> I always thought that xxx40xx CMOS chips were pretty much compatible.

No.
Simplistically -

B series are buffered.
A series are not.
A series are good mainly for pulling skins off rice puddings, lowest
power and using in  linear mode as analog amplifiers.
B series have more hair on chest but still hardly a "power" device by
most standards.

A series have a very useful place in life - even though eg Digikey
seem to sell only B series for eg 4049.

RCA felt the need to clarify the differences in 1983  and TI thought
it still important enough to rewrtite a Harris rewrite and reissue in
2001 in a 17 page paper. Worth reading

   http://focus.ti.com.cn/cn/lit/an/scha004/scha004.pdf

"Both buffered and unbuffered CMOS B-series gates, inverters, and
high-current IC products
are available from TI. Each product classification has application
advantages in appropriate
logic-system designs. Many CMOS suppliers have concentrated on
promoting buffered
B-series products, with applications literature focusing on the
attributes and use of the
buffered types. This practice has left an imbalance in the
understanding and application of
both buffered and unbuffered gates. In some instances, customers are
not using unbuffered
products when they are the best choice for the intended application.
This application report
offers clarification of the relative merits of the buffered and
unbuffered CMOS devices.
This application report was acquired by TI from Harris Semiconductor
Corporation and edited
and reformatted in December 2001. This application report is adapted
from AN6558.1,
published b Harris (1992), which, in turn, was adapted from ICAN-6558,
published by RCA
(1983)."

________

Also useful:

                    http://en.wikipedia.org/wiki/4000_series



  R
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