board (did this ever happen?)

> Surely the question original replacement vs. reengineered functionality 
> has some religious aspect, but if it is the question of whether
> an old system should be left inoperable in a museum in contrast to 
> actually switching its blinkinligths I favor reengineering.

Me too. Computers that don't work are not interesting. That said, I am in 
favour of doing as few mods as possible to the original machine. For 
example, I'd fit a turned-pin socket in place of the original chip and 
wire up a heeader to the replacement chip (maybe using stripboard or a 
PCB to make a plug-in module). That way, if I ever get the real part, I 
can go back to the original design.

-tony