As a learning experience, I am studying a
64K 80 COLUMN expansion card for the
Apple IIe. The model number is
820-0067-D for those interested.
Anyway, I'm sure the schematic is online somewhere but I just wanted to do this for fun.
The first thing I noticed, however, is that it appears the designers used a Hex Schmitt Trigger Inverter (
74LS14 PC) solely to enable another IC in the circuit (Octal Bus Transceiver
So on the
74LS14, they tied
!E on the
74LS245. And, since the
A1 tied to
A3 isn't connected to anything, it seems to enable the
Why would they do this? Why not just tie the
!E enable pin to ground?
In fact, I can't see why the Schmitt Inverter is used at all. The only other connections it has is
A6 to external pin 26 on the motherboard and
Y6 tied to
Y5 tied to nothing.
It just seems like a waste. Do you think it was used as some type of propagation delay? If so, seems like it would only be around 50ns or so.
Here is a crude schematic that I came up with. I might just try and dig up the real schematics to make sure I'm not crazy. lol
Looks like I was wrong.
A3 does indeed connect to
A2 is not floating, it is connected to