#A4000 motherboard
4 messages in this thread
Hello,
on the A4000 motherboard, underneath the CPU card, there is an IC socket
which contains a 74HCT244. I read somewhere that if you were having troubles
with expansion cards, replacing this IC with a 74F240 may improve matters. I
have a Emplant card and a Piccolo SD64 video board and with the 74HCT244, my
Mac emulation freezes after about 15-30 minutes, the IBM emulation after 1-2
minutes, and sometimes the Amiga itself freezes after a couple of hours. With a
74F240, everything seems to run O.K..
Does anyone know why this should be ? Why did Commodore put this IC in a
socket, the only one on the motherboard ? Are there any potential problems ?
From now on I'm leaving the 74F240 in, but I would like to know the
background to this modification.
Fred
Fred,
I don't know the specifics on the A4k but I do know the 'HCT244 and 'F240.
They're the same functionally but the 'F240 is a faster device, i.e. has lower
propagation delays. Sounds like the timing in the A4k may be marginal and the
temperature rise may be enough to cause a hardware lockup. Why use the 'HCT244
– maybe something as simple as a shortage in supply of 'F240s when your machine
was built. Maybe C= knew it was marginal and used the socket to allow for an
easy fix later.
Steve
Daring to be different, Amiga on the Internet, Lotus on the Interstatee
Hello Steve,
the chip shortage reason makes sense. Why put in a socket with associated
labour costs if you don't need to. After all, we all know how competitively
priced the A4000 is (g). Both chips are bus drivers, but isn't one inverting
and the other non-inverting ?
Fred
Fred,
You're right ! The '240 is inverting, the '244 non-inverting. I hope they're
not sending data through them 🙂
If its some sort of clock the inversion wouldn't matter, the net result being a
(180 degree) phase shift.
Steve
Daring to be different, Amiga on the Internet, Lotus on the Interstatee