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The specifications for TTL-compatible CMOS outputs usually have two sets of output parameters; one set or the other is used depending on how an output is loaded ..
A TTL load can consume much more sink and source current, up to 4 mA
from and HC/HCT output and 8 mA from a VHC/VHCT output. In this case, a
higher voltage drop occurs across the “on” transistors in the output circuit, but
the output voltage is still guaranteed to be within the normal range of TTL output
A CMOS load is one that requires the output to sink and source
very little DC current, 20 mA for HC/HCT and 50 mA for VHC/VHCT. This is,
of course, the case when the CMOS outputs drive only CMOS inputs. With
CMOS loads, CMOS outputs maintain an output voltage within 0.1 V of the
supply rails, 0 and VCC. (A worst-case VCC = 4.5 V is used for the table entries;
hence, VOHminC = 4.4 V.)
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